replacing concrete with suspended floor....

25 Jul 2011
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United Kingdom
Hi all, I've been looking at the site for a while and now need to ask a question if any one can help.....

We have a 200 year old cottage , we have done upstairs and now are starting on the ground floor. The floor was always damp and was constructed of a concrete pour over the uneven ground below with no dpc or any proper construction at all!!!!

I have removed the concrete and intend to dig out the uneven soil to allow for a suspended timber floor. I'm aware that we may have limited if any foundation and so have opted for a dwarf wall up the middle to keep the joist depth to a sensible minimum, allowing for DPC, oversite covering and ventilation.

Question is much foundation will be needed to support the dwarf wall/floor. The span is 2.4m. I also intend to create a similar wall along the paralell walls again to support the floor to prevent loading the house walls as they are of rubble filled stone construction.

Any help appreciated, thanks.
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Sleeper walls are usually built off the oversite, unless the slab is likely to heave.
1. if you dig out to some depth to allow ventilation of your now suspended floor(s) you risk undermining your structural wall foundations ( if any ). this cottage probably never had a suspended floor.
2. dig an exterior test hole(s) to determine the depth of the outer walls.
3. the answer to the footing dims. for a sleeper wall Q. is probably 100 - 150mmD x 3 times the thickness of your honeycombed brickwork.
4. there are a number of wait and see's in your information so do item 1. first then get back here.
Thanks for the replies, I have now dug down outside and as far as I can tell the bottom of the wall is no lower than I would need to go down. ie I wouldn't be risking undermining the wall base but I would be at the same level as it.....


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For the hassle of having to rebuild sleeper walls, insert airbricks and ventilation for the timber, fixing and cutting of the timber etc etc... you might as well just put down another solid floor, albeit with proper dpm and insulation.

How far down have you gone with the existing floor? If you've got about 250 - 300mm from what's there now to where the finished floor will be then you're good to go.
We are leaning towards a solid floor but it would have to be limecrete to be breathable as there is no dpm in any of the walls and an impermeable floor would surely push moisture out into the walls causing damp. I have got 300mm without getting to the bottom of the wall albeit within 75mm. Price now is going to be the issue...!

Well you'd lay about 100mm of hardcore, layer of sand over that, poly dpm over the lot, Inc. up the walls, then 90mm of insulation and 90mm of concrete and you're done. (Slightly skimpy with the depths there but will work)
You'd have to trowel the concrete smooth to avoid screeding, or put some self levelling compound down. Either way you're pretty much there and good to go. Floor for life.

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