We have 2 bedrooms in the loft in our new house built in 1970 but had crap artex on walls and slopping ceilings so decided to pull the plaster board walls and ceilings down to replace insulation rewire and reboard , but noticed on the sunward side of the roof the felt under the concrete tiles has slipped and pereshed in a lot of places, ive read that the felt is not really used as long as the roof tiles are in good condition is this true?