Oversite for suspended timber floor- doc membrane?

22 Aug 2010
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United Kingdom
Hi guys,

Just trying to get my head around the floor construction for a single storey rear extension. I have decided to go with suspended timber floor. The outside ground level is about 300mm lower than the inside floor level is now. I assume my trench foundations will need to be 900 deep and filled near to the top with concrete and I then build up with blocks from there. I believe I need 150mm gap between the base of the joists and the top of the Oversite. Does this mean that I,ll have to excavate the void under the floor to below the outside ground level (currently built up as a patio).

Also, can someone tell me whether I need doc membrane below the Oversite and how thick the Oversite should be? I have read that you can both use it and also that you don't need to. Does the membrane then just lap up the inside of the inner skin and tie into the dpc?

It would seem sensible to make this void sealed from potential damp.

Sorry for all the questions
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Traditionally, the oversite concrete was 4" thick directly on the hardcore; the top surface should not be below ground level.
Alternatively, you can use just 2" concrete (or inert material such as gravel) laid on a sheet of 500g polythene. There is no need to link the polythene to the dpc; the void would be kept dry by an adequate number of air bricks

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