# New workshop build on pier foundations



## Baker1983

*Part 1*

Having followed this forum for a while now and gained some really useful knowledge along the way, I thought it only fair to share my workshop build with others. I’ll be documenting the build as I go with photos and drawings, hopefully to help others and also to receive constructive feedback and maybe a little encouragement along the way.

*The Plan*

To construct a timber framed workshop measuring 5.4 x 4.2m at the rear of the garden as close to the boundary as possible. The back and sides will be clad in metal box profile sheets and the front face slatted timber (tbc). The roof will be a warm roof with an epdm covering. The workshop will be divided into a small storage area and the rest will be used as home office.






View down the garden. A little bit over grown, but mostly all clear now.




Planning permission drawings.

*Rewind to June 2021 - The Base*



I would have liked to construct a concrete base, however with no access to the garden other than through the house, along with constraints with parking, etc. this just wasn’t an (easy) option, so instead I settled on a timber framed base sat on top of concrete piers. I watched a lot of tutorials on this technique which seems to be widely adopted in the US, but less so in the UK.

The thinking behind using this technique for the foundation;

1. Less concrete, so I could mix up small batches, just enough at a time to fill each pier.
2. All materials could be brought through the house in small quantities.
3. Whilst maybe not as good a concrete slab base, it would still be very stable and not succumb to rot / damp.
4. The piers could be raised out the ground slightly to provide air flow and keep the timber away from the floor.
5. I wouldn’t have to completely level the garden which slopes from its highest point (top right) down to the lowest point (bottom left) by about 40cm.

After clearing the garden and marking out the size using string lines, I then marked out where and how far apart my concrete piers would need to be. 16 piers in total, 4 across and 4 deep. The maximum span between two piers across being 1.8m.

I levelled out the area roughly and then using an auger I drilled down as far as I could in each of the 16 pier locations. Approximately 1m deep. I’d seen in the US that they use a kind of cardboard tube that’s then filled with concrete and this had been my intention, however after more thought I was concerned that the diameter of the holes which was restricted by the size of the auger wouldn’t be sufficiently wide enough.

In the end I dug out each of the holes to approximately 40cm square and using scaffold board as shuttering, constructed a square frame around each opening, ensuring the tops of each one were level with one another.

With the holes for the foundation piers dug and the scaffold boards level, I then laid out the framework on top, ensuring it was square and level and temporarily fixed the frame together with each timber sitting centrally over the holes.

With the frame in place, I then drilled through the timber frame in each of the pier locations ensuring the hole was centred on the dug hole below. Once all drilled, I dropped in a steel threaded rod through the drilled holes in the frame and into the hole below. Each rod was held in place with a couple of nuts from above.

Next I poured concrete into each hole creating the piers. Once dry, I removed the frame and now had 16 concrete piers, each with a steel threaded rod sticking out the top.

With the frame removed, I used polo-shaped rubber rings that are typically used under plant pots to raise them off the ground and placed these over the steel rods. These would sit between the concrete pier and the timber frame stopping any moisture or water sat on the piers from rising up into the timber.





Model of pier construction.




Actual pier construction. 

The frame then went back on and was secured in place with washers and nuts that sat just below the top of the timber. Any protruding steel rod was cut level with the timber using a grinder.

The frame was constructed with 4x2 C24 treated timber with the outer frame doubled up and bolted together. I should have probably used a bigger timber here to reduce flex, however as I wanted to keep the height low, I opted for doubling up the timbers and there doesn’t appear to be any flex across the spans.

Using mini joist hangers, I placed floor joists at 400 centres and then inlaid rigid foam insulation and foil taped the joints. These were sat on timber battens beneath the frame to stop it slipping out.

The next step, which I wasn’t sure about, but had seen someone else do it and seemed like a sensible idea, was to lay breather membrane over the top. This was laid with the outer face, facing downwards to hopefully reduce any moisture coming up through the frame. Not sure this was necessary as theoretically there shouldn’t be any moisture rising up, but thought it wouldn’t do any harm.

Over this I laid 22mm P5 tongue and groove chipboard, secured with screws and 5min wood glue.





View of base prior to overlaying breather membrane and tongue and groove chipboard.
*Change of plan*

After I constructed the base, I re-thought about the overall design. I initially wanted to build the workshop under permitted development, however I really wanted a warm roof. Due to the size of timber I need for the roof span, plus the thickness of the insulation on top and the raised timber base, keeping within the permitted development height of 2.5m would create a very low internal ceiling.

As a result, I decided to apply for planning permission so that I could maintain a decent internal height and keep the warm roof. This is a fairly simple process and easy to do if you have some drafting / drawing experience. As I had built the base I applied for part-retrospective permission. Although an extra cost, for the extra head height I think it’s worth it.

Six weeks later I got confirmation that my planning permission had been granted. They didn’t even come out to visit, just asked for some photos to be sent to them.

*Fast forward to March 2022*

After being granted permission, I covered up the base with tarpaulin and didn’t touch it until nearly a year later and it’s from this point that I’ll be documenting the build on the forum.




All wrapped up ready to begin again nearly a year later. PS. Tarpaulin isn't water proof!


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## Lorenzl

Is your project marked as garden room instead of workshop to get it through planning 

Looking good and neat; the piers are a good idea. I don't think the breather membrane was required as the foil insulation should keep out any moisture.

Out of interest where is your power feed? Having a sink?


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## Baker1983

Lorenzl said:


> Is your project marked as garden room instead of workshop to get it through planning
> 
> Looking good and neat; the piers are a good idea. I don't think the breather membrane was required as the foil insulation should keep out any moisture.
> 
> Out of interest where is your power feed? Having a sink?



I tend to use workshop / garden room depending on who I’m talking to. Do you think it would have made a difference to the planning permission calling it a workshop?
My plan for power is to run up through the side wall. I didn’t show this level of detail on the plans, but it’s something I do need to think about before going much further. 
No running services such as a sink, although I did consider it, but felt it wasn’t necessary. Time will tell.


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## Sheptonphil

Couple of points

When putting in for my workshop planning permission I was advised by planning officer to call it a hobby room. Somehow it evokes less concerns from neighbours, so ‘hobby room’ was applied for and granted with no comments from neighbours.
The membrane you’re about to lay on the floor before the P5, I would use DPM plastic sheet, not permeable membrane. There will be damp passing through the timber, so DPM will stop any ingress up through floor.

I used the same pier method building my 7m x 4m summerhouse, but with 250mm cardboard tubes. It’s absolutely not going anywhere, and ground levels are 300mm sloped. Saved a terrific amount of excavation. 

looking forward to following progress, but do ask for ideas before doing things the hard way if you’ve no experience of this type of build. There is a vast amount of friendly, non judgemental advice available on this site.


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## Lorenzl

Sheptonphil said:


> Somehow it evokes less concerns from neighbours, so ‘hobby room’ was applied for and granted with no comments from neighbours.


Workshop potentially means noise whereas sunroom sounds peaceful

A friend of mine dropped himself in it as he descried the big workshop he wanted to build as a cow shed. He then got embroiled in farming regulations


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## Baker1983

Sheptonphil said:


> Couple of points
> 
> When putting in for my workshop planning permission I was advised by planning officer to call it a hobby room. Somehow it evokes less concerns from neighbours, so ‘hobby room’ was applied for and granted with no comments from neighbours.
> The membrane you’re about to lay on the floor before the P5, I would use DPM plastic sheet, not permeable membrane. There will be damp passing through the timber, so DPM will stop any ingress up through floor.
> 
> I used the same pier method building my 7m x 4m summerhouse, but with 250mm cardboard tubes. It’s absolutely not going anywhere, and ground levels are 300mm sloped. Saved a terrific amount of excavation.
> 
> looking forward to following progress, but do ask for ideas before doing things the hard way if you’ve no experience of this type of build. There is a vast amount of friendly, non judgemental advice available on this site.


Thanks. That’s interesting regarding the planning permission and what you call the space. 

With the floor. I’ve already laid the chipboard over the frame and think it’ll be a hard job getting it back up again. I have rigid insulation between the joists and all the joiners and tops of the timber are covered over with foil / aluminium tape. Do you think that will be ok? I could also lay a DPM over the chipboard before the final floor finish goes down - but assume this would cause problems with the chipboard sweating as it would essentially be sandwiched between the insulation and DPM. 

That’s good to hear someone else has used the pier method. Yes I think I worried too much about the diameter of the holes and as a result spent far to much time excavating.

Thanks for the advice. I think I’ll document what I plan to do first on here, then wait for feedback before I carry out the work and then show the actual construction.


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## Baker1983

Lorenzl said:


> Workshop potentially means noise whereas sunroom sounds peaceful
> 
> A friend of mine dropped himself in it as he descried the big workshop he wanted to build as a cow shed. He then got embroiled in farming regulations


I hadn’t really given it much thought about the naming of the space, so guess I got lucky calling it a garden room. To be honest though, it probably will be used as a garden room the majority of the time and if I do use it for any wood working projects, the neighbours will probably be happy that I’m indoors rather than on the garden making noise!


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## Cooper

Sheptonphil said:


> I used the same pier method building my 7m x 4m summerhouse, but with 250mm cardboard tubes.


A picture would be interesting. Does your roof have a large overhang to keep walls more or less dry?

Woops, I thought you meant to construct the garden room from cardboard tubes but on reading the dimension I presume it was to cast the piers.
Foolish me.
Cheers
Martin


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## Pedronicus

Baker1983 said:


> I hadn’t really given it much thought about the naming of the space, so guess I got lucky calling it a garden room. To be honest though, it probably will be used as a garden room the majority of the time and if I do use it for any wood working projects, the neighbours will probably be happy that I’m indoors rather than on the garden making noise!


The current neighbours may be OK with that assumption but all it needs is one (new) stroppy one to raise a complaint of noise and possible commercial use for a large can of worms to be opened.


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## Sheptonphil

Cooper said:


> A picture would be interesting. Does your roof have a large overhang to keep walls more or less dry?
> 
> Woops, I thought you meant to construct the garden room from cardboard tubes but on reading the dimension I presume it was to cast the piers.
> Foolish me.
> Cheers
> Martin


Yes, just the foundation piers.


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## Molynoox

Interesting thread 
What is the wall build up? You are over 15m2 so it gets interesting with the fire regs. stuff.
Martin


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## Baker1983

Molynoox said:


> Interesting thread
> What is the wall build up? You are over 15m2 so it gets interesting with the fire regs. stuff.
> Martin


Hi Martin. Yes it does. I really liked what you did with your build and really enjoyed the process. 

I know this particular topic gets discussed a lot. I think in part as there’s no real guidance in the building regs as to what constitutes ‘substantially non-combustible materials’. For example is it based on a volume, weight, surface area, etc? and this seems to change depending on the local authority. Some seem to include the overall structure including base and roof, for others just concerned with the walls adjacent to the boundary.

Interestingly I can’t find anyone that has been taken to court over this issue, which I assume is because the regs are ambiguous, even interpreted differently by those who job it is to enforce. 

On my the longest wall, the timber would equate to around 10% of the total surface area, so assuming I use a fire rated insulation and non combustible cladding such as box profile metal cladding, then technically it would be substantially made from non combustible materials, but again this is dependant on how you measure. Equally if I was to use metal cladding, normal insulation and line the internal faces with fire rated plasterboard, then based on surface area the non combustible materials would equate to around 66%. This would clearly be different if based on volume.


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## Molynoox

Baker1983 said:


> Hi Martin. Yes it does. I really liked what you did with your build and really enjoyed the process.
> 
> I know this particular topic gets discussed a lot. I think in part as there’s no real guidance in the building regs as to what constitutes ‘substantially non-combustible materials’. For example is it based on a volume, weight, surface area, etc? and this seems to change depending on the local authority. Some seem to include the overall structure including base and roof, for others just concerned with the walls adjacent to the boundary.
> 
> Interestingly I can’t find anyone that has been taken to court over this issue, which I assume is because the regs are ambiguous, even interpreted differently by those who job it is to enforce.
> 
> On my the longest wall, the timber would equate to around 10% of the total surface area, so assuming I use a fire rated insulation and non combustible cladding such as box profile metal cladding, then technically it would be substantially made from non combustible materials, but again this is dependant on how you measure. Equally if I was to use metal cladding, normal insulation and line the internal faces with fire rated plasterboard, then based on surface area the non combustible materials would equate to around 66%. This would clearly be different if based on volume.


haha, I didn't realise you had spotted my mammoth build thread  

everything you say about the interpretation of the regs. sounds theoretically sensible, however, like you already allude to, it doesn't really matter what the regs. say because it's up to the local planning officers to decide what is ok and not ok (as frustrating as that may be). For that reason I didn't really waste too much time getting into the definitions once I had realised that, and instead just setup a call with them to discuss my particular build.

they were very friendly, and actually gave me confidence in my build as they said it was a refreshing change from most of the stuff they see which isn't normally designed in a way that reflects any knowledge of the regulations. So I think you are already in a good position, having at least read through the regulations.
Its a bit like when you go abroad on holiday - if you at least make an attempt at speaking their language the locals are super friendly and usually help you out, even if its just a few words that you know

I think the regs are clear that you don't need to worry about the walls *not *on the boundary by the way, so I wouldn't expect any interpretation problems with that aspect.

In summary, I would recommend doing some sketches / CAD models showing your wall build up and setup a call with the local planning officers to explain it. Make sure you label all the materials precisely, even with brand names if that is relevant. I think if you go in with guns blazing like that they will be very open to your design ideas, rather than steer you down a 'standard' route

Martin


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## SamG340

Following


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## Yojevol

When I put in for planning for my completely wooden w/s I was advised I needed to provide some fire rated material on the wall adjacent to the boundary. Why's that? I enquired. "To protect your neighbours property" was the reply. I pointed out that the neighbours property is an open field only ever used for horses, it doesn't even have a stable. The inspector said "Yes, it's a bit ridiculous in this situation. Just put up a layer of plasterboard on the inside to show willing and I'll pass it. 
Brian


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## Baker1983

Yojevol said:


> When I put in for planning for my completely wooden w/s I was advised I needed to provide some fire rated material on the wall adjacent to the boundary. Why's that? I enquired. "To protect your neighbours property" was the reply. I pointed out that the neighbours property is an open field only ever used for horses, it doesn't even have a stable. The inspector said "Yes, it's a bit ridiculous in this situation. Just put up a layer of plasterboard on the inside to show willing and I'll pass it.
> Brian


Hi Brian. 
That’s crazy. This is the thing, there seems to be such a varied opinion. A friend of mine was told he would have to build using block work to meet the requirement, however I don’t think he put forward any other options for them to consider, so as Martin said above, I think it’s important to have a few options of how you think you can meet the requirement and then present these to the building officer and hope they agree.


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## Baker1983

Molynoox said:


> haha, I didn't realise you had spotted my mammoth build thread
> 
> everything you say about the interpretation of the regs. sounds theoretically sensible, however, like you already allude to, it doesn't really matter what the regs. say because it's up to the local planning officers to decide what is ok and not ok (as frustrating as that may be). For that reason I didn't really waste too much time getting into the definitions once I had realised that, and instead just setup a call with them to discuss my particular build.
> 
> they were very friendly, and actually gave me confidence in my build as they said it was a refreshing change from most of the stuff they see which isn't normally designed in a way that reflects any knowledge of the regulations. So I think you are already in a good position, having at least read through the regulations.
> Its a bit like when you go abroad on holiday - if you at least make an attempt at speaking their language the locals are super friendly and usually help you out, even if its just a few words that you know
> 
> I think the regs are clear that you don't need to worry about the walls *not *on the boundary by the way, so I wouldn't expect any interpretation problems with that aspect.
> 
> In summary, I would recommend doing some sketches / CAD models showing your wall build up and setup a call with the local planning officers to explain it. Make sure you label all the materials precisely, even with brand names if that is relevant. I think if you go in with guns blazing like that they will be very open to your design ideas, rather than steer you down a 'standard' route
> 
> Martin


Thanks for the reply Martin. I did really enjoy the mega build / post! I could read posts like that all day long.
Thinking about this a bit more, I think you’re absolutely right. The Building Regulations set out the requirement. There are many ways to achieve that requirement and this gives the architect, designer, builder and homeowner some creativity as to how they achieve that requirement. If you ask them what their view would be, it would most likely be the simplest (belt and braces approach) way to achieve that requirement which would most likely be block work, etc. If however you present a creative approach / alternative method, then that provides them which something to consider in more detail.
Will be sketching up possible solutions and arranging a call with them to discuss.


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## Molynoox

Baker1983 said:


> Thanks for the reply Martin. I did really enjoy the mega build / post! I could read posts like that all day long.
> Thinking about this a bit more, I think you’re absolutely right. The Building Regulations set out the requirement. There are many ways to achieve that requirement and this gives the architect, designer, builder and homeowner some creativity as to how they achieve that requirement. If you ask them what their view would be, it would most likely be the simplest (belt and braces approach) way to achieve that requirement which would most likely be block work, etc. If however you present a creative approach / alternative method, then that provides them which something to consider in more detail.
> Will be sketching up possible solutions and arranging a call with them to discuss.


I think you have summarised that spot on there 
Martin


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## Baker1983

Update 

Hi Everyone. Have been giving more thought to the base I’ve constructed and after a quick calculation of the overall weight; base, walls, cladding, doors and roof, etc. I’m concerned my pier foundations with the timber frame sat on top may not be suitable. 

I may be over thinking it, but perhaps a concrete base would be better. There is a significant amount of weight sat on the piers and I can’t help but think should one of those piers not hold up or sink slightly more than the surrounding piers, then the entire room will collapse. 

Taking up what I’ve already done and replacing it with a concrete base is no easy task and obviously a waste of time and expense to date, however where my heads at, at the moment I don’t think I’ll feel comfortable unless I put in the concrete base. Starting again, would also allow me to provide more access around the perimeter and also the overall height would be lower (although I have planning permission for a higher roof level, I’m conscious it will sit high above the fence line and don’t want to upset any neighbours). 

I guess without a structural engineer I’ll never know what’s best and I am a worrier about these things. Am I worrying over nothing?


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## Molynoox

I think if you are going to worry about something then the foundations are a worthy candidate, you could argue it's the most important part in terms of impact if it goes wrong.
I had a similar dilema to you actually, I was 95% sure that concrete piers would be strong enough, but by the time I had costed it all up, it wasn't much of a saving over groundscrews, and I fancied the idea of being 100% confident seeing as it was the foundations. I looked at a regular concrete base too and I think cost was similar ball park to groundscrews for a flat pad and more expensive for a gradient like mine (as the concrete pad gets big at one end thus needing more concrete). I also don't like the idea of wood sitting on top of a flat bed of concrete.

I think you are also wise to try and get the levels down at this stage - you can't make the building lower later  I excavated down about 150mm at one end of mine and I'm really glad I did for same reasons as you.
Martin


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## Inspector

Your piers are more than enough to hold up your building as long as they were deep enough to sit on undisturbed soil. You are below the frost line so that isn’t an issue. If it really bugs you waste some money on a engineer to sign off on it.

Pete


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## Molynoox

I've just taken a look at the details of your piers - you say you have gone down 1m, that's a pretty good depth, what is the soil like at that depth in your garden? I didn't realise you had gone down that far, seems fine to me.


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## Baker1983

Molynoox said:


> I've just taken a look at the details of your piers - you say you have gone down 1m, that's a pretty good depth, what is the soil like at that depth in your garden? I didn't realise you had gone down that far, seems fine to me.



Thanks for the quick response. I did go down a fair way, typically between 80-100cm. Beyond the top soil, it was clay all the way down. Quite soft clay at that. When I had my extension built, the building officer required 2m deep foundations, so that’s what I’ve been thinking about. As you say, better to make the change now rather than later if that’s the way to go.


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## Baker1983

Inspector said:


> Your piers are more than enough to hold up your building as long as they were deep enough to sit on undisturbed soil. You are below the frost line so that isn’t an issue. If it really bugs you waste some money on a engineer to sign off on it.
> 
> Hi Inspector. Thanks for the quick reply. After digging through the top soil, the rest was almost entirely clay and quite soft, which has got me worried. When I had an extension on my house, the building officer required is to dig down 2 metres.


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## Baker1983

Inspector said:


> Your piers are more than enough to hold up your building as long as they were deep enough to sit on undisturbed soil. You are below the frost line so that isn’t an issue. If it really bugs you waste some money on a engineer to sign off on it.
> 
> Pete



Hi Inspector. Thanks for the quick reply. After digging through the top soil, the rest was almost entirely clay and quite soft, which has got me worried. When I had an extension on my house, the building officer required is to dig down 2 metres.


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## Geriatrix

Perhaps you're in an area built on the infamous Essex (London) clay? Whether or not, the depth you should go for a given number of piers is dependent on the number of piers, the weight they are supporting and the characteristics of the ground in that particular area. Perhaps you should get some advice from someone qualified to give it? In my youth in SE Essex (now E. London), footings were generally dug to an inadequate depth, inspected, bottomed out with the cycle repeated until the site foreman was satisfied. You sometimes needed a ladder to get out afterwards.

If you did get any settlement in the future it would hopefully be consistent over the whole plot, but I fear that with one side open to the elements and the remaining three sides sheltered by adjacent fences, it would not be. I do think you need to get some qualified advice before going any further. You should perhaps create a test hole beforehand?


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## Heluvaname

As you've got the piers in place, and a concrete slab would be practically quite difficult to do, I guess you could consider making the pier heights adjustable if you're worried about potential subsidence (or heave) of one or more.

Either cut the top off the rod (or pier if you're stuck for height), and place an Easypad or similar on each one, to give you future adjustment. Not particularly cheap but in relation to a concrete slab may be quicker and easier (and possibly cheaper).

Depending on the diameter of the threaded rods you already have I guess you might be able to make up an adjustable collar to go on those to achieve the same thing? For info the threaded rods on Easypads are 30mm dia. EasyPads - The easy-to-use foundation system for modular buildings
(I've no connection with easypad, other than just about to start a build myself using them!)


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## RobinBHM

Baker1983 said:


> Update
> 
> Hi Everyone. Have been giving more thought to the base I’ve constructed and after a quick calculation of the overall weight; base, walls, cladding, doors and roof, etc. I’m concerned my pier foundations with the timber frame sat on top may not be suitable.
> 
> I may be over thinking it, but perhaps a concrete base would be better. There is a significant amount of weight sat on the piers and I can’t help but think should one of those piers not hold up or sink slightly more than the surrounding piers, then the entire room will collapse.
> 
> Taking up what I’ve already done and replacing it with a concrete base is no easy task and obviously a waste of time and expense to date, however where my heads at, at the moment I don’t think I’ll feel comfortable unless I put in the concrete base. Starting again, would also allow me to provide more access around the perimeter and also the overall height would be lower (although I have planning permission for a higher roof level, I’m conscious it will sit high above the fence line and don’t want to upset any neighbours).
> 
> I guess without a structural engineer I’ll never know what’s best and I am a worrier about these things. Am I worrying over nothing?



I built a shed 5.4m x 2.7m right next to a small brook at the bottom of my garden.

I built it on 8 piers - the 2 nearest the brook were 1.6m deep - I never reached hard ground, but it was as deep,as I could reach with hand tools. the others only about 600mm or deep.

After a year or so it moved a tiny bit in one corner - which I jacked up and adjusted. But given the brook is about 7’0 down to the bottom of the water (and water is shallow) it’s not surprising.

Youve got the weight spread over 16 piers - so if your building weight 3 tons, that’s only kg per pier, which is nothing.

Also don’t forget modern house foundations are designed to be extremely stable because modern bricks / blocks / cement mortar has almost no flexibility and will crack with just a few mm‘s of ground movement.

You are building a timber frame structure which can accept considerably more movement without causing any meaningful damage.

I would be very confident your piers will experience no real movement - and in fact being piers they won’t be really subject to ground heave or shrinkage due to roots or water - because sideways movement won’t impact in any way.

I bet the bottom of your holes will have quite stable ground of reasonable load bearing capacity - and 200kg per pier is really nothing.

Another thing to consider - if your ground was so unstable the 16 piers would move around, then a ground slab would have to be a massive reinforced raft to resist such movement, so changing wouldn’t benefit you.


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## MikeJhn

RobinBHM said:


> Another thing to consider - if your ground was so unstable the 16 piers would move around, then a ground slab would have to be a massive reinforced raft to resist such movement, so changing wouldn’t benefit you.


There is so much wrong with that statement, I don't know where to start.


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## Baker1983

*Part 2 – Virtual Build*

Firstly, thank you to everyone for your advice and support. Apologies if I haven’t directly responded to each one of your messages, however they are all very useful and very welcome.

Before continuing with the physical build, I thought I would confirm some of the build details. I produced basic outline drawings to submit for planning permission but hadn’t drawn up much detail apart from a few rough sketches.

I was fairly clear around the build up for the walls, but the roof was concerning me, so I thought it best to draw a 3D model to experiment with the roof build and come back to ask for advice.

Here's the virtual build in progress; My partner said the man accurately reflected what I tend to do a lot. Stand around thinking about what needs to be done!







I’m planning a warm roof. The maximum span from front to back is 3975mm. Looking at the span tables this would require a minimum timber of C24, 2x7 (47 x 170mm) based on an imposed load 0.75 kN/m2 and a dead load 0.5 – 0.75 kN/m2 and spaced at 400mm.

If I add 18mm for the bottom deck, vapour barrier, 100mm insulation, 11mm top deck and epdm (circa 130mm) this brings the total roof thickness to 300mm. All fine. Not overly large.

The problem I have is, in my wisdom I’ve decided to construct the walls all level – no raked walls. This decision was made to make it simpler to construct the walls without having to cut down board and cladding, etc. at an angle and I thought it would be easier to just slope the roof joists.

The roof with overhangs measures 5m from front to back. Based on typical flat roof falls I have planned for a 1:50 fall (I know typically this would be 1:40), achieving a finished fall of 1:80. This however requires a fall of 100mm over the length of the roof. If I add in 100mm fall to the roof build up, I’ll end up with a 400mm thick roof at the front of the building sloping back to 300mm at the rear. 300mm seems about right, however 400mm seems like it would look overly large and out of proportion with the rest of the build and probably require firring strips. Additionally, it would at considerably more weight. (See Option 1 below).






A thought I had was to incorporate a beam that would be partially supported by an internal wall and hung at the other end using a joist hanger (if this is appropriate).










Incorporating an internal beam, would reduce the maximum span of the roof joist to 2234mm which would allow me to then reduce my timber to C24, 2x5 (47 x 120mm). Adding in the fall required of 100mm would require a timber of 2 x 9 (47 x 220mm) which could be cut down to create the slope. Additionally, with the other materials added, the overall thickness would be 350mm at the front sloping to 220mm at the rear. (See Option 2 below)

Here's a section of the two options;






Hopefully this all makes sense, and the images help to explain what I’m talking about. What do you think? How would I calculate the size of beam required? I assume I can’t use span tables to calculate this beam as it’s standalone. Any other suggestions as to how I can reduce the overall thickness of the roof, whilst maintaining the fall and required timber thickness based on the maximum spans?

Thanks in Advance. ps. Have order materials to start walls and will post up progress photo's soon.


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## MikeJhn

Why not just add an additional 100mm wall plate one end and sit the joist on that, far less work, don't forget the noggins.


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## Torx

Are you planning to insulate the walls on the inside? Only mentioning as I had that in mind and it seems to be frowned upon.


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## Spectric

Mikes idea is the better way forward, why get involved with making odd shaped joist when just raising one end gives you the fall needed.

All I would suggest is that rather than just sitting them on the top plate cut a birdsmouth joint which sits better when the joist is angled.


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## Jameshow

Spectric said:


> Mikes idea is the better way forward, why get involved with making odd shaped joist when just raising one end gives you the fall needed.
> 
> All I would suggest is that rather than just sitting them on the top plate cut a birdsmouth joint which sits better when the joist is angled.


Hardly needed for such a shallow roof?


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## RobinBHM

MikeJhn said:


> There is so much wrong with that statement, I don't know where to start.


I see nothing wrong with my statement…at least not in the context of this thread.

a raft is designed to float on the surface - but given the OPs situation (poor access) a properly constructed raft is not realistic given the requirement for significant spoil removal and then a thick layer of type 3 MOT, reinforced concrete with a substantial skirt. A thin slab on shrinkable clay soil, especially if insufficient reduce dig leaving top soil…..would likely move and crack.

16 piers going down a metre would likely to be pretty stable even on poor ground, piers resist movement by both bearing on bottom and side friction.
piers are less likely to be impacted by tree roots.


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## MikeJhn

You have contradicted yourself by saying a raft would have no benefit if the ground was subject to movement, on suspect ground a reinforced raft foundation would be the first choice, only the top soil needs to be removed, a thick layer of MOT type 3 is not needed for a shed, just a sand blinding for the DPM, even on a piled foundation the pile caps would be tied together with ground beams, compared to a Raft foundation, piers are more likely to be impacted by tree roots not less.

I shudder at the pic's of joist on very small rubber supports, the metal bracket idea introduce's two point loads with the bolt heads, but what do I know as a retired Chartered Structural Engineer you would think I would have learned something in the fifty years I was in practice.


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## Spectric

Jameshow said:


> Hardly needed for such a shallow roof?


Don't make it to shallow otherwise you wont get sufficient run off.


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## pip1954

Hi baker just an idea if you are worried, another way would be to put more piers in help spread the weight out ,would be easier than putting base down cheers phil


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## Baker1983

MikeJhn said:


> Why not just add an additional 100mm wall plate one end and sit the joist on that, far less work, don't forget the noggins.


I had thought this and this would be the easiest way, however the reason for not going down this route is because I’m using box profile steel sheets down the side and thought it would be much easier to cut the wood to an angle / use firring strips than having to cut the steel sheets at an angle.


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## Baker1983

Torx said:


> Are you planning to insulate the walls on the inside? Only mentioning as I had that in mind and it seems to be frowned upon.


Hi. Yes I am. That’s interesting. Haven’t heard that before.


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## Torx

Baker1983 said:


> Hi. Yes I am. That’s interesting. Haven’t heard that before.











Shed build, many many questions...


Following on from this thread (thanks for the contributions), I've realised I'll be asking question after question so have decided to put it all together in one and hopefully document the build. This is the first of two outbuildings I'm hoping to build in the garden, this one is basically a...




www.ukworkshop.co.uk


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## RobinBHM

MikeJhn said:


> You have contradicted yourself by saying a raft would have no benefit if the ground was subject to movement, on suspect ground a reinforced raft foundation would be the first choice, only the top soil needs to be removed, a thick layer of MOT type 3 is not needed for a shed, just a sand blinding for the DPM, even on a piled foundation the pile caps would be tied together with ground beams, compared to a Raft foundation, piers are more likely to be impacted by tree roots not less.
> 
> I shudder at the pic's of joist on very small rubber supports, the metal bracket idea introduce's two point loads with the bolt heads, but what do I know as a retired Chartered Structural Engineer you would think I would have learned something in the fifty years I was in practice.


As I said, you need to consider my post in relation to the OP.

he stated he lives in a terrace house, making removal of top soil on a slab of 20 square metres unrealistic, If the top soil is 300mm deep that’s 9 cubic metres of soil out and 3 cubic metres back in. (I’ve assumed 50% bulking factor and 150mm concrete)


where you state “only the top soil needs removing” well how deep is the top soil? I’ve seen sites where top soil is 200mm deep and I’ve seen top soil 800mm deep.

You don’t know the soil type, top soil depth, it’s load bearing capacity, or it’s soil volume change potential, so I’m not sure how you can decide a raft needs no hardcore.

I agree that individual piers wont have the stability of piles with a ring beam, however we are talking about a timber structure which has considerable more flexibility compared to a masonry structure, constructed with cementitious mortar.

In my opinion, 16 piers would be better than using a thin slab on top soil with risk of cracking.


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## MikeJhn

Topsoil in the case of a shed construction would be defined as the depth of soil with sufficient nutrients to support ground growing plants, no need to go any deeper.


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