# Proposed workshop build



## DomP (27 Aug 2022)

I'm a longtime lurker on here but this is my first post.

I'm planning a workshop of roughly 7 x 4.5m, I'm thinking of a raft type foundation, 3 courses of brick then studwork framing clad internally with osb, insulation and then wrapped externally in membrane then either corrugated sheet or feather edge. My question is if I'm going to 2.5m at the eaves what size section of treated cls should I use for framing if I'm going for 600mm centres on my uprights? I was thinking 50×120mm, any help or thoughts much appreciated. Dom


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## DomP (27 Aug 2022)

One other consideration is a large beech tree that sits central on our land and is near to where I'd like to site my workshop, how close can I sensibly go to it or what measures can I take to safeguard my concrete pad? Felling it is not an option at this point


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## Jameshow (28 Aug 2022)

DomP said:


> One other consideration is a large beech tree that sits central on our land and is near to where I'd like to site my workshop, how close can I sensibly go to it or what measures can I take to safeguard my concrete pad? Felling it is not an option at this point



Plenty of reinforcement in the pad? And thicker too say 300mm. 

Others will be along with more concrete advice!


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## DomP (28 Aug 2022)

300mm, wow I wasn't banking on that. Ive looked at resiting it but this area makes most sense other than the tree situation


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## DomP (29 Aug 2022)

Any thoughts on this? I'm keep to proceed with my plans but the realisation that a large beech tree is going to cause me issues has stopped me in my tracks. Repositioning is an option but doesn't work so well with garden layout or the contour of the land as its not an overly flat site.


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## Seascaper (29 Aug 2022)

DomP said:


> I'm a longtime lurker on here but this is my first post.
> 
> I'm planning a workshop of roughly 7 x 4.5m, I'm thinking of a raft type foundation, 3 courses of brick then studwork framing clad internally with osb, insulation and then wrapped externally in membrane then either corrugated sheet or feather edge. My question is if I'm going to 2.5m at the eaves what size section of treated cls should I use for framing if I'm going for 600mm centres on my uprights? I was thinking 50×120mm, any help or thoughts much appreciated. Dom


Hello,
Reference your workshop dimensions - if you buy 4 x 2 in bulk to can get a discount it comes in 6m lengths so that the length of the building may be better belong 6m, something that can be considered. Your timber yard will give a better discount if you ask for whole lengths of 6m 4x2. Work out the roof pitch to save wastage.
I built such a workshop and went for a portal frame building, so that it is a completely free open space, that is, no roof trussing. With a portal frame building of this size you would need one large central support frame floor to roof apex made from something like 8 or 10 x 2 bolted together with metal knee plates at both eaves and apex.
The advantages are enormous when using long lengths in your workshop.

Start of with 4 to 5 courses of bricks on to a trench type foundation about 1ft wide, damp proof membrane and bolt 4x2 wood to bricks and build up from there.
For the floor get some sheets of metal reinforcing steel, raise of ground and then pour concrete over and smooth. If you want it reasonably warm put some sheets of 2 inch polystyrene on ground before you por concrete. Remember to have fun doing it....

Regards


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## DomP (29 Aug 2022)

I want to maximise the permitted size allowed without planning and without it being a long narrow space so I initially thought along the lines of 7x5 given that the internal space would then be nearer to 30m². I've no close neighbours and rural so I'm not too fussed on being strict to actual size as a few around us have just done as they please with outbuilding builds.


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## pauljhaigh58 (29 Aug 2022)

Hi have you considered ground screws you won’t have problem with roots and can get closer to the tree than with a concrete pad


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## DomP (29 Aug 2022)

pauljhaigh58 said:


> Hi have you considered ground screws you won’t have problem with roots and can get closer to the tree than with a concrete pad


I need a concrete pad due to a 250kg planer thicknesser and motorbikes that will be occasionally stored/maintained in it. I'd also like to have 3 courses of brick or a block wall to get above splashback


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## Jameshow (29 Aug 2022)

Seascaper said:


> Hello,
> Reference your workshop dimensions - if you buy 4 x 2 in bulk to can get a discount it comes in 6m lengths so that the length of the building may be better belong 6m, something that can be considered. Your timber yard will give a better discount if you ask for whole lengths of 6m 4x2. Work out the roof pitch to save wastage.
> I built such a workshop and went for a portal frame building, so that it is a completely free open space, that is, no roof trussing. With a portal frame building of this size you would need one large central support frame floor to roof apex made from something like 8 or 10 x 2 bolted together with metal knee plates at both eaves and apex.
> The advantages are enormous when using long lengths in your workshop.
> ...


6.0m? Most timber is 4.8 around me unless you pay extra! 

What are you putting in the workshop ? 

If only light machinery then why not use piers? 400mm sq dense concrete blocks? Overcomes the roots issue?


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## Inspector (29 Aug 2022)

My table saw is 310Kg and I have a couple hundred board feet of hardwood, about 400kg, in a stack near it and that doesn't include other machines and more wood. The floor is 2x10 on 1' centres with 1" OSB sheets. There is no deflection that I can detect walking on it or looking up at from the garage below. You would put your walls directly on the wood deck and run the siding down to the bottom edge of the deck. The ground screws keeping you off the ground. There would be no spalsh back. A ramp would let you drive the bikes in and roll out.


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## DomP (29 Aug 2022)

Don't get me wrong, I appreciate your advice and will have a think on it but I'd rather a solid concrete base to work from as it gets really wet where I am so the idea a wooden floor failing once I've got an expensive structure on top doesn't appeal. Also oil and fuel getting onto a chipboard floor doesn't seem a good idea. 

I'd definitely get splashback unless I have it a good 3-400mm off the deck as my current shed has shown which is why I'd like to get a few courses of brick


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## mikej460 (29 Aug 2022)

DomP said:


> Don't get me wrong, I appreciate your advice and will have a think on it but I'd rather a solid concrete base to work from as it gets really wet where I am so the idea a wooden floor failing once I've got an expensive structure on top doesn't appeal. Also oil and fuel getting onto a chipboard floor doesn't seem a good idea.
> 
> I'd definitely get splashback unless I have it a good 3-400mm off the deck as my current shed has shown which is why I'd like to get a few courses of brick


deeper eaves and guttering will take care of splashback


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## Inspector (29 Aug 2022)

I am not offended that you want to go a different route. Some of it is with the conventions you have around you. 

As for the oils, fuel and some solvents on the floor. I had assumed you would be using an appropriate finish on the floor much as you would with concrete. 

As Mike mentioned wide eaves and gutters go a long way to keeping walls dry. 

I also would not consider the shop floor extending past the outside of the shop even if being made of concrete. Your practice of a bigger slab, then bricks on it and the wall perched on top is something never done here. I see it as asking for leaks under and through it as well as being prone to failure in earthquake regions. Except for a few areas in Canada (West Coast where I grew up) rafts or slabs as we call them are rarely used because of the cold sucking the heat out of the edges and frost heaving them. The usual practice is a footing several feet deep at least with a concrete foundation wall poured to get above ground with the house on top. Then foam insulation placed around the inside edges of the foundation with a slab poured on top of foam and vapour barriers to keep dry. Here because of the cold virtually all structures have that footing and foundation wall are deep enough for basements. Otherwise foam slabs around the outside have to go almost as deep or out horizontally 6' or more to keep the frost from heaving the place. All our walkways and driveways etc are poured after the foundation etc is made, slope away and usually lower than the floors inside.

With concrete or screw piles your structure is above the ground and a skirting placed around it to keep critters and such out from under. That would also stop splashing from getting under and with treated wood at least for the perimeter a little dampness isn't an issue. Venting dries out the space. 

Construction approaches are different due to an areas history and what works there. Sometimes tradition is good and sometimes not. Thing is to keep an open mind. Good luck with your build.

Pete


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## DomP (29 Aug 2022)

Surely by the time I've got ground screws or piers and floor joists I'm going to be quite far out of the ground and losing valuable height at the eaves


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## Inspector (29 Aug 2022)

If that is a restriction then yes. A couple feet give or take, depending on the height of the ground screws, slope etc. I don't have to contend with heights. I was told for under 100 sq ft (no permits) garden sheds I can go three stories high if I like. Anything with a bigger foot print needs permits and inspections and being rural height wouldn't be an issue either. In the city they look at each build case by case if outside the general rules but someone I know was granted permits to build his shop and he has 10' high walls inside and is within 3' of the property line at most. Your rules are different and you must abide by them which can dictate/limit what you can practically do. If that means a slab then so be it. Trees won't like it but who listens to them anyway?  

Pete


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## DomP (29 Aug 2022)

I was more worried about the tree cracking my pad


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## PDW125 (30 Aug 2022)

The issue with that size will be cracking of any slab that is long and narrow where heave occurs. As it is only a timber structure, I would go with a thickened raft and use 300mm at the edges, then use 150mm Jablite insulation in the centre as insulation but also as back fill. Laid across the slab to leave a 300mm deepened edge all around, and then use sheets of A142 mesh to tie in to the overall slab and ring beam. Positioning a DPM may be a challenge with this as you’ll end up with it wrapped outside the concrete structure however you could also go with a DPM on top of the slab and then a floating chipboard or ply floor.


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## Jameshow (30 Aug 2022)

Also perhaps 200mm reinforcing squares in the 300mm edge beam wouldn't hurt?!


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## MikeJhn (30 Aug 2022)

I always leave my Jack hammer out of reach of the trees.

A slab of that length to width ratio will crack without heave, best to put some crack inducers into the slab and locate the crack where you want it, at the edge form a step that will take the DPM and attach to the DPC, or consider a suspended timber floor.


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## Jones (30 Aug 2022)

Regarding a slab foundation ask near a tree ask someone who might be able to give a definitive answer which may well save you overspecifying and wasting money or worrying you've done it wrong. Though you may not need building regs they may help or even recommend a local architect or technician who can. Paid for advice you can trust may be money well spent.
Construction softwoods are in nominal 2x5 etc the actual size will be 47x 120 mm and lengths are in 0.3 m increments from 3.6m to 5.4m. Longer lengths up to 7.2m may be available usually more expensive though . The meter price is the same regardless of length. A four inch stud will be structurally adequate for that size shed doubled up by wider than door openings. A five inch stud will give room for 4" insulation plus an air gap which is better than no air gap.


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## DomP (30 Aug 2022)

PDW125 said:


> The issue with that size will be cracking of any slab that is long and narrow where heave occurs. As it is only a timber structure, I would go with a thickened raft and use 300mm at the edges, then use 150mm Jablite insulation in the centre as insulation but also as back fill. Laid across the slab to leave a 300mm deepened edge all around, and then use sheets of A142 mesh to tie in to the overall slab and ring beam. Positioning a DPM may be a challenge with this as you’ll end up with it wrapped outside the concrete structure however you could also go with a DPM on top of the slab and then a floating chipboard or ply floor.


I like that idea as I can still stay 50mm out of the ground at the higher end of the site so not causing accessibility issues or reduced internal height.

Presumably as long as tree roots don't cause an issue when digging down for the base all should be fine.


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## davethebb (31 Aug 2022)

Dom P, Take a look at my workshop build thread "......Grade 2 listed building". The overall size is similar and we had a tree issue. We used a concrete pad, 200mm thick**, RC30 concrete poly fiber reinforced as this eliminates using A142 mesh and also reduces the risk of cracking. We have approx. 5t of clay tiles on the roof. ** I will check on the depth for you.


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## DomP (31 Aug 2022)

davethebb said:


> Dom P, Take a look at my workshop build thread "......Grade 2 listed building". The overall size is similar and we had a tree issue. We used a concrete pad, 200mm thick**, RC30 concrete poly fiber reinforced as this eliminates using A142 mesh and also reduces the risk of cracking. We have approx. 5t of clay tiles on the roof. ** I will check on the depth for you.


Thanks mate, that sounds ideal especially the fibre reinforcement.


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## DomP (31 Aug 2022)

davethebb said:


> Dom P, Take a look at my workshop build thread "......Grade 2 listed building". The overall size is similar and we had a tree issue. We used a concrete pad, 200mm thick**, RC30 concrete poly fiber reinforced as this eliminates using A142 mesh and also reduces the risk of cracking. We have approx. 5t of clay tiles on the roof. ** I will check on the depth for you.


One detail I can't decide on is how I get the DPM to finish, I intend doing it the same as you but how did you get continuity up to the DPC between brick and frame? Great build by the way!


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## davethebb (31 Aug 2022)

The DPM was wrapped over the former (scaffold boards) and then cut - ensure as the DPM runs up the inside of the scaffold board you create a fold into the concrete as this creates a good seal so damp can not get in between. I didn't extend the DPM up the brickwork but I used DPM between the top brick (class "B" engineering bricks) and the timber. This should be sufficient for the dampness. 

Think also about the finished floor level and the door cill. I will be using UPVc doors/windows and probably stay with a concrete floor. So when I made the form work Inserted a section of timber where the door will be so the Cill will be level within the concrete floor - easy to do now and can be adjusted later if need be. I can always alter this if I put a different floor in later.

Now is a good time for concrete pouring - you don't want it to set too fast e.g you don't want it too hot.


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## Terry - Somerset (31 Aug 2022)

Tree will cause problems over time - the only question is when. In fairness it could be decades - although simply putting a building where once the ground could absorb moisture could make a difference.

Roots suck moisture from the ground. A mature tree they may be in reasonable equilibrium with its surroundings - but drought or very heavy rain will cause ground movement. Trees may need radical surgery (depending on location) and eventually die completely changing soil moisture.

The impact is also affected by the nature of the ground - clay, rock, etc etc.

Personally I would try to locate the workshop away from the tree to de-risk the project.


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## DomP (31 Aug 2022)

Right then, I've seen sense and looked at a better location, better in several senses I think. This is only near to a hedge and a small Hazel which is a good 5m away but also has better more level ground.

I'm trying to keep costs down where I cam so would a 150-200mm thick slab with fibre reinforcement work out more cost effective than using A142 mesh?

Old location first photo, new one second photo.


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## davethebb (31 Aug 2022)

DomP, As it really needs to be a continuous pour, not a DIY mix and lay, you probably need to contact your local concrete delivery companies and get some quotes for RC30 with and without Fibre mix and then compare with using A142 mesh. There probably won't be a lot of cost difference, but laying the concrete with mesh is harder and you need to keep checking the mesh has not moved off the blocks and seek advice on how to set the mesh (the height etc. within the 200mm concrete is important). look to hire a vibrating tamper that is wide enough to span the formwork.

Ensure you thoroughly compact (hire a big wacker plate) the soil down and each layer of Type 1 (lay in multiple layers) and sand.


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## DomP (31 Aug 2022)

Yes Dave, I think I need to get on the phone to a few concrete suppliers near me, I've got near free access to two wackers thankfully so they'll get a good workout! I need to have a think about formwork too


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## davethebb (31 Aug 2022)

I used scaffold boards for the formwork as they can be useful afterward.


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## johna.clements (1 Sep 2022)

davethebb said:


> DomP, As it really needs to be a continuous pour, not a DIY mix and lay, you probably need to contact your local concrete delivery companies and get some quotes for RC30 with and without Fibre mix and then compare with using A142 mesh. There probably won't be a lot of cost difference, but laying the concrete with mesh is harder and you need to keep checking the mesh has not moved off the blocks and seek advice on how to set the mesh (the height etc. within the 200mm concrete is important). look to hire a vibrating tamper that is wide enough to span the formwork.
> 
> Ensure you thoroughly compact (hire a big wacker plate) the soil down and each layer of Type 1 (lay in multiple layers) and sand.



Fibre is more for anti crack rather than strength. Good enough for smaller slabs if you do not think the ground will move and lightly loaded.

Concrete is good in compression not so much in tension. The slab will act as a beam with the top compressing in most cases and the bottom in tension where the steel should go. The mesh needs to be spaced off the bottom (cover) to stop it rusting, I would go with 40 or 50mm. Don't go more than this as you will be reducing the effective depth and hence strength of the slab (think going from a 3 by 2 to a 2 by 2). You can get plastic chairs, or premade mortar strips "mars bars" or spacer blocks but you would have to tie them on individually. Alternatively you could get some concrete paving slabs and break them up or concrete block pavers; not what you would want under a bridge but most likely ok for a shed if you can not gets some chairs or mars bars.


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## MikeJhn (1 Sep 2022)

With A142 mesh you only need a 100mm thickness of slab with drop edges, bottom cover should be minimum of aggregate size, generally 20mm, but cover to the mesh is according to what it is there for and its location in the tension or compression zone of the slab, not to stop it rusting. In reality its a shed not the Taj Mahal, but do get proper spacers for the mesh, anything else is a jobbing builder bodge.


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## DomP (1 Sep 2022)

Thanks, all food for thought, so am I right in saying if I use A142 then I won't need to cut the slab (7x5) to induce cracking?

Thickened edges, a 200mm edge sloping up at 45° to meet the 100mm main slab ok or more?


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## Jones (1 Sep 2022)

A142 is a light mesh and fibres or more cement in the mix may be just as good, ask the concrete supplier to confirm what's best for you, also consider a retarder if it's a hot day. Wether the slab subsequently cracks will depend on site conditions and ground preparation so I wouldn't worry about it. In a shed it won't matter where any cracks appear.


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## akirk (1 Sep 2022)

DomP said:


> I want to maximise the permitted size allowed without planning and without it being a long narrow space so I initially thought along the lines of 7x5 given that the internal space would then be nearer to 30m². I've no close neighbours and rural so I'm not too fussed on being strict to actual size as a few around us have just done as they please with outbuilding builds.


I don’t think that size is a planning issue (as long as you remain at under 50% of your garden) - it is a building regs issue which I think cuts in at 30m2 floor space

So your premise for 7x5 might be flawed


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## DomP (1 Sep 2022)

Plenty of garden space thankfully, it's a 1/3rd of an acre site.


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## Jameshow (1 Sep 2022)

akirk said:


> I don’t think that size is a planning issue (as long as you remain at under 50% of your garden) - it is a building regs issue which I think cuts in at 30m2 floor space
> 
> So your premise for 7x5 might be flawed


Why not go for 7.2 x 4.8 using standard material sizes. 

Unless you need it under 30m then 6x 4.8. round up to imperial on will just get you under.


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## DomP (1 Sep 2022)

Not a bad idea that James, 7x5 was really just a figure that I thought would be a good size but 7.2 x 4.8 would be fine


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## johna.clements (1 Sep 2022)

MikeJhn said:


> With A142 mesh you only need a 100mm thickness of slab with drop edges, bottom cover should be minimum of aggregate size, generally 20mm, but cover to the mesh is according to what it is there for and its location in the tension or compression zone of the slab, not to stop it rusting. In reality its a shed not the Taj Mahal, but do get proper spacers for the mesh, anything else is a jobbing builder bodge.


 Cover is very much to stop rusting. Why do you think different cover is used in different situations. If you look at old reinforced concrete structures you will see that where the concrete has blown and the rebar is exposed it is normally close to the surface, not enough cover.

Your suggestion of 20mm cover is very low and is only suitable for mild conditions were the concrete is not exposed to the weather and there is nothing nasty in the soil. Concrete inside buildings.

The edge of the slab will certainly be subject to the weather and could well freeze at times.. That sounds like a serve exposure or at least moderate. 

My old texts books give a minimum nominal cover (there is a -5 +20 tolerance) for severe with C40 concrete of 40mm or 25mm with C50.
For moderate it is 35mm with C35, 30 with C40 and 20mm with C50.

Your suggestion of using of 20mm nominal cover with 20mm aggregate concrete is wrong. The top of the type 1 under the slab will not be perfectly flat and the mesh will sag slightly between the chairs/ spacers. You will certainly be losing 5mm in places maybe 10mm if the type 1 is particularly bumpy. A 20mm stone will not go through a 15mm wide gap and you will get places under the mesh wires where there is just sand and cement. There is a current thread about dusty floors where there is no aggregate in the top of a slab. If you use 20mm nominal cover you need to reduce the aggregate size to 15mm but this will reduce the strength of the concrete so you may have to up the cement content and so the cost.

I would agree that it is a shed and not the Taj Mahal so if the edges of the slab do start failing in twenty years time it most likely is not a big problem. But what do you gain by building it that way in the first place. If the mesh is just in the slab to stop early cracking rather than using fiber it does not need to be near the bottom so why deliberately put it closer to the bottom. The tension face of the slab, where we want the rebar close too, will most likely be at the bottom of the slab but it may not be.


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## MikeJhn (1 Sep 2022)

Oh dear, you always read what you want instead of what is posted, you obviously know a little about this, but using type 1 under a floor slab shows your lack of real knowledge, and off at a tangent with 15mm gaps, the fabric should be sufficiently supported to prevent sagging between chairs, I said aggregate is generally 20mm the depth of the reinforcement in the concrete structure is designed as the d1 depth of the moment arm and is not to prevent rusting even though if is important for the cover to be consistent, concrete cancer is usually due to bad workmanship, the relationship of reinforcing cover to concrete strength is a new one on this Ove Arup trained ex Chartered Structural Engineer, cover is always related to size of aggregate, the designed depth required by the calculations taking into account any shear links, main bars and the tension or compression laps.


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## DomP (1 Sep 2022)

So can someone clarify for me then what spec I need for my slab?

I want a slab roughly 7m x 5m for a framed workshop clad in either feather edge or corrugated sheet with corrugated roof on 3 courses of engineering bricks.

The ground is pretty level so I intend digging out to 200-250mm to that my type 1, blinding sand and slab bring me to 50mm above ground.


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## Skydivermel (1 Sep 2022)

Why don't you go for ground screws. Simple, effective and relatively cheap. See here or here


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## DomP (1 Sep 2022)

Skydivermel said:


> Why don't you go for ground screws. Simple, effective and relatively cheap. See here or here


Because that means a floor level well above ground which will lose me internal height or increase overall height of the build. I'd also like a very firm base which a suspended floor can't offer

There is also several very huge stones below topsoil level one of which is in my veg patch that as much as we've tried we can't find an edge to get under it. Another was removed when we had our soakaway dug which is now on the end of my parents lane 4ft high bt 3ft diameter.


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## johna.clements (1 Sep 2022)

MikeJhn said:


> Oh dear, you always read what you want instead of what is posted, you obviously know a little about this, but using type 1 under a floor slab shows your lack of real knowledge, and off at a tangent with 15mm gaps, the fabric should be sufficiently supported to prevent sagging between chairs, I said aggregate is generally 20mm the depth of the reinforcement in the concrete structure is designed as the d1 depth of the moment arm and is not to prevent rusting even though if is important for the cover to be consistent, concrete cancer is usually due to bad workmanship, the relationship of reinforcing cover to concrete strength is a new one on this Ove Arup trained ex Chartered Structural Engineer, cover is always related to size of aggregate, the designed depth required by the calculations taking into account any shear links, main bars and the tension or compression laps.



If you read the thread you will see that people before me talked about placing type 1 under the slab. It seems to be a common recommendation / practice on this forum. I would assume it is recommended because it is free draining, so will not be susceptible to frost heave which is why it is used under roads. It is also widely available in small loads unlike something like 6N which is tested for sulfate as well as being free draining. If people are going to lay concrete on a granular material or the soil with some dpm over it you have to counter the unevenness of the surface.

You proposed 20mm cover. The tolerance for cover is -5mm therefore with your proposed 20mm cover you are allowed 15mm cover before it is out of tolerance. If you want a minimum cover of 20mm you need to specify a nominal cover of 25mm to allow for deflection.

I think you may have forgotten how a simple reinforced concrete beam is designed. In a simply supported beam that is acting like a floor joist spanning between two walls the top of the beam is in compression and the bottom is in tension. We are assuming that the slab in the shed will act in the same way with the top of the slab, where you stand in compression and the bottom in tension (this may not be the case but most likely is).



You may want to watch the video but there is a helpful sketch at 10 seconds which is all you need to watch. In the sketch the cover is the gap between the steel (sown as As Area of steel) and the bottom of the beam (or slab). The lever arm from the steel up to the center of the concrete which is in compression is marked as z on the sketch rather than d1. If you increase the cover to and do not also increase the overall depth of the slab the lever arm will be reduced and so the slab will be weaker in tension.




I am sure you have worked with BS8110 which was replaced by eurocode 2 in 2004 but they are similar. There are tens of thousands of structures built to these standards in the UK.

From EC2
""4.4.1.2 Minimum cover, Cmin
(1)P Minimum concrete cover, Cmin, shall be provided in order to ensure:
the safe transmission of bond forces (see also Sections 7 and 8)
the protection of the steel against corrosion (durability)
an adequate fire resistance (see EN 1992-1-2""


https://www.phd.eng.br/wp-content/uploads/2015/12/en.1992.1.1.2004.pdf



What do you think corrosion of steel is!

from EC2""
) For concrete cast against uneven surfaces, the nominal cover should generally be
increased by allowing larger deviations in design. Tile increase should comply with the
~difference caused by the unevenness, but the nonlinal cover should be at least k1 nlm [email protected]
concrete cast against prepared ground (including blinding) and k2 mm for concrete cast directly
against soil. The cover to the reinforcement for any surface feature, such as ribbed finishes or
exposed aggregate, should also be increased to take account of the uneven surface (see
4.4.1.2 (11)).
52
Note: The values of k1 and k2 for use in a Country may be found in its National Annex. The recommended
values are 40 mm and 75 mm""

BS8110 gives 40mm nominal cover for concrete cast on blinding (thin layer of concrete to work off) and 75mm nominal cover when cast against the earth. This is why I suggested 40 to 50mm nominal cover.


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## Molynoox (1 Sep 2022)

Are you trying to keep it under 2.5m in height?
If so, how will you span 4.8m?

Martin


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## DomP (1 Sep 2022)

Molynoox said:


> Are you trying to keep it under 2.5m in height?
> If so, how will you span 4.8m?
> 
> Martin


2.5 at eaves


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## Molynoox (1 Sep 2022)

DomP said:


> Because that means a floor level well above ground which will lose me internal height or increase overall height of the build. I'd also like a very firm base which a suspended floor can't offer
> 
> There is also several very huge stones below topsoil level one of which is in my veg patch that as much as we've tried we can't find an edge to get under it. Another was removed when we had our soakaway dug which is now on the end of my parents lane 4ft high bt 3ft diameter.


I think there is a perception that concrete pads sit flush and groundscrews sit on stilts.

With screws you set the height you want them. Yes you need 50mm air gap, but if that's an issue you can dig down 50mm beforehand. The only way to get a concrete pad flush is to dig down by the depth of the pad, which is more than 50mm anyway.

So from height perspective they are pretty similar aren't they?

Martin


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## Molynoox (1 Sep 2022)

DomP said:


> 2.5 at eaves


Ah ok. Within permitted development?


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## macca (1 Sep 2022)

In regards to building height I'm guessing you are restricting yourself to 2.5m for permitted development, last i checked you can go significantly higher if you were to have a dual pitch roof. This would also reduced the depth of timber needed to span your workshop which will also reduce internal height, and we haven't even gone into warm roof/cold roof. its worth noting the height at eaves is also considered at the highest point of natural ground level directly under the eaves of the building. If your new site is within 1M of your property boundary your going to need fire proof wall on those sides of the build (corrugated metal roof sheets usually get around this) and building regs will kick in at 15m2.

You can get treated cls but at 38mm i'd advise your go for the industry standard treated '2 x' material which comes in around 45-47mm, you'd thank me when you're bonding 2 sheets on the uprights! go for 4.8M lengths with doubled plates and stagger the joins, timber scarcely comes straight and the longer it is the more issues you'll get and pay more for the privilege.
be aware your courses of brick will soak in water like nothing else, the simplest way around this would be bringing your cladding down over them to screen out any water but this would bring you into your proposed splash zone. I agree that large overhangs will be a key to solving many of your issues, its just good practise for any building IMO.


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## DomP (1 Sep 2022)

Yeah dual pitched which I believe allows for 4m at top of pitch and 2.5 at eaves, within 1m of boundary too but when I checked it was 30m² before regs kick in.

Regarding pad height, I would be getting it in the ground to 50mm above surrounding area, ground screws would be that plus the height of floor joists and flooring


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## Molynoox (1 Sep 2022)

DomP said:


> Yeah dual pitched which I believe allows for 4m at top of pitch and 2.5 at eaves, within 1m of boundary too but when I checked it was 30m² before regs kick in.
> 
> Regarding pad height, I would be getting it in the ground to 50mm above surrounding area, ground screws would be that plus the height of floor joists and flooring


I recommend reading the regs, what you are proposing is not allowed.

Martin


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## DomP (1 Sep 2022)

Molynoox said:


> I recommend reading the regs, what you are proposing is not allowed.
> 
> Martin


What aspect of my proposal? Everything I've read on the planning portal suggests it is


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## Molynoox (1 Sep 2022)

You said within 1m of boundary, therefore max height is 2.5m.
Regardless of single or dual pitch


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## Molynoox (1 Sep 2022)

DomP said:


> What aspect of my proposal? Everything I've read on the planning portal suggests it is


Also your comment about building regs applying to 30m2 only is incorrect


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## DomP (1 Sep 2022)

That's not what the planning portal suggests, I have no dwellings or domestic properties on any of my boundaries if that makes any difference


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## DomP (1 Sep 2022)

Molynoox said:


> Also your comment about building regs applying to 30m2 only is incorrect


As long as 1m from boundary upto 30m² doesn't require regs is what I've just read


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## macca (1 Sep 2022)

haha i really should stop handing out advice whist watching tele, you are entirely correct, but moving away from the boundary it certainly does make a difference.


Molynoox said:


> You said within 1m of boundary, therefore max height is 2.5m.
> Regardless of single or dual pitch



Regs is a different ball game and apply based on varying factors not just the internal footprint of the build, once you actually decide on a design i would just call a local building inspector and get their opinion


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## MikeJhn (2 Sep 2022)

johna.clements said:


> If you read the thread you will see that people before me talked about placing type 1 under the slab. It seems to be a common recommendation / practice on this forum. I would assume it is recommended because it is free draining, so will not be susceptible to frost heave which is why it is used under roads. It is also widely available in small loads unlike something like 6N which is tested for sulfate as well as being free draining. If people are going to lay concrete on a granular material or the soil with some dpm over it you have to counter the unevenness of the surface.
> 
> You proposed 20mm cover. The tolerance for cover is -5mm therefore with your proposed 20mm cover you are allowed 15mm cover before it is out of tolerance. If you want a minimum cover of 20mm you need to specify a nominal cover of 25mm to allow for deflection.
> 
> ...



And the point of this diatribe is?


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## rs6mra (2 Sep 2022)

DomP said:


> As long as 1m from boundary upto 30m² doesn't require regs is what I've just read


The last time I checked building regs which was ages ago if you are within 1m of your boundary as you are proposing, the max is 2.5m. If you were to exceed 15m² then the material must be non-combustible and it was suggested earlier that you can get away with corrugated metal sheets; otherwise building regs kick in.......................


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## DomP (2 Sep 2022)

I can easily move it further than 1m


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## Jones (2 Sep 2022)

If you have not got neighbors or a highway on the boundary then I think the poster is correct about the 30m3 footprint for regs and 4 m to ridge. I have certainly built garages and sheds for people on that basis. The fire regs aren't easy to interpret and height is a factor, you can certainly have a six foot wooden fence next to an escape route


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## Molynoox (2 Sep 2022)

DomP said:


> As long as 1m from boundary upto 30m² doesn't require regs is what I've just read


Don't know where you read that but it wasn't the regs. Gets interesting above 15m2.


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## DomP (2 Sep 2022)

Molynoox said:


> Don't know where you read that but it wasn't the regs. Gets interesting above 15m2.


Planning portal, I'll call my local planning office to confirm though


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## johna.clements (2 Sep 2022)

MikeJhn said:


> , the relationship of reinforcing cover to concrete strength is a new one on this Ove Arup trained ex Chartered Structural Engineer,



I missed out this point

from EC2
""
SECTION 4 DURABILITY AND COVER TO REINFORCEMENT
4.1 General

4) Corrosion protection of steel reinforcement depends on density, quality and thickness of
concrete cover (see 4.4) and cracking (see 7.3). The cover density and quality is achieved by
controlling the maximum water/cement ratio and minimum cement content (see EN 206-1) and
may be related to a minimum strength class of concrete.""

and you can look at the previous standard BS8110 Table 3.4 which gives nominal covers for various conditions of exposure against the grade of concrete. 


You state that you are unaware of "the relationship of reinforcing cover to concrete strength", and state that it "is a new one on this Ove Arup trained ex Chartered Structural Engineer". I learnt this in technical college back in the 1970s. 

I do not know if we were working to a new code that had introduced the relationship between concrete strength and required cover and you have not undertaken any concrete design since the you were initially trained. You could have been designing other things.

Alternatively you could have just forgotten, thats a sad fact of ageing. I certainly can not do some of the maths that I could do in University.

I think it is unlikely that you are deliberately giving bad advice. But you could have said something like with "20mm cover it will be 20 years before there is a problem in MikeJhn's experience" or the like "40mm is the standard but not needed for a shed". 

I doubt you will explain yourself but I will take everything you say with a pinch of salt.


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## akirk (2 Sep 2022)

Molynoox said:


> Don't know where you read that but it wasn't the regs. Gets interesting above 15m2.








Exemptions from building regulations - Building Regulations - Planning Portal


Details of your responsibilities in regard to building regulations in England




www.planningportal.co.uk









The Building Regulations 2010


These Regulations revoke and replace the Building Regulations 2000 (S.I. 2000/2531) (“the 2000 Regulations”), and consolidate all subsequent amendments to those Regulations with minor amendments. The amendments include removing the requirement, where building work involves inserting insulation...




www.legislation.gov.uk





*CLASS 6
Small detached buildings

1. A detached single storey building, having a floor area which does not exceed 30m2, which contains no sleeping accommodation and is a building—

(a)no point of which is less than one metre from the boundary of its curtilage; or

(b)which is constructed substantially of non-combustible material.

2. A detached building designed and intended to shelter people from the effects of nuclear, chemical or conventional weapons, and not used for any other purpose, if—

(a)its floor area does not exceed 30m2; and

(b)the excavation for the building is no closer to any exposed part of another building or structure than a distance equal to the depth of the excavation plus one metre.*

_*3. A detached building, having a floor area which does not exceed 15m2, which contains no sleeping accommodation.*_


That is the actual legislation - so, point 3 is a generic default if below 15m2
point 1 allows you to go up to 30m2 as long as it is not used for sleeping and also above 1m from boundary (or built substantially from non-combustible material)
point 2 is probably not relevant
All of this is class 6 exemptions from building control and is the legislation
not hugely complicated...

build up to 30m2 if you are 1m from the boundary or substantially non-combustible
above that, building regs come in...


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## Spectric (2 Sep 2022)

davethebb said:


> I used scaffold boards for the formwork as they can be useful afterward.


And even better if you use polythene sheeting on the inside of the boards as you get a nice finish on the concrete and the boards are kept clean.


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## Molynoox (2 Sep 2022)

akirk said:


> Exemptions from building regulations - Building Regulations - Planning Portal
> 
> 
> Details of your responsibilities in regard to building regulations in England
> ...


Yup. As I say, just have a read through the regs. But also speak to your local BCO because their interpretation of non combustible is what matters (which you are doing so that's all cool)

Also think about that 4.8m span and how to achieve it without loosing all your internal headroom.

M


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## Molynoox (2 Sep 2022)

Jones said:


> If you have not got neighbors or a highway on the boundary then I think the poster is correct about the 30m3 footprint for regs and 4 m to ridge. I have certainly built garages and sheds for people on that basis. The fire regs aren't easy to interpret and height is a factor, you can certainly have a six foot wooden fence next to an escape route


Where did you read that?


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## johna.clements (3 Sep 2022)

Molynoox said:


> Where did you read that?


I am no planning expert but if you where next too the beach or a large river you would have no neighbours on the boundary. (apart from the crown on the foreshore and who ever owns the river but!). Other wise you always have a neigbour is just a matter of what they are currently using the land for. 



Not sure how the builder and owner of a garage would fare if a farmer had his corn field burnt if the garage did not comply. Would their insurance pay out, I don't know.


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## DomP (3 Sep 2022)

johna.clements said:


> I am no planning expert but if you where next too the beach or a large river you would have no neighbours on the boundary. (apart from the crown on the foreshore and who ever owns the river but!). Other wise you always have a neigbour is just a matter of what they are currently using the land for.
> 
> 
> 
> Not sure how the builder and owner of a garage would fare if a farmer had his corn field burnt if the garage did not comply. Would their insurance pay out, I don't know.


Thankfully not arable land  and what I meant by no neighbours was that I have no domestic property neighbours


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## Molynoox (3 Sep 2022)

I totally agree that the regs make no sense in certain circumstances but with all due respect that's irrelevant  the only thing that matters is what's written down in the regs and how your local BCO interprets it thus making your building legal or not.

Martin


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## DomP (3 Sep 2022)

My new intended location will be 8-10 metres from a field and can be over 1 metre from a hedge that runs along a country lane. Again I will contact my local planning office but I don't see an issue. I'll post a plan


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## Molynoox (3 Sep 2022)

DomP said:


> My new intended location will be 8-10 metres from a field and can be over 1 metre from a hedge that runs along a country lane. Again I will contact my local planning office but I don't see an issue. I'll post a plan


Needs to be *2m* away from boundary if you want to go above 2.5m height, that's according to the permitted development guidelines.

*1m* from boundary is how building regs are defined, and that governs how you build not what you build.

HTH

Martin


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## MikeJhn (7 Sep 2022)

johna.clements said:


> I missed out this point
> 
> from EC2
> ""
> ...


You deliberately miss interpreted my previous post on purpose I assume as it meets your agenda, you have latched onto cover and I SAID 20MM AGGREGATE, sorry to shout, but you seem you have difficulty reading what is written.

And you insults are not appreciated whether meant tongue in cheek or not.


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## DomP (7 Sep 2022)

Can you all just agree to disagree so that my thread doesn't turn into slanging match, I'd prefer it was kept to just advice to get me nearer to my build


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## DomP (10 Sep 2022)

Well having spoken to my local planning folk it transpires the ditch the other side of my hedge is my boundary, said hedge is hugely deep so well over 2m from my planned pad


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