Need to create a 50-25mm fall in new screed

stevo_sideways

I have a couple of terraces that are built from old poured concrete. It’s causing pooling and has cracks in it etc….so not suitable for us putting new tiles down onto.

This means we need to lay a new screed but I’m getting differing advice from tradesmen. Once is saying he can pour an unbonded sand and cement screed on top of polyurethane over the current concrete, but then the tiler said that won’t work and instead he wants to use a bonded levelling compound instead.

there’s a substantial difference in pricing with these two approaches so any advice would be appreciated.

Tks

10-10-20

Hi, what depth are they proposing to pour in each case - both bonded and unbonded? That's the important detail here as unbonded will obviously sit over the existing cracked concrete as a monolith, but needs to be thick enough and strong enough not to crack itself once the base shifts, and the bonded one needs to be thick enough to bond the base entirely and not risk cracking later on… so it's really a case of who's ugly baby you want to live with, if I can put it that way. 😁

stevo_sideways

Thanks for the reply. I can’t go past 70mm in finished height at the highest point as it’ll bring the finished tile level above the vents in the bottom of the double doors.

So the suggestion was to pour around 55mm down to about 25-30mm

builder working across the road said he thinks the unbonded version would give in and pop the tiles after a while. Was looking at the sika200 or Larsen Primadry screeds which hopefully can have a sharp sand aggregate added to it in order to reduce costs because trying to get a 55mm to 30mm screed over 45m2 is going to be pricey if I don’t add an aggregate. .

stevo_sideways

https://www.boards.ie/discussion/comment/122945087#Comment_122945087

hi 10-10-20

just wondering if you’ve any suggestions based on my last post pls?

Tks

Stephen

10-10-20

https://www.boards.ie/discussion/comment/122964009#Comment_122964009

Sorry, I was hoping that somebody else would have an input as I know some of the theory but not the practice!

Lumen

I'd guess unbonded would be better because there's less chance of cracks propagating, but I don't know how anyone could offer much of an opinion without seeing it, otherwise how could the cracks be properly assessed?

I doubt a screed is going to hold cracked concrete together.

GreeBo

Are the cracks stable, or still moving?

How deep is the pooling? If its really 20mm (based on your depths above) you are at the limits of what you can build up with fast setting tile adhesives.

It seems to me that if the cracks are stable enough and the pools are shallow enough, they could just tile directly over and use tile cement to ensure you get a level surface. Using an externally rated decoupling membrane might help avoid issues if the cracks are still active. IF they are active then IMO no screed is going to help things, you would need a new subbase.