Laying a concrete floor over concrete and soil???

12 Oct 2012
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United Kingdom

Having finally removed all the suspended timber floors we have discovered a mixture of sub bases in the void.

2 rooms + hall way are just soil and the remaining rooms have a concrete oversite. Now i have a couple of questions on how to proceed.

Is it ok to pack rubble/fill in on top of the bare soil? (Sand/DPM/slab/insul/VCL/screed to follow)

Would i be better breaking up the exisiting oversite concrete and compact the whole lot down into the soil beneath? There are a couple of hollow sounding spots.

The rooms with soil will need approx 500mm of fill and the other rooms just 200mm.

Thanks for any info.

Cheers Neil
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There is no need to remove anything in the sub-floor provided it is not contaminated (sulphate etc).

You will however, have to fill and compact in layers particularly where the depth of infill is 500mm.
Thats great thanks, had thought it should be ok to compact direct onto soil.

Currently digging out the exisiting solid floor (again of various constructions!) and using it to backfill the voids then work out how much clean stone we need to get the levels we want.

Just have to worry about the weather now, was hoping santa would bring some concrete but its touching 0 every night :(
Thats great thanks, had thought it should be ok to compact direct onto soil.
By 'soil' you don't mean dark matter i.e. fertile growing stuff?

Or do you mean the residual stuff left over from the original oversite?
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1. FWIW i would be interested to know why you intend to replace your suspended floors?

2. Are you in the bldg trades, or are you a good diy'er? as what you propose can sometimes be a bit tricky for newcomers. eg. late/darkening afternoon delivery of concrete, support of stair flight etc. Depending on your experience it might be better to shutter up openings and pour one room at a time. Apologies if you'r well up with the game.

3. in-filling with clean rubble only, essentially bricks and conc, is fine.All wood and cellulose debris to the skip

4. set ledger datums.

5.FWIW: why not cover the rubble with a thin layer of stone, then sand, then membrane - flashed up the walls, the insulation to be under the slab and at the edges. Top the insulation with 6"min. conc. to a finished surface. There's no need for a S&C screed.
Apologies for the delayed reply.

Thanks for the advice :mrgreen:

Got a good bit done over the holidays, dug down to required depth and filled up were required to get the correct level throughout the ground floor and compacted. Just to get the final blinding ordered, in and then compacted again.

Found a few things that need sorting before proceeding. Such as a 3ft unsupported portion of wall (was built ontop of the floor between old footings), where/when the plumbing is going to go, damp proofing and insulation.

The "soil" was just clay oversite although there were quite a few large field stones within the floors which makes us think there was either a wall or stone building before the original brick house was built.

Initial reason for replacing floors was for peace of mind due to varying levels/ constructions and some creaks. As we removed the suspened floors we found that the wall plates in 2 rooms were rotten and some of the joists were in a bad state. Cost wise it was working out similar price to concrete it as it was to replace joists and floorboards etc.
We are wanting to install UFH which is why i was going for the slab/insulation/screed to keep response times good but still have some thermal mass in the screed/tiles

Im a keen diy'er so any advice welcome, i am however having contractors do the concrete and screeding due the requirement to get it right before it sets lol.

thanks again

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