Hi everyone, I need advice and hope someone can help. I am building ne extension and flat roof on top of it. roof is 3mx3m, there is six 6" joists 45cm apart, on top there is 18mm plywood, then 115mm insolation, underlay and one layer of felt on top. BR is telling me to put another 3 layers of felt. do I need so many things on one little roof? ne roof is merging existing one and old roof has only 5cm depth. many thanks