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.