Damp between layers of roof felt.

Needed some basic remedial work to my bay window (to introduce a gutter), and the roofer has overlaid the exisiting felt with new.

This isnt a problem in itself, however the new felt does not completley cover the old, and water is able to penetrate between the two layers. As the old roof wasnt leaking, this isnt causing me too much concern... YET.

My question is, if moisture is allowed between 2 layers of felt (its damp, not wet), does this increase the decay of both layers of felt, or is it largely unaffected ??

Thanks