Screed thickness / UFH

3 Nov 2010
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United Kingdom

Relative newbie here, but have had to dig up my living room floor due to cracks and damp issues. Those are now cured permanently, and now we have laid a new DPM and concrete subfloor.

Due to issues, we now have an 11inch drop from the desired floor level and the top of the concrete subfloor.

3" of that will be insulation as approved by Building Control. I can whack another inch on top...
Then I plan to put warm water UFH pipework in, and plonk a flowable screed on top. Are there any limits to the thickness of the screed - ie could I have a 4.5" thick screed (allowing half an inch for the floor covering, which I suspect will be tiles).
I know that the recommended thickness for screed is 65mm, so the proposed 4.5" screed is a little thicker, and would result in a longer warm up time for the UFH.
Any other issues anyone might foresee? Or do I need to whack more insulation on the floor?
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Alternatives I've thought of...

Increase insulation, then put the required 50-60mm flowable screed, then put another layer of self levelling compound (inch thick too thick) to bring it up to the required level (we will be using Tiles on this floor, 1cm thick)
You need to check with the tech details of the screed you want to use. Mine had a max thickness that was a lot less than 115 mm. The nominal depth was 6 cm and every extra centimetre was charged extra.

I would put in insulation to raise the level. Why are you thinking of using SLC ? The screed is self-levelling anyway, so SLC is just extra work and expense and won't self-level as well as the screed ( if my experience was typical )
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4.5" screed + 3" insulation + .5" tiles = 8 inches. What about the other 3 ?

Increase your insulation (ring manufacturer for advice) or lay a sand blinding before insulation.

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