Insulating Warm Roof - Multiple layers of celotex?

[phillarby]

We are just about to start insulating our roof, which is a warm roof. Our building regs docs say 120mm of celotex between the rafters with an further 20mm over the whole lot.

Would it matter if we used a 100mm and a separate 20mm between the rafters, or does the 120mm need to be a single layer (We have a lot of 100mm celtoex available as we bought it for the floor and don't need it yet)? Would using multiple layers between the rafters affect the U value or could it cause any other problems?

Cheers
Phil

 

[SurreyRoofingEstimator]

Would have thought you could do that without issue as long as moisture can't get between the layers by using a good VCL below the rafters. Odd spec though