Workshop strip foundation

Hi

For my upcoming workshop build I'm edging towards Mike G's suspended timber floor route. I have a some questions if anyone can help.

How deep and wide would a concrete strip foundation/footing need to be?

Could I use concrete blocks laid flat onto the foundation to raise up the floor from ground level? Rather than bricks.

I'm sure there will be more questions after!

Thanks, Patrick

If you lay the concrete blocks on their side won't you have to do something to keep rodents etc from making a home under your floor? If I were going to use the blocks I would lay them as they are meant to be with mortar. Then just fill the block holes with cement where you are going to put J hooks (threaded rod bent into a J) to bolt the sill down to.

Pete

Solid concrete blocks (not the hollow ones) would be fine. They will not look as good though unless you render them.

Re foundation....how long is a piece of string? How soft is your soil? Is it sandy or clay? Is it well away from trees? What loading will the building have (ie is it going to be full of heavy machines). If it's just a shed on decent ground and not full of Wadkin stuff, 450mm width is what I would do. However, personally I prefer to lay a slab rather than strip. In the end a concrete floor is useful.

Inspector said:

If you lay the concrete blocks on their side won't you have to do something to keep rodents etc from making a home under your floor? If not clear, I meant the solid concrete block type!

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Apologies if not clear, I meant the solid block type.

AJB Temple said:

Solid concrete blocks (not the hollow ones) would be fine. They will not look as good though unless you render them.

Re foundation....how long is a piece of string? How soft is your soil? Is it sandy or clay? Is it well away from trees? What loading will the building have (ie is it going to be full of heavy machines). If it's just a shed on decent ground and not full of Wadkin stuff, 450mm width is what I would do. However, personally I prefer to lay a slab rather than strip. In the end a concrete floor is useful.

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I have clay, a good distance from large trees. The workshop will be roughly 30sqm with machinery but not wadkin weight. I plan to add block piers on pads under the middle of the building to support the joist span. The problem with a slab for me is it'll need to be poured 100 metres from the road, I don't fancy doing it by hand myself and it's on a bit of a slope.

Hire a petrol powered mixer for the day?

Or buy one ,use it once,then sell it as a more cost effective alternative....

If this building is outside the remit of building control I can give an opinion, and would stress that it's a guess as I don't know the ground conditions....and that I'm an architect, not an engineer.

Firstly, your excavation needs to be below the top soil and any vegetative matter. The minimum width of the trench is 300mm, but if the excavation is deeper the width will have to increase, simply to allow access for a trowel to lay the blocks. The concrete depth..........hmm....... there are actual engineers here on the forum..........but I would be uncomfortable with anything less than 250mm, and I would be laying rebar 50mm up from the bottom to make any clay movement less catastrophic. Given that it will be a flexible timber-framed building, minor cracking of the foundation or masonry plinth is probably more tolerable than if it were a stiff masoonry structure. If you find roots anywhere in the sub-soil, or if the surrounding area is boggy in the winter, or if you find lenses of peat or similar, then the excavation will need to be substantially deeper (and possibly wider).

Sorry that that isn't prescriptive, but as soon as you put a spade in the ground you are into a whole other world of "ifs".

fuse said:

Hire a petrol powered mixer for the day?

Or buy one ,use it once,then sell it as a more cost effective alternative....

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Still feels like a mammoth task pouring a slab of that size having not done it before

Mike G said:

................... as soon as you put a spade in the ground you are into a whole other world of "ifs".

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Isn't that the truth.

I'm a retired builder not an engineer though I was also a part time lecturer and taught some of the aspiring building inspectors back in the day.
I could give very accurate estimates for building jobs at the time except for underground works where it's rarely what you expect to find so it's just educated guesswork.

What Mike said about clay heave and soggy areas is very relevant as is the possibility of frost movement if a strip found is too shallow. You can certainly lay concrete blocks flat on top of each other with mortar beds but I'd personally dig down at least 400mm and the slope of the ground will be another issue., you'll have no possibility of rodent under the floor as long as the founds are solid.

My preference if building that founds would be to ground level with ideally engineering brick and a DPC above ground to support the joists. Rememmber you can fix anything that goes wrong with the actuall workshop but extremely difficult and expensive to repair foundatioon issues.

1. Full fill trench which means no laying and no need for wider access and rebar or better reiforcement mesh cut to width and linked will give a solid found that's suitable. It's not a difficult job with a mixer, best if the materials are close to the site but not the end of the world to mix elsewhere and barrow it, especially if you can get some help.

2. Trench blocks that are usually 300 or 355 wide, 440 long and 215 high plus a motar joint. so if you want a trench that's 450 deep you need two courses but if you wanted it to be only say 300 or 355 deep by 215 wide if happy with that then it's one row of blocks laid on their sides.
They're lightweight celcon and soft so you need to handle them carefully but are made for the job underground and have been around a long time. I've used them extensively. Google "foundation trench blocks"

3. (but only if I absolutely had to) use lightweight 7n concrete blocks laid either traditionally or flat. Perfectly acceptable but I guarantee you'll get aches where you didn't know you had muscles to ache.

All that said it's a just wooden workshop after all not a habitable building.

BTW
If you were hiring a mixer and had to mix 100 mtrs away then possibly worth hiring a powered barrow or small dumper truck. You could ever get readymix dropped on plastic sheets and move it that way or one of the companies that mix your exact requirement while you barrow it. Cost is a factor of course.

You never know...
We recently built a kitchen extension and during the dig we found no less than 3 unexplained sewerage pipe runs. They were redundant but it did cause a lot of hassle with cameras and discussion with Thames water to satisfy the build over agreement.

Mike G said:

If this building is outside the remit of building control I can give an opinion, and would stress that it's a guess as I don't know the ground conditions....and that I'm an architect, not an engineer.

Sorry that that isn't prescriptive, but as soon as you put a spade in the ground you are into a whole other world of "ifs".

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I appreciate it's guess work but this is all helpful. Yes it'll be under permitted development.

Lons said:

1. Full fill trench which means no laying and no need for wider access and rebar or better reiforcement mesh cut to width and linked will give a solid found that's suitable. It's not a difficult job with a mixer, best if the materials are close to the site but not the end of the world to mix elsewhere and barrow it, especially if you can get some help.

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If I were to go full trench I can then lay bricks or blocks straight on there for plinth which my suspended floor can sit on? This sounds a lot easier than laying below ground level!

Thanks

Raymedullary said:

You never know...
We recently built a kitchen extension and during the dig we found no less than 3 unexplained sewerage pipe runs. They were redundant but it did cause a lot of hassle with cameras and discussion with Thames water to satisfy the build over agreement.

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I'd hope there's nothing like that so far down the garden from a 1920s house but I guess you never know!

ptomharper said:

Still feels like a mammoth task pouring a slab of that size having not done it before

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Possibly
I once did the entire ground floor of a cottage.
Two helpers two small Belle mixers and sixteen tons of aggregate.
Took less than a weekend with all the shuttering and dpc done the week before.

You can now get readymix concrete pumped in on a "pay for what you accept" basis

Im an advocate of the Pad method these days but often get the feeling that it wouldn't have been much more of a job to have done a full slab

fuse said:

Im an advocate of the Pad method these days but often get the feeling that it wouldn't have been much more of a job to have done a full slab

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I originally wanted to do pads but don't want anything getting underneath. There's rats and foxes about and that's just what I know about. What would you put between your pads to stop intruders?

ptomharper said:

I appreciate it's guess work but this is all helpful. Yes it'll be under permitted development.

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Permitted development means it doesn't need planning permission. Whether building control is required is a separate question that mostly hinges on the final internal floor area and distance to a boundary.

What would you put between your pads to stop intruders?

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A course or few of bricks perhaps

Mice can get through a 7mm gap and its around 10mm for rats.
Rodents wouldnt neccessarily be a big problem other than just being there.

Its a bit like the swarm calls I get in springtime.
Bees have set up shop in a roofline or disused chimney.
if its difficult or impossible to remove without damaging the colony then I will ask the householder what harm are they doing to you up there and do you think you can co habitate?
Upon consideration the answer is usually yes.
.
Theres a brook at the bottom of our patch so I know Ill never ever be rid of the rats.
We just co exist until they overstep the line and then the traps are deployed for a few months.
Wish it were as easy with the deer.

The regular presence of humans around would deter most rural foxes.
The problem with what we call vermin be it wasps,foxes,rats,seagulls whatever is that we provide food for them .

One option you could consider would be screw/helical piles. There are small ones a do it your selfer can put in with bars or a big drill, you need more and there are the big ones put in with a hydraulic skidsteer attachment, you'll need as few as four. Your shop is roughly the size of a one car garage. I built a deck using 10' long screw piles (need to get below the frost line here) and the guy had them driven in less than two hours from arrival to leaving. Deck is 14' x 16' with the piles about 12 feet apart in the short dimension. A built up beam of three treated 2x12s at each end and then 2x12 joists every foot in joist hangers down the long dimension. Blocking across in two places made the deck solid with no bounce or wiggle. Composite decking went on top but in your case plywood would do the job. Once the piles are in it is a one man job to build the rest. Only downside is it is open underneath but most animals won't live there because it being open it doesn't offer protection. One upside is you can make the shop floor vehicle floor height for unloading heavy things like machines easier from a van.

Pete

Inspector said:

One option you could consider would be screw/helical piles. ......

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As soon as you do that, Pete, you have the problem of sealing access to your underfloor void, and to keeping your floor low enough to fit your roof in under the permitted developments heights.

Mike G said:

As soon as you do that, Pete, you have the problem of sealing access to your underfloor void, and to keeping your floor low enough to fit your roof in under the permitted developments heights.

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Fair enough. I am rural so don't have many of the restrictions you do. ptomharper never gave any information about the lot size, placement relative to property boundaries or any building size limitations so any suggestions I give still leave the legalities for the OP to consider. I should probably skip threads about construction since the approaches used in our two countries are so different. Don't take my response as hurt feelings as that isn't the case.

Pete

If ypu possibly can, go the extra mile and put in a solid slab and make the shop as big as you can. Makes sure there is easy ramp access for machines. It makes life much easier when positioning machinery if you have a decent slab. Extra cost is fairly minor but long term gains are significant.

AJB Temple said:

If ypu possibly can, go the extra mile and put in a solid slab and make the shop as big as you can. Makes sure there is easy ramp access for machines. It makes life much easier when positioning machinery if you have a decent slab. Extra cost is fairly minor but long term gains are significant.

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I'd go further and say that if you feel constrained by Permitted Development limits, then get planning permission. It's not onerous, and you'll get the workshop you want.

I have 9.6m of garden width so if I go for a 5m(width)x6m shop the internal floor space will be under 30sqm and over 2 metres from the boundaries of the property. That way I will be able have a dual pitched roof of 3m (planning portal says up to 4m) which will help with head room. Still looks like most would opt for a concrete slab over any other option.

ptomharper said:

....... Still looks like most would opt for a concrete slab over any other option.

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In my view it's the best solution. It removes a whole lot of awkward detailing, and it keeps the headroom as high as possible.

Can I suggest that you go for 4.8m of width, rather than 5m. It might seem trivial, but 4.8m is a standard timber length, and many timber yards will charge a lot more for anything over that as it is a "special". And don't forget, the 30 square metres is INTERNAL floor area. To get to 30 sqm assuming a 102mm wide brick plinth, your shed can be 6.52m long internally, so 6.72m to the outside of the plinth. 6.52 x 4.6 =29.9.

That makes a lot of sense @Mike G. Thank you. I'm fighting between not making it too long because of the sloped ground but keeping 2 metres from the boundary so I can get a dual pitch roof above 2.5m. Its Swings and roundabouts as they say

ptomharper said:

That makes a lot of sense @Mike G. Thank you. I'm fighting between not making it too long because of the sloped ground but keeping 2 metres from the boundary so I can get a dual pitch roof above 2.5m. Its Swings and roundabouts as they say

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If that's a difficult decision to make, it sounds like it might be worth just going for planning permission and not being constrained by the limits of PD. You only need to be one metre from the boundary to maintain the building regulations exemption, which is the one that will really save you hassle.

ptomharper said:

so I can get a dual pitch roof above 2.5m.

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Terminology question. What is a dual pitch roof? Gambrel?

Pete

Inspector said:

Terminology question. What is a dual pitch roof? Gambrel?

Pete

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Roof has an apex ridge and two slides slope away from it to meet the walls.

Not a gambrel - that has two slopes on each roof side and is rarely seen in the UK.

spb said:

If that's a difficult decision to make, it sounds like it might be worth just going for planning permission and not being constrained by the limits of PD. You only need to be one metre from the boundary to maintain the building regulations exemption, which is the one that will really save you hassle.

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Oh really, that's good to know. Thank you.

AJB Temple said:

Not a gambrel - that has two slopes on each roof side and is rarely seen in the UK.

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Our form of that....the mansard roof......is far more common than you'd think. Many village houses with mansard rooves were originally thatched.

This may help ? For those who are not familiar with roof styles.

duke said:

This may help ? For those who are not familiar with roof styles.
View attachment 39906

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Who’d have thought there would be so many, ( apart from the obvious contenders of course).
They seem to try and design houses over here to have as many different layers and levels as possible.
Keeps the roofers on their toes I suppose lol.



Getting a bit out of hand really!

Same here Ian, guess it gives the impression of a large expensive home for those who like to say , look what I have.
And a $ 60,000 re-shingle job in 20 years.

Aiming for a clean aesthetic like the image below but with a gutter. I assume this has a concealed gutter or maybe just runs off... Anything I should consider when doing away with fascia and soffit?

Even though it means complicated valleys and more expensive construction I like the look of multiple roof on a building and find plain versions just boring and with no character. Personal taste of course.

duke said:

This may help ? For those who are not familiar with roof styles.
View attachment 39906

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There's some weird ones there. "Lerkinhead" is a half-hip, or, colloquially, a granny hip. "M shaped" is a double pile house. "Saltbox" is a pitched roof with catslide. "Pyramid hip"...the the "hip" part is redundant because you can't have a pyramid without the hips. "Gambrel" is American, I believe, and the dual-pitch nature of it is covered by the term "mansard"....which DOESN'T require the ends of the roof to also be double pitched. The "Dutch gable" diagram is pure nonsense. A Dutch gable has the gable wall extending above the roofline. "Shed" is just a mono-pitched roof, and "Gazebo" would be described as a hexagonal pitched roof.

ptomharper said:

......Anything I should consider when doing away with fascia and soffit?

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Yes.......your sanity! There's very good reason rooves overhang walls. Unless you're into pressed metal rooves and walls, this idea is a recipe for a years-long headache.

I thought pent was the term for a mono pitched , shed, roof

I used to live in a top floor flat in a building with a variation of the M-shaped roof where the M was periodically blocked along the length - one of these houses:



There was a narrow slit (about 10 mm by 50 mm) in the middle of the bottom of each trough and then an open-topped gutter that ran through the attic space and out to the outside gutter. Suffice to say it didn't take very much in the way of leaves and such-like to cause a blockage and water leaks.

The roof types posted are from a USA site, I would imagine one from another country will be different.
Yea some there I have never heard of.

Mike G said:

Yes.......your sanity!

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Understood! I was going to ask also is the brick plinth in your drawings purely to raise the timber wall off the floor? Does it cause a cold spot as the wall is insulated and the bricks aren't? Sorry, so many questions but best to ask before I start

Yes, and yes. The only sensible way to build a timber framed building is to raise all of the timber clear of the ground (min. 150mm), so that even if it gets wet it will dry off completely. And yes, a single skin of brickwork is a thermal bridge....but in the context of an irregularly heated outbuilding the inefficiency is trivial. If you try to get into cavity plinth walls you have no end of complications to overcome, and my design should be seen in comparison to sheds built directly on slabs on the ground.