New workshop build on pier foundations

[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

 

[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.

 

[Baker1983]

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.

 

[Baker1983]

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.

 

[Baker1983]

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.

 

[StephenArfer]

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?

 

[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!)

 

[RobinBHM]

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.

 

[MikeJhn]

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.

 

[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.

 

[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.

 

[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.

 

[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.

 

[Jameshow]

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?

 

[RobinBHM]

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.

 

[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.

 

[Spectric]

Hardly needed for such a shallow roof?

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

 

[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

 

[Baker1983]

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.

 

[Baker1983]

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.