Breaking up and Re-laying Concrete Floor - Some Questions...

[jonespr1]

Hi

We've recently purchased a property which has a large 1980's extension with a downstairs bathroom which we want to refurbish.

The external wall is 4" block, 4" gap and 4" thermolite block, the plan is to add insulation to bring that up to current building regs, we're also changing the lights, installing an extractor, along with new shower, bath, basin and toilet, skimming walls/ceiling and re-tiling - all fairly standard stuff.

However, we wanted to install wet underfloor heating (UFH) to have a warm tile floor, this is where things have become more complicated. Upon raising the current chipboard floor we were surprised to find timber battens laid over a fairly rough concrete floor (1st image). I had hoped to add some insulation and biscuit mix with UFH, but we have 50mm on one side, going to 80mm on the side nearest external wall, which isn't really enough height to work with and obviously the reason the timber battens are down.

I've broken up a small area of the concrete near the external wall (image 2), which is 4" thick and was surprised to see a void underneath the floor of approximately 4" (image 3), which may again explain the battens and can only assume the ground underneath settled over time. There is a sheet DPC in place which runs under the entire slab and I can see the internal walls which have being built on the slab have a secondary DPC in them (image 4). My plan was to break up the concrete and start afresh which will give me the height for insulation, concrete, UFH, screed etc without any build up floor height.

My question is how do I do this with the internal walls laid on the slab, should I cut around the walls with a concrete floor saw a couple of inches out and then re-incorporate this in to the upper screed layer? What about the DPC, when I cut through this with the floor saw, how will I then be able to re-instate the DPC, there will be an un-joined area?

Apologies for the long post, wanted to ensure there was sufficient information to consider.

Thanks

Robert

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[ree]

The whole floor plan of the extension will have been a single concrete slab poured on a "single sheet" membrane. The membrane should lap up at the perimeter of the single slab - it wont lap up at each interior wall.

Starting at the present "hole", use a hired Kango to cut and nibble away the field of the concrete, and the edges of the bathroom floor - dont undercut the wall base. It will almost crumble under a Kango steel. No saw work necessary.

Be cautious of damaging the membrane but if it is holed or ripped then custom adhesive, and overlapping, will allow you to patch repair pieces in. Self adhesive water proofing materials will do the same.

Something amateur about all the pics: the plumbing and joists and esp. the hardcore dropping (perhaps hand tamp it when its exposed) - check if the hardcore has dropped under any loadbearing wall that backs on to the bathroom?

 

[jonespr1]

Thanks for your response Ree

Correct, it laps up external walls and under internal as one slab, I think your approach of the Kangol is best as ther is least chance of damaging thee membrane.

I agree, it does look like a 'bodge job', once I have broken the slab up, will be able to check extent of the situation, if it matters internal walls are of Thermalite block construction.

 

[jonespr1]

Apologies for the delayed reply, it's taken me a while to get down to the job!

So, I've taken out most of the concrete floor now as I've got closer to the edge of the internal wall I was been more careful as not to damage the membrane.

At this point I am fairly close to one of the internal walls and have ceased any further concrete breaking as can't prevent the concrete from breaking up underneath the actual walls which are built on top of the slab - I don't want to undermine them! I think I'm close enough to the one wall, but have a reasonable gap to the other wall - should I go closer?

My plan now, was simply to slab and insulate the rest of the room and will level out with screed onto the existing slabs near the wall - these are 50mm below finished floor level so screed will be slightly shallower here than rest of room - I assume that won't be a big issue?

Thanks

Robert

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