Floor construction - refurb & extension

[DavidBJ]

Hello,

I’m refurbishing a Victorian terrace including removing chimney to create open plan space at the rear. Existing solid floor is being dug up and replaced to include underfloor heating.

Existing quote and spec was hardcore, sand binding, DPM, 150mm concrete slab, 120mm PIR insulation, 75mm sand cement screed.
Builders are now suggesting hardcore, sand binding, DPM, 120mm insulation, then 150mm flow screed i.e. no concrete slab.
Aside from issues this might create with the heating (less responsive due to thicker top layer), is anyone able to comment on the structure of this build-up?
I've seen specs for similar build-ups (i.e. insulation directly onto hardcore instead of slab), but that's when the top slab is 150mm concrete, not flow screed.

I'm not convinced in not having a concrete slab somewhere in the flooring structure, but would be good to get some thoughts...
Many thanks,

David

[nod]

I would stick with plan A

The beauty of flow screed is that is can go down 50 mil thick Which most jobs are 

I wouldn’t be relying on it as a structural element 

[PeterW]

Flow screed isn’t structural. You could move it around and go hardcore/sand/120mm insulation / 100mm concrete and then take the imperfections out using self leveling compound. 
 

150mm oversite is a thick lump too - any reason they specify this as it is only an extension. Is any of it loadbearing ..?

[bassanclan]

There is a difference between flow screed and strucrural liquid concrete e.g. agilia

[oldkettle]

@DavidBJ Have you been quoted separately for this work? Curious to get a rough idea of the cost. 

[DavidBJ]

Thanks all for your replies. 
 

On 25/04/2020 at 14:07, PeterW said:

Flow screed isn’t structural. You could move it around and go hardcore/sand/120mm insulation / 100mm concrete and then take the imperfections out using self leveling compound. 
 

150mm oversite is a thick lump too - any reason they specify this as it is only an extension. Is any of it loadbearing ..?

- No loads on the floor. Total size of floor being replaced / new to extension is ~45m2.

 

On 25/04/2020 at 21:42, bassanclan said:

There is a difference between flow screed and strucrural liquid concrete e.g. agilia

- their proposal is an anhydrite flow screed, not concrete.

 

11 hours ago, oldkettle said:

@DavidBJ Have you been quoted separately for this work? Curious to get a rough idea of the cost. 

- afraid not, all in for GF works including extension, chimney removal etc.

I’m tending towards asking them to stick to original build-up but maybe switch top layer to flow screed.

 

Thanks all,

David