Laying a thin screed

[CrazyFool]

Hey guys

How thin can you go with an underfloor heating screed bonded to a concrete slab?
I've got 28-30mm of flooring to raise.

I know that a normal sand cement screed will curl but I've read that screeds reinforced with polypropylene fibres can go as thin as 25mm.

Does anybody have any experience with this?
Please can somebody give me any advice?

I know Mapei renovation screed will work, but its expensive and I already have lots of sand and cement that I can use, will just need to buy a small bag of polypropylene fibres.

From bottom to top this is what my floor looks like:

100mm PIR
100mm concrete
16mm UFH pipe
28mm screed (including UFH pipes)
Porcelain tiles

My concrete has been laid. Just need to figure out what to do next before I can put down pipes + screed.

Thanks in advance

CF

 

[^woody^]

I've read that screeds reinforced with polypropylene fibres can go as thin as 25mm.

Where have you read this? Fibres are to control initial shrinkage and nothing else.

Your thin [standard] screed will debond and then crack, so a modified product such as from Mapei may do. But you should really refer to the underfloor heating manufacturers specification and adhere to that.

 

[CrazyFool]

Thanks for clearing that up for me. I'll ditch the idea of fibres.

Are there any additives, plasticisers, or hydraulic binders that can allow me to lay a thinner sand + cement screed?
Or do you suggest any alternative recipes?
Would drilling holes into the concrete slab provide reinforcement for the screed?

What are my options if I want a 28mm screed?

Grateful for any advice

CF

 

[^woody^]

Screed around heating potentially goes through a lot of movement stress, so you would need something specific and tested to modify the screed, rather than trying to guess.

A quick google and www.screedgiant.co.uk/product/ronascreed-fast-drying-underfloorheating-screed-additive/

Expensive, but shows that additives are available.

As to the 28 mm depth, as this is building and not engineering you can't nortmally gurantantee that there wont be humps in the subbase that reduce that 28mm to much less. So be sure about the real depth available, and not just at the one or two places measured at the edges.

 

[CrazyFool]

Screed around heating potentially goes through a lot of movement stress, so you would need something specific and tested to modify the screed, rather than trying to guess.

A quick google and www.screedgiant.co.uk/product/ronascreed-fast-drying-underfloorheating-screed-additive/

Expensive, but shows that additives are available.

As to the 28 mm depth, as this is building and not engineering you can't nortmally gurantantee that there wont be humps in the subbase that reduce that 28mm to much less. So be sure about the real depth available, and not just at the one or two places measured at the edges.

Thanks for the reply friend.

Yes, I've considered the highs and lows, so its 28-30mm of screed that I'll need.
I've heard that screeds can go down to 25mm on a normal slab provided its bonded properly, so was wondering if in a UFH application a little bit of SBR + the extra 3-5mm would be sufficient to counteract the additional movement stress.

But like you said there is probably limited experience/documentation with something like this.

Appreciate the advice

CF

 

[Leofric]

Never heard of a screed as thin as 28mm , I thought 50mm was an absolute minimum thickness for a self levelling screed.

 

[catlad]

I was at a trade show last year and there was a dunlop stall raving about their new flexible levelling compounds, maybe give their tech department a call.

 

[^woody^]

I suspect that there are two issues to cater for - thin screed, and thin screed over heating pipes. This will need a researching the manufacturers for a dedicated product, or shelling out for that Mapei screed, as the risk of failure and consequences from some Frankenscreed is too high.

 

[23vc]

Our local readymix co do some kind of pumped self levelling screed that they’ll either supply only or supply and level. I’m sure that’s down to something very thin like 30mm.
The 50mm+ ones are the semi dry traditional jobbies that you rule off, float, trowel etc (not self levelling).

 

[Leofric]

I am not up to date on the latest developments in screeds but 50mm used to be the minimum thickness recommended for self levelling screeds and 65 to 75mm for cement/sand traditional types.

 

[CrazyFool]

Thanks for all the insightful comments guys.

The Mapei renovation screed seems the best option.

https://drive.google.com/file/d/1HGhvzfIa8xtPFn7WXEfpfP7l_8Ja_yX0/view?usp=drivesdk

I'll batten the white area with MDF and pipe/screed/tile in between.

No point having any of that underneath the kitchen units.

Thanks for all the advice

CF