Proposed workshop build

[Jones]

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.

 

[DomP]

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.

 

[davethebb]

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.

 

[DomP]

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.

 

[DomP]

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!

 

[davethebb]

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.

 

[Terry - Somerset]

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.

 

[DomP]

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.

 

[davethebb]

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.

 

[DomP]

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

 

[davethebb]

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

 

[johna.clements]

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.

 

[MikeJhn]

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.

 

[DomP]

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?

 

[Jones]

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.

 

[akirk]

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

 

[DomP]

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

 

[Jameshow]

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.

 

[DomP]

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

 

[johna.clements]

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.