Roofing Felt (#15 and #30)

Roofing felt is a sheet of organic or inorganic base material saturated with bitumen (asphalt or, historically, coal-tar pitch). It has been used for well over a century as an underlayment beneath shingles, tiles and slates, and as plies in built-up flat roofs. It is also known as tar paper, felt paper or, in Britain, sarking felt.

Grades

In North America the familiar names are #15 and #30 felt. The numbers come from the weight of the original organic felts: about 15 lb or 30 lb per 100 sq ft (roughly 0.7 or 1.5 kg/m²). Modern bases are lighter, so the products no longer weigh what their names suggest, but the names remain in use.

• ASTM D226 (asphalt-saturated organic felt used in roofing and waterproofing) covers Type I (#15) and Type II (#30), perforated or non-perforated.
• ASTM D4869 covers asphalt-saturated organic felt underlayment for steep-slope roofing in four types, from #8 to #26. Its 2003 revision added a shower test for liquid water and limits on wrinkling.

Base materials

Early felts were made from recycled rag fibre, hessian or cellulose. Organic bases had fallen to about 5% of the market by 1987. Fibreglass bases were developed in the 1950s; asbestos bases were used earlier and later banned.

Use and limits

Heavier felt is generally used under longer-lived coverings and on lower slopes; for example, double layers of #30 under slate or tile versus a single #15 under shingles on a steep roof. Felt can wrinkle when wet, tears more easily than synthetics, and is degraded by prolonged heat and UV exposure, so it should be covered promptly. In many markets synthetic underlayments have largely replaced it on new steep-slope roofs, though felt is still widely sold.