details for oversite concrete beneath suspended timber floor

14 Oct 2008
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United Kingdom
My rear extension will have a suspended timber floor to match the existing 30's floor. How is the oversite concrete beneath it meant to be finished? can it be 2" thick? must it have a dPM beneath it? the extension will be 4m deep, so I will have to put a sleeper wall in to support the joists.
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Yes put dpm under your oversite. Finish wants to be smooth but isn't important, all in Approved docs Part C. Building regs require 100mm (which seems excessive to me), really all you need is enough weight on the dpm to stop it lifting. You don't have to have a sleeper wall if you use beefy enough joists, if you do want a sleeper wall then bed it on something a bit deeper than 2" of concrete (blocks on their side will do, as long as you're on good ground there's no need to dig too deep, shouldn't get frost problems under the house)
Down this neck of the woods the dpm under the oversite forms the Radon barrier and needs to carry on through the external walls. But maybe Radon isn't an issue where you are. If I was building a sleeper wall I'd be sticking a foundation under it but as mentioned I would just beef up the joists which would be cheaper than a sleeper wall and foundation.
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We don't have Radon in Surrey, There isn't enough space for deeper joists if I have to bung down 4" of concrete unless I dig out.............don't want to do that!
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Similar issues with my project ruin. Depends how good your ground is- my place is on firm clay so for sleeper wall I dug down about 300mm below the rest of the subsoil, chucked 175mm of concrete in and then 1 course of blocks spaced with 200mm ish between them to give me honeycomb and 150mm air gap below the joists (with DPM and a 50 x 100 on top of the blocks). Any particular reason for not wanting to dig down (my excuse was the foundations stop about level with the finished oversite so didn't want to disturb the ground. BCO was quite happy with that)

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