In article , Andrew Gabriel
writes
In article ,
fred writes:

Watch out using breathable membranes over boarded roofs, they don't like
getting scuffed and will leak if they do.

Breathable membranes don't like the back being touched by anything,
or they can become leakable membranes. I didn't understand how that's
supposed to work when they rest on a rafter (except that the natural
sag between rafters will direct any water away from them), so I used
non-breathable when I needed to replace some of the old tar ones (which
were non-breathable until they wore out). In my case, there's a vent
formed at every sarking overlap, so there's no shortage of loft
ventilation.

A relative who is a building surveyor (working in Scotland) has reported
that some contractors have expressed a preference for using tougher, non
breathable sarking even though it requires a greater ventilation space.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs