Garden office foundations

[jonnypron]

I'm building a 4.8m x 2.8m (ideally) garden room and need some advice on the base / subfloor and possibility of overhang on the floor joists.

As you can see in the image, I've got a concrete slab (not insulated) that I was going to sit the 3x2 treated joists directly on with DPC underneath each length for extra protection against moisture from the concrete.

I would then put 40mm insulation between these joists, supported so there was an air gap underneath. OSB then flooring on top of this. I cannot find much about this sort of structure, but have been advised it's fine. I can't have much more height as trying to keep headroom in 2.5m max limit.

As for the overhang, the concrete was built at 2.4m but now want the room 2.7, so looking up allowances for the 3x2 joists and spread every 400mm the overhang should be ok for 200mm either side. Obviously we would clad lower than the concrete for rain protection.

is this ok, firstly for the floor joists directly on concrete and also the overhang? The roof seems to be standard structure but have included on there incase.

thanks in advance.

 

[oldbutnotdead]

Your joists will rot. You shouldn't get a huge amount of moisture coming up through the concrete (if it has DPM underneath it), but with no airflow round the timbers any water that does get in the space will lurk for a long long time. Putting them on DPM would probably make matter worse (it'll be a nasty sweaty space).

I'd be building 3 dwarf walls on the slab (standard concrete blocks on their sides will do), 1 wall each side and 1 up the middle). Put DPM on top of the walls. Then I'd span those with treated 4 x 2s. Then stuff 100mm insulation between them. Then clad to ground level with 4 vents in the cladding so the void is ventilated.

Your roof is very shallow pitch- you probably don't get much snow down there but if you do get any then the buildup will be impressive. Plus it'll look rather ugly. Presumably you're on a boundary so need to stay below 2500 overall height? Is your slab raised as in the drawing? If so then we have;
Slab 100mm
Block 100mm
Joists 100mm
Floor 25mm


SPACE 2050 at the highest point internally. Bit poky..... with 10% pitch roof you'd be down to 1770 internally on the low side.

Rafters 100mm
Roof covering (ply plus felt?) 25mm
TOTAL 2500

Oh yes, your overhang plan with the 3 x 2 is a bit dicey. If you had a uniformly distributed load on them then you'd be fine. But a lot of the load is going to be on the unsupported ends (the walls and thus the roof). It isn't ideal, even with 4 x 2. Much better to extend your slab (or just put a trench foundation in on the side you can get at) to give solid support for the thing.

 

[jonnypron]

thank for the useful advice.

just to confirm re overhang, it's not essential for the extra space. I could extend the slab or put adjustable feet under the ends?

Wish we'd just insulated the slab instead or used timber frame on posts!

re floor joists: I've seen stud wall floor plates screwed directly into the concrete base before, and insulation laid on top of concrete inside the wall frame, then 3/4" timbers then flooring. I was hoping this would be another solution, but you're saying any timber resting directly on the concrete is at risk of rotting?

Need to maximise the headroom inside as you mention. I forgot to mention drilling ventilation holes in the air gap under the insulation in the floor joists, or would this still be an issue?

Thanks in advance

 

[oldbutnotdead]

Ventilation is the key in a dead space like that. Yes I've fixed sole plates to concrete before now- but that bit of timber gets some heat from the room which helps prevent condensation & thus rot. Perimeter timbers are at less risk (one side has fresh air usually). If you covered the slab with insulation you could just put a floating floor down (if you can support the walls by some other means).

So actually almost your plan- perimeter sole plates (or one course of bricks, dpc on top of bricks then sole plate. Gives the moisture somewhere to go to). DPM on the slab, 100mm Celotex or similar on the slab then floating floor on top of the Celotex. If the floor void is full of insulation you don't have to ventilate it. And you win a bit of headroom by only needing 125mm total for floor instead of 225. Flooring- heavy duty click-together stuff is the future, don't go putting machine tools in the place though .

 

[jonnypron]

Ventilation is the key in a dead space like that. Yes I've fixed sole plates to concrete before now- but that bit of timber gets some heat from the room which helps prevent condensation & thus rot. Perimeter timbers are at less risk (one side has fresh air usually). If you covered the slab with insulation you could just put a floating floor down (if you can support the walls by some other means).

So actually almost your plan- perimeter sole plates (or one course of bricks, dpc on top of bricks then sole plate. Gives the moisture somewhere to go to). DPM on the slab, 100mm Celotex or similar on the slab then floating floor on top of the Celotex. If the floor void is full of insulation you don't have to ventilate it. And you win a bit of headroom by only needing 125mm total for floor instead of 225. Flooring- heavy duty click-together stuff is the future, don't go putting machine tools in the place though .

This sounds like a great solution. More insulation (as no joists) less height needed and less work. So wanted to check DPM on slab underneath insulation will still allow the concrete to dry out ok as it's still drying (only 4 weeks old) considering there's DPM underneath the slab as well?

Thanks for your help. (nothing heavy on the floor btw)

 

[oldbutnotdead]

How thick is the slab? Keeping it damp will help it cure to be massively solid, if its been down 4 weeks and hasn't got saturated with rainfall it'll be OK.

 

[jonnypron]

How thick is the slab? Keeping it damp will help it cure to be massively solid, if its been down 4 weeks and hasn't got saturated with rainfall it'll be OK.

It's been pretty dry for the first 3/4 weeks but has rained a bit recently in the 4/5 week.

It's about 80-90mm with DPM and 70mm MOT under that.

 

[oldbutnotdead]

Put a plastic sheet on it (now would work well), weight the corners down. Tomorrow morning before the sun hits it too hard lift it up, see how damp it is under the sheet? (PS A binbag works well for this). A bit dark= nothing to worry about. Actual wetness= give it another week or so

 

[jonnypron]

Ok will give this a try. Thanks for your help.