Is this a bit mad?

[MustyandConfused]

We are in the early stages of our extension.  
 

builder has dug and poured the strip foundations to the structural engineers spec. All good. 
 

my expectation and understanding was the next stage would be building the first few course of the cavity wall up to at or around dpm level. 
 

Then they would look to prep and pour the concrete base within that ‘perimiter’ (including 130mm layer of hardcore, sand blinding, dom, 130mm insulation, more dpm and then pour the slab on top. 
 

however builder is suggesting they could prep and pour the concrete slab before building up the cavity wall (by using shuttering). This seems weird and I can’t get my head round it. 
 

is it weird? 😃

[SBMS]

I’d want to make sure they’ve got the dpm continuity right. Our DPM is turned up the inner face of the blockwork and lapped over the DPC, how do they do this if pouring the slab first? I suppose they could leave it long and then integrate it into the inner block after?

in any event I’d check with your BC first. 

[nod]

There’s absolutely nothing to stop them doing this I’m guessing the brickie isn’t quite available 

[Mike]

It's not normally done because using the wall as formwork is cheaper. But, subject to some conditions (you're not involuntarily paying extra, the formwork is adequate; the edges of any hardcore are retained in place after the formwork is removed), it's possible.

[Oz07]

Why would they do it like this just sounds like aggro. I can understand the other way round waiting till roof is on and pour a slab/screed that is pretty common

[MustyandConfused]

Thanks all - so it sounds like it’s not weird, but is unusual. 
 

Will try to get straight answer from them tomorrow.