Summer house base and timber

burts · 12 May 2021

Evening, being pestered to do a summer house for a family member and have a couple of questions.
Base: There is a current slab of 100mm and relatively level. Best way to begin build? I was thinking Concrete block laying on its side with a DPC underneath, then screw floor plates to them which I can build off and rest joists on for a suspended floor. Suggestions please for what’s the norm ....

Also, what size and grade timber for walls, floors and roof.
Is it all C24?

Scarlet Pimpernel · 13 May 2021

You do not want the dpc level with the slab.
See this thread for why:

I have a brick shed, built on a concrete slab.
Read more: https://www.diynot.com/diy/threads/damp-course-fail.570940/#ixzz6ukJDQ67q

Better to lay the blocks and have the dpc on the top of the blocks.

burts · 13 May 2021

Bobby Dazzler said:
You do not want the dpc level with the slab.
See this thread for why:

I have a brick shed, built on a concrete slab.
Read more: https://www.diynot.com/diy/threads/damp-course-fail.570940/#ixzz6ukJDQ67q

Better to lay the blocks and have the dpc on the top of the blocks.

Thanks for the reply, I did think this but the DPC will be pierced when I screw the timber plate to the concrete block

Scarlet Pimpernel · 14 May 2021

Better that than ending up with the problem shown in the other thread.
It's not usually a problem, the weight of the timber, etc will tend to cause a closure of the dpc around the screw.

burts · 14 May 2021

Bobby Dazzler said:
Better that than ending up with the problem shown in the other thread.
It's not usually a problem, the weight of the timber, etc will tend to cause a closure of the dpc around the screw.

Yeah I get it’s the better option.
Would you put the floor joist on to the wall plate, or rest and fix it to the other half of the concrete block ( as will be laid on side) to achieve a lower floor?

Notch7 · 15 May 2021

burts said:
Yeah I get it’s the better option.
Would you put the floor joist on to the wall plate, or rest and fix it to the other half of the concrete block ( as will be laid on side) to achieve a lower floor?

If you start by setting out the perimeter sole plate, you can then run the floor joists in between.

And the wall framework can sit on top flush to the outside.

For the mostvweathertight construction, pin on breathable membrane to the outside of the framework, then 50x 25 battens then the cladding.

C16 timber is fine, treated of course.
Get some water proof treatment like the no nonsense stuff from screwfix and splash it on every end you cut.

burts · 15 May 2021

Notch7 said:
If you start by setting out the perimeter sole plate, you can then run the floor joists in between.

And the wall framework can sit on top flush to the outside.

For the mostvweathertight construction, pin on breathable membrane to the outside of the framework, then 50x 25 battens then the cladding.

C16 timber is fine, treated of course.
Get some water proof treatment like the no nonsense stuff from screwfix and splash it on every end you cut.

Thanks for the reply. I’m going to 18mm board the outside, then membrane, batten, weatherbord so that’s all good.

Regarding sole plate, I might be being thick. It will be flush with outside of blocks so everuthihbis flush outside , but are you saying put the joists on top of the sole plate, but in between framework uprights. That’s what i was going to do but I didn’t know whether they can sit on the other half of the block that didn’t have the sole plate on, to just lower the floor slightly. Don’t suppose It matters, it’s minimal.

Scarlet Pimpernel · 15 May 2021

I think that was what Notch was suggesting.
You really need 100mm bearing for your joists. So blocks laid flat should be ample bearing. Don't forget they also need protecting with dpc.

burts · 16 May 2021

Bobby Dazzler said:
I think that was what Notch was suggesting.
You really need 100mm bearing for your joists. So blocks laid flat should be ample bearing. Don't forget they also need protecting with dpc.

DPC ...But the joists will rest on the sole plate? Not the block

What ventilation will the floor need

Would a screed floor on top of slab be better, membrane, insulation then 70mm screed ‍

I don’t even want to do it but prices are mental in Essex .

Scarlet Pimpernel · 17 May 2021

burts said:
DPC ...But the joists will rest on the sole plate? Not the block

What ventilation will the floor need

Would a screed floor on top of slab be better, membrane, insulation then 70mm screed ‍

I don’t even want to do it but prices are mental in Essex .

I think we're getting mixed up.
As I understand it, and you seem to confirm it, the blocks would be laid on their side, giving a potential for 200mm bearing surface.
About 100mm of this would be covered by the perimeter sole plat, on the outside edge of the block, to allow for the exterior sheeting to slightly overlap the outside edge of the blockwork.
Inside of this, you would place your floor joists, resting on the remaining 100mm block available.
With dpc between all blocks and timber.

Ventilation: air vents at opposite ends of the building. There is a formula for estimating the number of airbricks/airflow, but I don't know what it is.

You can choose your floor type, according to your ability, access, materials availability, etc.
With a slab, there is always the danger of rain water accessing the join between wall and floor, and this needs to be considered at the design stage to eradicate that possibility, (like Notch7 suggested your timber walls on the outside edge of the blockwork)

burts · 17 May 2021

Yeah, we are getting mixed up a little. My fault.
So
The perimeter sole plate on outside of block, correct. With DPC between.
I then thought the joists would rest on top of the sole plate, so they can be fixed Securely in to it to prevent movement.
If they sat on the remaining 100mm of the concrete block, (which is better)Would a screw on the angle through sole plate in to joist be enough to secure in place?

Would vents be needed, as the bottom of the weatherboard will be open allowing air to flow through the floor and down the 25mm gap created from batten.

My mate had his done and builder just laid 4x2 tantalised joists directly on the slab with sole plate perimeter screwed on top then built up, all floating and no DPM. This is incorrect right?
If I wasn’t laying the blocks, how could it be done ?

Where does floor insulation come in to it?

Scarlet Pimpernel · 18 May 2021

burts said:
Yeah, we are getting mixed up a little. My fault.
So
The perimeter sole plate on outside of block, correct. With DPC between.
I then thought the joists would rest on top of the sole plate, so they can be fixed Securely in to it to prevent movement.
If they sat on the remaining 100mm of the concrete block, (which is better)Would a screw on the angle through sole plate in to joist be enough to secure in place?

Blocking between all the joists, and the end walls, and the floor/joists can't go anywhere, even though they will be virtually floating between the sole plate.

burts said:
Would vents be needed, as the bottom of the weatherboard will be open allowing air to flow through the floor and down the 25mm gap created from batten.

That gap is the ventilated space between membrane and cladding. The membrane/ cladding will go all the way down to the blocks. You could even take it down past the top of the blocks a little. Your membrane would be stapled to the outside edge of the sole plate.
If you're worried about insects/critters accessing that space, use an eaves vent strip

https://www.google.com/url?sa=i&url...ved=0CAIQjRxqFwoTCMi35Z7C0vACFQAAAAAdAAAAABAI

burts said:
My mate had his done and builder just laid 4x2 tantalised joists directly on the slab with sole plate perimeter screwed on top then built up, all floating and no DPM. This is incorrect right?

The sole plate can easily rot.

burts said:
If I wasn’t laying the blocks, how could it be done ?

Ideally a dpc would be 150mm above ground level, so top of your slab would be 150mm above ground level. It's to stop rain splashing up above dpc.
You also need to ensure the rain can't run down the wall and sit on the slab, or go under the sole plate.

burts said:
Where does floor insulation come in to it?

It depends on how efficient, warm and cosy the summer house needs to be.
Insulation in order of priority: roof, walls, floor.
For the floor, either rigid insulation between joists, or netting stapled to joists to support rockwool or similar. Your underfloor area will be ventilated.

Notch7 · 18 May 2021

burts said:
Would vents be needed, as the bottom of the weatherboard will be open allowing air to flow through the floor and down the 25mm gap created from batten

It's just closed off with insect mesh.

Notch7 · 18 May 2021

Sorry I wasn't clear.

If you put down the outer joists, then put a timber closing off the ends, that forms your base, then all this joists in between and screw in place - if they aren't sitting on a block they need joist hangers.

Then you can put your stud walls on top.

I did double joists at the end with a spacer, so there was something for the floor OSB sit sit on.

burts · 18 May 2021

What I mean is, will the air gap between breather membrane and cladding also provide adequate floor ventilation.... if I’m using 150mm joists, then the floor decking will be 100mm above the top of the sole plate and should ventilate out and down the cladding gap. If that makes sense

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Summer house base and timber

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