pir insulation advice

hi guys
basically im almost at the point of starting to get the flat roof on my extension its going to be a warm deck sandwich design
i have 5 sheets of 100mm kingspan left over from a previous project which i intend to use and plant a 50mm sheet on top of this bringing it upto 150mm thickness to satisfty bc
however it is going to take more than that to cover the whole roof deck
i have available to me quite a few sheets of 75mm pir board used under screed by a floor screeder mate of mine which i can buy at cost
my idea being i can double up the (celotex) (quinn boards) to get to my desired depth of 150mm
But my question is this
is the pir board used under screed any different to that used on flat roofs or are they virtually all the same and can i use the 75mm doubled up to 150mm
without causing any future problems?
thanks for any advice.

The insulation values per mm will be the same, though won't you end up with 50 wide gaps above joist level that you'll have to faff about infilling?

hi freddy thanks for the reply
im not sure what you you mean by 50 mm gaps so heres what i plan to do.

fix 9 x 2 c16 joists at 400 centres
board over with 18mm ply
lay vapour barrier
lay pir insulation boards made up of 100mm 50mm and 75mm boards to a thickness of 150mm
(basically im using up pir boards i have left over from other projects and boards i have access to at cost)
lay 18mm ply board on top of pir insulation screwed and glued
single ply membrane over the top (firestone rubber)
foil back plaster board ceiling beneath

does that sound like a reasonable plan ?
i cant see how id get any gaps using this method.

Ahh ok, thought you were proposing sticking your100 ins between joists but if it's all going on top then fill your boots, obviously stagger any joints.

Thankyou Freddy as always much appreciated