wet under floor heating???

[lcjohno]

Hi all

Currently talking to a customer through emails regarding a bathroom he wants tiling, he's been asking me questions regarding best prep for travertine etc.

He has told me he has installed wet ufh which i have no experience of. Here is what he's writen in regard to floor: We have had some different views on how to treat the floor of the bathroom ready for tiling and would appreciate your views so we have prepared the floor appropriately.

We have the studwork of one wall done now and we have put in the wet underfloor heating as follows
75mm of Kingspan insulation supported on 2” x 2” battens on the sides of the joists
50mm of 3:1 cement screed around the heating pipes giving a finished surface level with the top of the joists
It is very solid but the cement screed is likely to expand and contract at a different rate to the timber joists so we are told the concrete may expand and rise up relative to the joists.

My thoughts are to lay a 12mm ply sheet over the floor There will need to be a join as the area is too big for a single sheet. My thoughts are to but the 2 pieces together over a joist and screw those mating edges down but leave the others free to move with any expansion or contraction. What are your thoughts?

Can anyone explain to me how this kind of floor works, i just dont get where the joists come into it. I just thought wet floors were just for concrete floors. I am pretty sure the plywood idea is a no no! but what is the best way? I am going to look at it soon but want some advice on this type of floor before i do.

Really appreciate responses :smilewinkgrin:

 

[DHTiling]

Why oh why would they do it that way..?

IMO it is not suitable to tile.. the heaving will deffo crack tiles.

 

[Mike]

it seems a very risky job to take on. i was asked to quote on a large travertine floor recently which was far from straightforward so i refused to price it as its not worth the risk with such expensive tiles. it's a very strange set up there.

 

[lcjohno]

I must admit it sounded strange to me but having no experience of wet ufh i thought that must be one way of doing it. Might have to give it a miss.

 

[Ajax123]

this type of floor is becoming quite common although sand cement screed does not generally have the flexural strength to cope with the level of deflection involved in a joisted floor. The joists need to be engineered to take the weight without bouncing and the screed needs to be reinforced with fibres or d49 steel mesh. If these factors have not been considered my opinion would be don't tile it.

 

[White Room]

Screed weighs about 108 kg m2 at 50mm thick, that can be some downward weight

 

[Ajax123]

Screed weighs about 108 kg m2 at 50mm thick, that can be some downward weight

1.08kn/m2 approximately. Domestic floors are usually designed for 1.5pm/m2 loadings but the deflection that this downward force creates is the important bit. Too much deflection and the sheer stress at the ends of the beams can be exceeded and the floor Falls down. Taking account of the adhesive weights and the tile weights I would think that this one needs cnfirmation from building control at least if not a structural engineer