Flat roof - is underlay essential?

[Oscar123]

Planning on using a torch on cap sheet direct to fresh osb without using the recommended underlay...is this a bad idea? If the cap sheet is bitumen backed and therefore fully water proof and has good adhesion to the osb I'm struggling to work out what the underlay actually does...unless I'm missing something?

Cheers

Oscar

 

[Alastairreid]

Underlay is tacked on to the osb to allow for differential movement between the boards and felt.

 

[chappers]

usual method now with torch on is 3 layers.
1st perforated layer which is loose laid then torched over, gives light adhesion, then underlay is torched over, the perforations in the base layer mean that there is incomplete adhesion to the deck to allow for movement and then the capping sheet is torched over the top.