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I want to build a shed and it might make financial sense to roof it with a in roof solar panels. I'd have 2,5kW facing S and the same again on the north side.

If I am building a timber shed (2x4s) approximately 8m by 4m, what sort of foundations would I require?

I was looking at a concrete slab, but I think the loading with all the snow would mean a straight 100mm slab would probably not have enough strength at the edges. The hard grade is about 30-50 cm down so I'm assuming I could dig down at the edges of the slab to the hard grade and fill with cement giving a deeper lip? I'd be stripping off top soil and putting down hardcore.

The alternative might be to make strip foundations which finish above ground level and then just perch a timber floor frame on top.

Most of the YT vids deal with smaller buildings. Anyone got some pearls of wisdom that might enlighten me please?

Andy wrote: ↑Mon Nov 11, 2024 8:47 pm I want to build a shed and it might make financial sense to roof it with a in roof solar panels. I'd have 2,5kW facing S and the same again on the north side.

If I am building a timber shed (2x4s) approximately 8m by 4m, what sort of foundations would I require?

I was looking at a concrete slab, but I think the loading with all the snow would mean a straight 100mm slab would probably not have enough strength at the edges. The hard grade is about 30-50 cm down so I'm assuming I could dig down at the edges of the slab to the hard grade and fill with cement giving a deeper lip? I'd be stripping off top soil and putting down hardcore.

The alternative might be to make strip foundations which finish above ground level and then just perch a timber floor frame on top.

Most of the YT vids deal with smaller buildings. Anyone got some pearls of wisdom that might enlighten me please?

8m x 4m is 32m² which requires building regs, therefore you’ll need drawings, engineering calculations, visits, and everything else! You can avoid all this if it is 30m² or less and more than 1m from a boundary (or 15m² or less and within 1m from the boundary).

Anyway to answer your question: 100mm of MOT1 whacked down, followed by DPC and 100mm of 6:1 concrete with A142 mesh will be fine for a simple timber structure such as the one you describe. The solar panels are light so make no difference to anything. However if you’re doing something unusual such as putting concrete tiles on the roof or you live somewhere with increased snow load allowances (eg you live in in Scotland, The Pennines, or North East England, or you are above 100m) then you should go to 150mm deep for the outside 200mm of the slab.

Stinsy wrote: ↑Mon Nov 11, 2024 10:11 pm

Andy wrote: ↑Mon Nov 11, 2024 8:47 pm I want to build a shed and it might make financial sense to roof it with a in roof solar panels. I'd have 2,5kW facing S and the same again on the north side.

If I am building a timber shed (2x4s) approximately 8m by 4m, what sort of foundations would I require?

I was looking at a concrete slab, but I think the loading with all the snow would mean a straight 100mm slab would probably not have enough strength at the edges. The hard grade is about 30-50 cm down so I'm assuming I could dig down at the edges of the slab to the hard grade and fill with cement giving a deeper lip? I'd be stripping off top soil and putting down hardcore.

The alternative might be to make strip foundations which finish above ground level and then just perch a timber floor frame on top.

Most of the YT vids deal with smaller buildings. Anyone got some pearls of wisdom that might enlighten me please?

8m x 4m is 32m² which requires building regs, therefore you’ll need drawings, engineering calculations, visits, and everything else! You can avoid all this if it is 30m² or less and more than 1m from a boundary (or 15m² or less and within 1m from the boundary).

Anyway to answer your question: 100mm of MOT1 whacked down, followed by DPC and 100mm of 6:1 concrete with A142 mesh will be fine for a simple timber structure such as the one you describe. The solar panels are light so make no difference to anything. However if you’re doing something unusual such as putting concrete tiles on the roof or you live somewhere with increased snow load allowances (eg you live in in Scotland, The Pennines, or North East England, or you are above 100m) then you should go to 150mm deep for the outside 200mm of the slab.

Hi Stinsy, I rounded it slightly to take into account the 100mm for the base plate . 7.8*3.8=29.64m2 In Scotland the 30m2 is based on the internal area from my reading of the rules. It has to be built to regs but not notified as I understand it. Thanks for that, so I don't actually need as crazy a lip as I thought I'll need planning as the pitched roof will put it above the max height for permitted development.

So the DPC would wrap round with the timber base plate straight on the concrete sandwiching the DPC?

I was looking at a flat roof, but there doesn't seem to be any easy solution to attach the panels. I'm intending to order in the premade 45 degree trusses and use some form of in-roof solution to build out most of the roof. I want to make it as easy as possible to get the snow off. I'm fed up raking it off the barn and I'm scarred from 4-5 years ago when we had meters of the stuff.

It's funny how I've got to a point in life where the whole build of anything centers around the PV.

I’ve seen the DPM under the slab finished off in a few different ways. Folded over the slab and the frame built on top of it, stapled to the outside of the frame, and simply cut off at the top of the slab. I guess it doesn’t really matter.

I’m assuming we are talking an unheated space here?

Round my way “flat roofs” are all the rage. Primarily to avoid planning/building regs. However I can’t remember us ever getting more than 6” of snow. A 45° roof seems sensible if you get a lot of snow.

I personally wouldn’t overthink the solar panel mount on a shed! When I put them on my garden office I screwed 3x2s vertically through the felt and screwed the panels to the 3x2s.

We've built loads of timber framed garden buildings, the majority of them avoid PP if they have a flat roof that is less than 2.5M high if within 2m of the boundary, check your council website but it is almost certainly permitted development if less than 20% the garden area. Building control wont be interested unless you are building a habitable space, a shed is below their radar.
Regarding the floor, what are you putting in it? if heavy lathes and mills etc then yes a concrete slab is probably best, if it is just a general storage workshoppy type shed a slab is a bit over the top IMO
You could use screw piles in a grid format to support timber joist and build off that much quicker than mixing up 3 cubes of concrete and you wont turn the garden into a swamp doing it.
Another option is to use screwpiles just on the perimeter to support the wall base plates, and then within that level some crushed hard core and slab over the top, much quicker and cheaper. This is pretty much what I did for my workshop, to be fair the floor is not perfect, one or two of the slabs have cracked where my temporary bench goes mainly, but it is flat enough to sweep clean and roll stuff about on with no problem. I would keep the roof flat so you can walk about on it to clean and check the panels and as Stinsy says it's really easy to fix them down.

I actually didn't use screw piles for the wall support but dug pile holes (think fence post sized holes) so I could bridge over various tree roots and mixed up the concrete on site (1 bulk bag of ballast all in) and then placed concrete lintles across the tops of the piles just above ground level and then built off that, no DPM/C needed, just bring the breather membrane down just below the top of the lintles and you wont have any damp problems.

Good luck, I'm off to measure another one up later today funny enough.

Desp

Andy wrote: ↑Mon Nov 11, 2024 8:47 pm I want to build a shed and it might make financial sense to roof it with a in roof solar panels. I'd have 2,5kW facing S and the same again on the north side.

Sorry if I'm interferring, but could you build it with a monopitch, all south facing? Just increase the height on the north side, and maybe lower the height on the south side a bit?

Stinsy wrote: ↑Tue Nov 12, 2024 7:36 am I’m assuming we are talking an unheated space here?

*cough*. Yes because otherwise building regs need a ridiculous amount of insulation. I'll have some insulation and frost free heating.

Stinsy wrote: ↑Tue Nov 12, 2024 7:36 am Round my way “flat roofs” are all the rage. Primarily to avoid planning/building regs. However I can’t remember us ever getting more than 6” of snow. A 45° roof seems sensible if you get a lot of snow.

I've done the calcs and could make them work. And with the panels I already have, flat pitch would work as I really need cloudy weather generation.

Stinsy wrote: ↑Tue Nov 12, 2024 7:36 am I personally wouldn’t overthink the solar panel mount on a shed! When I put them on my garden office I screwed 3x2s vertically through the felt and screwed the panels to the 3x2s.

Are you not worried about tiny amounts of moisture getting through and rotting from the inside? Or did you put a squirt of sealant around the penetration points?

spread-tee wrote: ↑Tue Nov 12, 2024 9:04 am
Regarding the floor, what are you putting in it? if heavy lathes and mills etc then yes a concrete slab is probably best, if it is just a general storage workshoppy type shed a slab is a bit over the top IMO

There might be some gym stuff and heavy boxes of honey stacked.

spread-tee wrote: ↑Tue Nov 12, 2024 9:04 am You could use screw piles in a grid format to support timber joist and build off that much quicker than mixing up 3 cubes of concrete and you wont turn the garden into a swamp doing it.

When I've priced up piles, they have come in more expensive than building a cement foundation. I'll be digging the ground to remove trees and have access for a cement truck (if its not raining).

spread-tee wrote: ↑Tue Nov 12, 2024 9:04 am I actually didn't use screw piles for the wall support but dug pile holes (think fence post sized holes) so I could bridge over various tree roots and mixed up the concrete on site (1 bulk bag of ballast all in) and then placed concrete lintles across the tops of the piles just above ground level and then built off that, no DPM/C needed, just bring the breather membrane down just below the top of the lintles and you wont have any damp problems.

Good luck, I'm off to measure another one up later today funny enough.

Desp

Thanks, I'll look at that too. The Americans often do something similar. They have cardboard tubes and make concrete piers and then lay the joists on them. I only found one supplier in the UK where the tubes where so expensive it was cheaper to lay a whole slab.

Mart wrote: ↑Tue Nov 12, 2024 9:48 am

Andy wrote: ↑Mon Nov 11, 2024 8:47 pm I want to build a shed and it might make financial sense to roof it with a in roof solar panels. I'd have 2,5kW facing S and the same again on the north side.

Sorry if I'm interferring, but could you build it with a monopitch, all south facing? Just increase the height on the north side, and maybe lower the height on the south side a bit?

Not at all, I have thought of that. It would be crazy high with a 45 degree pitch and I'm not clued up on multi level timber framing so decided against that. It is also forming the protection for the back of my ground mount system so it will start quite high up. Currently there are leylandii which are a pain, so this build would mean they can go.

Andy wrote: ↑Tue Nov 12, 2024 10:08 am Thanks, I'll look at that too. The Americans often do something similar. They have cardboard tubes and make concrete piers and then lay the joists on them. I only found one supplier in the UK where the tubes where so expensive it was cheaper to lay a whole slab.

Pity this hadn't come up about 3 years ago as I could have got you plenty of 300mm diameter tubes.