Do I need a primer for SLC onto a slip membrane?

[Great_scot_selfbuild]

We’re covering a small area of UFH (which was too shallow to lay the dry screed) with self levelling compound. In searching about SLC I’ve seen mention of a primer - is this always needed? The SLC will be onto a slip membrane which is over the pipes.

This is the stuff I’ve been recommended to use. 

[Russell griffiths]

The primer is for priming up the surface if you are putting the SLC onto another concrete or screed surface. 

If you are going over a plastic membrane nothing is going to stick to it, your SLC will always be an independent slab on top of the ufh. 

[Nickfromwales]

How thick (or thin) is this going to be?

 

Also, you cant use primer over a membrane.
 

I can’t recall your foundation type, but if theres a DPM under the slab you should cut the slip membrane out and put the SLC down without any such decoupling.

[torre]

Isn't the membrane usually laid below the pipes?

[Great_scot_selfbuild]

33 minutes ago, torre said:

Isn't the membrane usually laid below the pipes?

@torre The vapour membrane is below the pipes, the slip membrane on top.

[Great_scot_selfbuild]

4 hours ago, Nickfromwales said:

How thick (or thin) is this going to be?

 

Also, you cant use primer over a membrane.
 

I can’t recall your foundation type, but if theres a DPM under the slab you should cut the slip membrane out and put the SLC down without any such decoupling.

probably end up being about 30-40mm.

Foundation is a suspended beam & block floor, DPM, 150mm PIR, Vapour membrane, UFH, slip membrane.

[Russell griffiths]

What’s the slip membrane for

do you not need the SLC to find its way all around the ufh pipes for good heat transfer, with no big voids. 

[torre]

I agree with @Russell griffiths that this extra layer is just reducing surface contact with the pipes. 

 

Your vapour membrane is enough of a slip layer, allowing the screed and pipes to expand slightly together. An extra layer sounds like it'll do more harm than good, it's not the screed will then slide separately over the top of the pipes, so what can it achieve? 

 

How large is the area? You won't even get 0.5m2 per bag at that depth so will have your work cut out mixing quickly enough to ensure it ends up as a contiguous slab. Buy more than you need as you don't want to run out. 

 

Have you enquired with liquid screed companies? Some can be poured very thin and will give you a guaranteed result.