Shed foundation

laugh

Hey I'm starting on a DIY 25 sqm shed.

I plan on building it myself with timber in a similar way to this:



My intended layers from inside to outside:

• Plywood
• Vapour barrier
• Insulation (maybe Kingspan)
• OSB
• Breathable membrane
• Batten
• Wood cladding

I want this "shed" to be a comfortable environment to work in at any time of the year.

How would you build the base for this type of shed?

Perimeter foundation with some rows of blocks and put barrier / insulation inside this and cover with concrete?

A lot of videos on youtube show sheds built on concrete peers with a wooden platform built on top but I don't really fancy that due to 4 legged friends etc.

I see some are built on a one pour slab with J bolts in the concrete sticking up around the perimeter to bolt the timber frame work to it. But it doesn't look like they bother with any insulation.

Rows Grower

Build your form work, throw in steel and pour your concrete in.

There's a few lads on here will be along shortly that will baffle you with science and laws, don't overthink it.

hesker

Planning required or not?

If going the planning exempt route the timber on piers option will eat into your head height.
Insulated concrete slab will give you more options in that regard.

You need to do more research on your wall makeup. OSB on the cold side is not great choice. You should look into breathable options.

Calahonda52

hesker wrote: »

Planning required or not?

If going the planning exempt route the timber on piers option will eat into your head height.
Insulated concrete slab will give you more options in that regard.

You need to do more research on your wall makeup. OSB on the cold side is not great choice. You should look into breathable options.

Why?


in passing the kingspan, assuming alu both sides is 100% non breathable

Lumen

As I understand it, sheeting on both sides is required for fire resistance in a dwelling, but in this case I don't see the point. Your rainscreen can be supported by battens/counterbattens on the studs, so what's the OSB on the outside for?

I would also prefer rockwool (pref. high density like Metac) to kingspan since it's breathable, but neither has much thermal mass, so...

I want this "shed" to be a comfortable environment to work in at any time of the year.

Think specifically about roof buildup and solar gain. Sheds in direct sunlight in summer can be uncomfortably hot.

Of course thermal mass works both ways. If it's only occasionally occupied, your thermal mass will mean that it takes longer and more energy to make it comfortable in winter.

Apols if this fails Rows Grower's "bafflement with science" test :pac:

selfbuild17

I would do the following for a simple, non habitable shed:

Dig out a level platform, put down and compact layer of 804 stone if the ground conditions require.

Lay a flat layer of DPM/ radon barrier

Lay insulation board, say 50mm

Set out your formwork in standing timber 9x2s, screw a flat 6x2 horizontally to the top so that it forms a rebate in top of the slab when you pour.

Check that all is level and square

Pour the concrete up to the top of the horizontal 6x2, level and smoothen.
Float again after a few hours to get a nice smooth finish

Set the outside walls down into the rebate so that the floor level is higher than the base of the shed walls. Run a DPC under wall first.

Ensure that ground level outside is lower than bottom of wall. Install drainage outside if necessary.

Lumen

Putting aside the objections to OSB on the cold side, how about using PIR-insulated loft walk boards, e.g. Xtratherm 86mm? You could screw them to the studs with ~150mm partial thread screws, tape the edges, and then mount the cladding battens (over a membrane) with ring shank nails or double threaded screws into the outer layer.

Do same for the roof and you have a continuous layer of PIR with no way for heat soak to make it uncomfortable in summer.

Only concern would be whether fixing to the outer sheet would be enough to hold up the battens and cladding....

hesker

Calahonda52 wrote: »

Why?


in passing the kingspan, assuming alu both sides is 100% non breathable

100%. That’s impressive. Should be fine so I guess.

Calahonda52

hesker wrote: »

100%. That’s impressive. Should be fine so I guess.

Iff he uses the alu coated rigid, and tapes the joints, he does not need either membrane