Depth of concrete floor slab for underfloor heating.

4 Jan 2011
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United Kingdom
I've been hunting for the answer all over but can't seem to find it.

Starting from scratch - redoing floor, room is 3m x 5m currently there's a hole in the ground. Its going to be a livingroom.

So in order from top to bottom:

Floor covering
Screed with UFH pipework (65mm)
Insulation (100mm)
Reinforced Concrete slab (???mm)
Blinding sand
Hardcore/rubble infill

What thickness of concrete slab should I be aiming for to have a hard level surface to lay the insulation ontop of.

100mm seems overkill, even 75mm to me seems like overkill since there will be 100mm of insulation (Very rigid stuff!) with 65mm of screed over the top of it.

Does anyone know how to calculate it? As I can only seem to find calculations for concrete that is hanging in mid air rather than completely supported by the ground below it.

All drawings I can find on internet state exact depths for insulation and the screed, but never the concrete slab its all resting on.

What thickness should help to support the whole floor and keep it all together whilst it slowly settles down, without cracking.
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The slab is the floor, its a structural part.

75mm may not deal with any movement, so should be at least 100mm

Its got nothing to do with the heating
The 65mm screed is "structural" as well, no?

Without the underfloor heating I'd do DPC 100mm insulation 100mm concrete and 1-5 mm self levelling screed

Since the UFH needs such a large amount of screed on top of it, doesn't that count towards the overal structure of the floor?
A screed is not structural. Its just a means of getting a level finish, and can often be done without if the slab itself is levelled smooth

A screed is needed as part of an UFH system as that is how it works, and to cover the pipes. It has no structural function

The thick slab is intended to control any movement or cracking, as said, it is the floor
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Ok thank you very much, I guess I best get digging some more then! :)

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