DPC and DPM options

[Adman101]

I'm in the process of planning my extension. Now, I have done a few building projects and helped out on a few extensions so have a fairly good grasp of how everything goes together (or so i think). I have seen a video online, building an extension where the don't cast the concrete slab until after the roof is on. They use what looks like a 2ft wide piece of DPM as the DPC (with the extra hanging inside) and then build up the walls and then pour the slab later.
Is the common and if so I presume you would just lay a sheet of DPM over the top of this then cast your slab. Are there any drawbacks to this? I think this would make my build much easier.
TIA
Adam[/img]

 

[freddiemercurystwin]

Why would casting the slab at the end be easier?

 

[Adman101]

I have two steels that will be resting on existing internal walls and the new exterior walls. I could get these in first and then this would allow me to run the new drainage pipes in to the centre of the house under walls to be demolished before casting the slab. Also It means I will be able to cast the slab as one (including a section of the existing house the other side of existing walls to be demolished) and avoid any potential reflective cracks coming up to the tiles where the old slab joins the new.