concrete floor slab help please

[no-more-cowboys]

hi all,

this is my first time here so please be gentle!!

long story short-i have been ripped off by a builder and i'm now left trying to pick up the pieces on a now tiny budget.

my first query (of many no doubt) is, i've got/will have a 3x3 metre utility room. foundations done and walls up. in the inside is currently a long standing concrete walkway (1/4 of width) and long standing (with no movement at all or cracks) 3 inch thick paving sitting directly on top of gravely clay.

would it be possible to use this as a base for the concrete slab floor?
if i've got it right, put on top a layer of sand, then the DPM, then insulation, then the concrete (unsure of depth) followed by a screed.
would that be ok or does it all need digging up and using hardcore etc?
also, i've been searching through info and have seen mentioned a few times about 2 lots of DPM, do i need 2?

[welsh brickie]

you need well compacted hardcore usually 150mm but it depends on conditons,50mm celotex/kingspan.Insulation on the harcore will stop the visqueen being pierced,then 100mm min concrete then screed 50mm

[no-more-cowboys]

hi thanks for the reply.

so you're saying no DPM under the insulation?
wouldn't the insulation then get damp?

[welsh brickie]

yes the insulation would get damp,Unless a double dpm is installed.Its stated on some plans, I have heard,But I have never seen it or been asked by the building inspector to install it .