Underlayments
There are two major types of shingle underlayment: water-resistant and waterproof.
Water-Resistant Underlayment
This is commonly referred to as roofing felt and is available in #15 (standard) and #30 (heavy duty) grades. The installation of standard underlayment beneath shingles is required by many shingle manufacturers. It is a product that consists of organic felt impregnated with asphalt saturant. Some also contain a fiber glass reinforcement which increases tear strength. The main purpose of roofing felt is to keep the roof decking dry until shingles can be applied and is thus known as "drying-in the roof." It is also useful as a barrier between the decking and asphalt shingles. It's ability to be water-resistant is temporary because once the shingles are installed, the holes created by the fasteners destroy its protection.
Waterproof Underlayment
This is a self-adhesive underlayment that should be installed in all valleys, along eaves, and around skylights prior to shingle installation. It prevents leaks in extreme conditions such as high winds, heavy rains, and ice dams. It is excellent underlayment for low-slope shingle applications and required by some manufacturers for these situations.
This product is waterproof because it seals
around the nails that puncture it.