Built-Up Roofs Roof Felts

(1)  Bitumen roofing felts shall be not less than No.15 felt.

Article Ontario Regulation 332/12 Building Code,   Information published by oncodes.ca for educational purposes only.

Note : Roofing felt or felt paper, also called underlayment or roofing tar paper, goes between the roof deck and the shingles. This additional layer of protection helps to prevent water from coming directly on to the shingles and acts as an insulator against heat loss through your roof. Felt works best for wooden shingles, but it can also be used for other styles of shingles such as slate and clay. Underlayment has a base made from natural materials like wood cellulose, or synthetics like fiberglass/polyester