Roofing felt, also known as tar paper, serves as both an extra layer of protection between the shingles and the plywood and also consequently a temporary moisture barrier till all of the shingles are in place. The felt’s Water immunity is temporary and limited.
Everything in Moderation
Roofing felt can get wet and still maintain its integrity, as long as it is not exposed to the elements for over a few days. It will break down in sunlight and with large quantities of continuing moisture. Roofing felt that’s gotten wet can be covered with shingles as long as it’s permitted to dry out first, the surface underneath didn’t get wet, and it didn’t wrinkle, rip or bubble. If the felt is still wet, it might rip when shingles have been installed. If the surface underneath is wet, mold and mildew can form, leading to the disintegration of the flux and the roofing felt. Wrinkles and bubbles at the felt will create heat pockets that damage the shingles as time passes, while rips compromise the ability of the felt to safeguard the roof.