UFH pipes going between slab and screed

[Tom]

Hi All, hoping someone can reassure me that this is alright. We have UFH pipes exiting our polished concrete slab below floor level and going in to an area that is going to be (liquid) screeded tomorrow. I'm concerned that at the vertical join where the slab and screed meet there will be movement as the two material expand/contract and that this will put the pipes here under repeated stress. Should I be worried? I'm guessing there should be an expansion gap here - but would this make the differential movement of the slab/screed even worse and put the pipes under even more stress? 

Thanks all

[Mr Punter]

Can you put some compressible insulation around the pipes where they pass through the 2 layers?

[Tom]

Hmm, I can spray a little expanding foam around the pipes at the concrete/screed interface, but I guess I would still have differential movement of the two pulling/pushing the pipes around?

[Tom]

Just spoke to the chap doing the screed tomorrow and he said not to worry about it, he has done hundreds of floors with pipes crossing under expansion joints etc and never had a problem. Perhaps I'm just over thinking this. Wouldn't be the first time. 

[Thorfun]

3 hours ago, Tom said:

Perhaps I'm just over thinking this.

It’s what we as self-builders do. 

[Nickfromwales]

Just install a flexible cable conduit over them and tape up the joint. It’ll be fine, as there’s actually very little actual movement, ask people who’ve tiled over ‘expansion points’ and not even had so much as a hairline crack. 
If the slab is of a reasonable thickness with anti-crack mesh etc then it’s just going nowhere, and if it’s a low energy home the slab should see big temps shifts anyways?

[Tom]

Thanks all, feel reassured. Just a slight mid-afternoon panic.

 

The UFH pipes I'd laid everywhere were finally hooked up to the emanifold today, and one was a few feet too short, FML, so it rides over several to cut a corner. Luckily the liquid screed is so sodding deep, and expensive, FML, that it will all be covered.

[Tom]

Actually, just remembered, the screed guy says we need to start getting some heat through the screed in about a week, and suggested hooking up an old immersion somehow. What is the best way to do this? Cheep second-hand immersion/tank, or a willis heater perhaps??

[saveasteading]

A well laid screed will have contraction formers at intervals, forcing shrinkage cracks at these  points. This is a standard process so we would have heard if the cracks damaged ufh pipes.

 

Our 70mm screed cracked where encouraged  to, but also a random crack, surprisingly about 3 months later. It is about 1mm wide. 

Try stretching a piece of pipe..its strong and ..I'm sure it's designed for this.

[saveasteading]

11 hours ago, Tom said:

the screed guy says we need to start getting some heat through

Any idea why? Normal practice is to keep slabs and screeds damp as long as possible.

[Tom]

its an anhydrite screed which it seems you can put heat through after a week or so

[saveasteading]

4 hours ago, Tom said:

you can put heat through after a week or so

But he says you 'should'. 

 

I googled it and this says the opposite. https://dunloptrade.com/technical-team-blog/top-tips-for-tiling-to-anhydrite-and-hemi-hydrate-screeds#:~:text=This means that a typical,days for Ordinary Portland Cement.

[Tom]

Sorry, I don't follow you, the info in the link says you can turn the ufh on after a week, which is what I was told. Am I missing something?

[saveasteading]

Can turn it on, not should turn it on. The main point was it saying that it takes longer than cemeng based screeds.

im not saying it is wrong, just wanting to learn and able to give besf advice...and these screeds ive only used once.

and in our place a big crack appeared after several months. I think it was missing a crack inducer, but the delay was a big surprise, so a good job it wasn't tiled.

Maybe your man has a point. Make it crack if it is going to.