An asphalt shingle underlayment is typically dry felt that may be impregnated or coated with an asphalt saturant, or a synthetic sheet. The use of an underlayment is recommended for the following reasons:
Resins may exude from the wood board decking. Underlayment protects roof shingles from the resins that may be released.
Underlayments protect decking material from wind-driven rain that may penetrate the shingle layers.
To validate their limited warranties, many manufacturers require the use of underlayment.
The use of underlayments, particularly heavier grades, reduces “picture framing.” According to CASMA, picture framing is the visible outline of deck panels caused by irregularities in roof decking thicknesses.
To obtain a Class A fire resistance rating, underlayments should be used underneath shingles.
The underlayment should conform with CSA 123.3-M (No. 15 / ASTM D226 Type 1 No. 15 felt), ASTM D 4869, and/or CAN 2-51.32 (Breather Type Sheathing Paper) industry standards.