Do you need screed on top of raft foundations

[Barny]

We are having a raft foundation and the UFH is going into the concrete. Do we need screed on top if the slab is flat? We are planning to either resin or tile the floor.

[markc]

No need for screed if UFH is in the slab

[saveasteading]

1 hour ago, Barny said:

If the slab is flat? 

That is quite a big IF.

It is a specialist skill to lay, level and smooth a oncrete slab, especially in outdoor conditions.

Some companies do nothing but.

 

But a latex levelling  screed may be enough to smooth it out and prep for tiling, ie not a full screed, but something a tiler would do first. I woukd cost that in.

 

There are formal specifications to describe what "flat" means  Does your drawing mention any?

 

As a bad example, our groundworker's slab work was so wonky that the joiner had to pack up wall plates by up to 50mm. He was dismissed, and amazed that we didn't think he was great. No spec would tolerate that. 

 

Just for clarity, what do you mean by a raft, and what superstructure method are you having?

 

 

[JohnMo]

We have a concrete over UFH with no screed. But had to self level the low spots and it took a day to grind down a high spot.

 

But cost a couple of hundred pounds including the grinding attachment for an angle grinder. My labour was free.

Edited July 4, 2023 by JohnMo
Added costs

[Kelvin]

I thought my groundworks team did a great job and the slab was fairly flat overall with an 8mm variance from the highest point to the lowest point. The screeders told me about a recent job where there was a 60mm variance between the high a low point. Too great to pour the screed in fact.