Construction of Ground-Supported Floor

30 May 2013
Reaction score
United Kingdom
I'm about to lay the floor slab in my extension, and the area has been excavated, levelled and the sub-base formed with well broken brick & block debris, together with MOT 1 stone and sand blinding.

I'm trying to decide which method of make up to finished screed will be best. Looking at Approved Document C, Diagram 4 (& para 4.7), I was originally going to put the DPM beneath the concrete, then insulation below screed (as 4a), but I'm a bit concerned about how long it will take for the concrete to dry, given that it will be covered with insulation and then screed.

Instead, I'm considering DPM above the slab (as 4b), as I anticipate being able to lay the DPM, insulation and screed just as soon as the concrete has hardened, and then only have to wait for the screed to dry out before laying finished floor coverings. I need to lay pipes for H&C water & for CHtg in the layer where the (80mm) insulation will sit, so the DMP will need to be below the insulation, rather than above it (as 4c).

What are the relative merits of methods 4a & 4b? Are there important issues of which I should be aware before going ahead? Thanks.
Sponsored Links
DPM above the slab is the way to go. Concrete doesn't 'dry' as such, it cures chemically and depending on the thickness ( min 100 mm ) shouldn't take more than a week or so before you can lay your screed. Minimum 1200 g dpm beneath your insulation ( min 75 mm celotex or similar ) and screed on top to required depth. Screed takes approx one day per mm to 'dry' so take that into account before laying flooring. Water and heating pipes need to be accessible if there are any joints below floor level so you would either need to lay them in channel in the screed or consider using a continuous run of plastic pipe in plastic conduit fixed to your insulation before screeding over. This will meet building regs requirements. Don't forget to add a border of 25mm insulation along all internal walls inside the dpm. This can be trimmed flush once screed is installed. Hope this helps
Concrete doesn't 'dry' as such, it cures chemically ...

It does both, surely (if not in a permanently damp situation)?

I'm going to lay continuous lengths of polypipe conduit pipe under the screed for water (and copper pipe with soldered joints for gas), so I'm hopeful that I won't have any maintenance issues.
All joints in screed must be accessible, soldered or not, building control won't allow you to permanently cover any joints in your screed whatsoever. I know it sounds crazy, especially if you're going to tile over it but that's just how it is. Run your heating/water supply in plastic buried in the screed and if you have no choice other than to run your gas in the floor then you'll have to do it in screed channel mounted flush with the top of your screed. You could always give BC a call and ask them but regs say any joint below floor level has to be accessible. This is so you can get your certificate of completion and some peace of mind. Copper needs to be protected when run in screed to prevent corrosion. Is it possible to run your gas supply via another route? Along a wall or above your ceiling perhaps? If not then screed channel is the only way.
Sponsored Links
Is it possible to run your gas supply via another route? Along a wall or above your ceiling perhaps? If not then screed channel is the only way.

It's awkward, as I'm having a vaulted ceiling, but I suppose I could take the gas pipe around the base of the rafters, just above the wallplate, and leave a break in the 35mm insulation layer between the rafters & the plasterboard. I was going to run 28mm pipe from the meter to half way to the boiler (with a branch to the gas hob) and continue in 22mm from there.
The concrete is ordered for delivery on Thursday.

A last question: how should I deal with the cavities in the external walls up to DPC (floor level) at the doorways? I assume they have to be filled.
So concrete for the floor slab was pumped in today, and now I'm finalising my plans for the insulation & screed.

I'm having very little success in finding anyone who wants to even quote for a sand & cement screed, let alone LAY it, so I'm going to put down EPS 100 @ 120mm and have an anhydrite screed at 40mm. I can have it done early next week, which gives me just enough time to get the insulation down. As it should be hard enough to walk on within 48 hours, that also means that I can get the electrical first fix done too, and then get on with insulating the ceiling and then there's just plasterboard & skim to go. I might yet be 'in for Christmas'.

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

Sponsored Links