7 Nov 2020
Reaction score
United Kingdom
Hi all, having searched and searched for a clear answer I’m hoping some one can help.

I’m going to try and build a 3m x 3m garden room over lockdown.

There is already a concrete base down which I am going to put it on which was laid in 1950s I think, very flat and level.

I planning 2” x 3” joist directly on the concrete to allow for insulation below the floor so I can use it all year round.

My wife has given me orders to minimise the height of the step up into it... so I need to find the right combination from concrete up.

I know ideally I’d have a course of blue bricks or raise it with blocks and suspend the floor etc but I’m trying to find a way to avoid this that won’t cause problems for 10+ years.

Been thinking about
1) Concrete
2) one of the following:
(a) wrapping joists in Damp course membrane
(b) full 3x3 DCM layer below floor
(c) paint timber with bitumen

3) 2 x 3 treated timber joists & noggins - doubled up at perimeter to support walls
4) 60/50mm PIR insulation (foil on both sides) & Seams taped
5) no air gap above / below insulation
6) plywood floor / T&G - tbc

I’ve got no idea what is best for #2 or if an air gap is needed.

The base will go to the edge of the concrete so the cladding will over hang it slightly so rain can run off. There will also be an over hang on the pent roof that slopes to the back where I’ll have guttering to keep water away.

Any suggestions / info really appreciated.

Sponsored Links
2, a

Build base then timber frame 4x2 all-round, osb board the outer skin then clad, insulate the inside then put whatever skin you want, plaster board, osb etc, have a year round room then.
Thx SxTurbo, appreciate quick reply.

2a -DPC wrapped joists

no need for air gap above/below the PIR and floor.

more straightforward than I’d hoped for
Sponsored Links

After much googling, I found another article that hinted on close cells membranes. From that article I found out that in the US they put FoamsealR between concrete and timber to act as a weather / water / airtight barrier.

it’s a barrier strip of closer cell 1/4 - 3/4” deep.

They don’t have it in Uk, but found something very similar on eBay which looks equivalent, so going to try that as logically it ticks all the boxes.

Article on closed cell membrane

Video on sill gasket / membrane

What I bought

Thanks for help and hope this helps others / remember untested!!!!
Links in this post may contain affiliate links for which DIYnot may be compensated.
Based on what you've described, here's what I suggest: 1) Start off with a solid concrete base, which sounds like a good plan. 2) To prevent any moisture issues, consider using Damp course membrane around the joists or a full 3x3 DCM layer beneath the floor. 3) For extra support, use treated timber joists and noggins, and double them up at the perimeter. 4) Insulate the floor with 60/50mm PIR insulation with foil on both sides, and make sure to tape the seams. 5) You shouldn't need an air gap above or below the insulation. 6) Finish it off with a plywood floor or T&G, whichever floats your boat. When it comes to rainwater, it's great that you're thinking ahead and planning for overhangs and guttering. That should definitely help keep the water away from the base. By the way, if you're looking for some awesome summerhouses, check out Scotts of Thrapston. They've got a great selection that might be just what you're looking for. You can find them at I hope this STILL helps.
Last edited:

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local