Tiling over screed wet underfloor heating

21 Mar 2005
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United Kingdom
I read another post about an unsuccessful attempt at tiling over a suspended timber floor with wet underfloor heating.
One suggestion was to aloow the heating on for a few says befor tiling the floor.
I wondered if this applied to tiling over a screed floor with wet underfloor heating.
The floor slab consistes of concrete base -dpm, 75mill hih performance insulation with pipes laid over it and a 75mm screed, which was laid about 7 months ago, but has never has the heating on( as the boiler hasn't been installedYet).
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The screed will be fully cured after 7 months but it will still have some residual moisture & it needs to be conditioned to the expansion & contraction of normal operation so the heating system should be fully tested & commissioned before tiling. After a gradual start up so you don’t thermal shock the slab, you should leave it running on a normal operating cycle for several days. Turn it of & wait two days (not much longer) before tiling, leave it at least 10 days before any attempt to switch it back on & when you do, run it low for the first few days then slowly increase the temp over several more days.

You need to prep the floor correctly & use the appropriate quality trade materials, SPF (single part flexible) adhesive & flexy grout. If you fully commission the UFH as above & no cracks develop in the screed, you shouldn’t need an uncoupling membrane but you can fit one as an extra safety precaution, probably worth while. You also need to allow for expansion, as a minimum this should be a gap around the edge of the room but a large area could involve expansion joints. Do not tile through door thresholds.
just to second what karis is saying. make sure you have commissioned, tested and ran for a few days, then leave until the slab is completely cold. Then after fixing turn the heating on gradually, bringing up to temp over several days.

Make sure its a concrete screen we are talking about, if its an Anhydrite screed then you need a modified procedure.

some manufacturers specify an s2 adhesive (usually a 2 part flexible) dependant on the type of screed so its worth checking their specs too. both bal and granfix have technical advice lines and they are worth using.
Thanks for the replies.
The boiler is not fitted so no commissioning has been undertaken, We are using BAl single part flexible adhesive wit mapei grout I think i will have to introduce the UFH on a very gradual basis as the extension has a large door opening and a surface area of approx 35 sq meters which has only been protected by 8x4 sheets of ply over the7 month period and has only had the opening closed by DG doors for the last 3 weeks before tiling,
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Oh dear, are you saying you’ve already tiled it? Did you instal an uncoupling membrane? If not you will run a big & very real risk of it cracking up. Very unwise to tile without first commissioning the UFH & conditioning the screed.

That's also a large area, hope youve allowed for expansion!
No decoupling membrane. Is a decoupling membrane always required and what sort of expansion gap would be required and where? Around the wall edge ?
To save writing it all out again, I’ve reproduced this from an UFH specification document;

Expansion Joints-Crack inducers
All screeds expand and contract to some degree so allowance has to be made for this. Perimeter insulation should be fitted with under floor heating which allows for some of this, however large areas will need to have expansion joints or crack inducer cuts in the screed itself to allow movement and avoid screed cracking. Expansion joints will also be required to mirror any expansion joints in the floor slab. These should be specified by the screeder or architect.

Floor tiles, marble floors, stone floors
Where the screed is to be finished with a rigid tile, marble or stone topping the expansion provisions are very important as screed movement can crack the floor finish. The flooring should be designed by the floor installer detailing expansion provision in the floor tiles themselves. This can then be mirrored with a separate bedding layer with expansion joints or in the screed below. Alternatively, an uncoupling membrane for the floor finishes can be installed above the screed.

An uncoupling membrane allows the tile base a certain degree of movement below the tiles without affecting the tiles above, especially where there are cracks in the screed where you can get differential movement. If you commission the heating & condition the screed & there are no cracks, you can get away without a decoupling membrane but it’s wise to fit one as an insurance against future problems that UFH systems can create with tiling. You should have at least an expansion gap around the room perimeter (as should the UFH screed), 10mm will suffice & you can hide this gap with the skirting - assuming you have one! Expansion joints will depend on your room layout & linear tile run & are normally recommended where this exceeds 10m but where UFH is installed, this should be reduced to around 6m.

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