New Flat Roof

A

andyf82

Hi I'm looking to spec a flat roof with insulation between the new joists. I've come up with the following from top to bottom:

EPDM sheet
19mm WBP ply
Firrings (75mm max down to 25mm min)
150x50mm strength class C24 joists @ 600mm c/s
100mm Kingspan K7 insulation between joists with 50mm air gap over
Vapour control layer
32.5mm K18 insulated plasterboard

Can anyone advise if seems ok or if there is a better way to do it? I cant insulate over the new joists for lack-of-height reasons.
 
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Taking into account that I don't know your span I cant comment on the joist size but the rest looks ok, you will need to vent the 2" void.
 
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If using EPDM membrane i would consider using a tongue and groove deck - OSB3 for example, as it provides a perfectly flat substrate for the membrane to adhere to. Being only 1.14mm thick it will show up all the lips of the boards if they arent super flush.
 

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