Traditionally, a slate roof will have #30 (thirty-pound) asphalt saturated organic felt unperforated roof underlayment. Required by many building codes, a slate roof’s underlayment serves many purposes and application may vary depending on thicknesses of slate, weather conditions of the area and size of slate.
Some key functions of a roof underlayment:
- Protects the roof and building until the slate is installed.
- First layer of waterproofing and will provide extra protection for leaks should slates become damaged, missing over time or poor installation.
- Assist in fire resistance rating of a slate roof as Class A
- Good surface for chalk lines when laying out the courses
- Cushions the slate when nailed into the roof deck
- Some should not be left exposed longer than 30 days and should be smooth and free of tears.
- Minimum requirements for slates 3/16” to ¼” and slope 8:12 or greater will require a single layer of #30 felt
- Minimum requirements for slate 3/8” or thicker will require a double layer of #30 felt
- Minimum requirements for slopes 4:12 – 8:12 will require a double layer of #30 felt
- A double layer may be required in areas where hard driven rain more common
- Order an extra 10% of underlayment to account for overlaps and waste. Underlayment is sold by squares similar to roofing slate. A 3’ x 72’ roll will cover 216 square feet which would cover 2.16 squares.
- Coated Felt Underlayment ASTM D 2626 asphalt-saturated and coated organic felt, mineral surfaced, unperforated
- Self-Adhering Underlayment ASTM D1970 rubberized asphalt membrane, internal reinforcement, and back plastic release film to meet the fastener seal requirements of ASTM D7349
- Self-Adhering Underlayment: ASTMD 1970, rubberized asphalt membrane, internal reinforcement, and back plastic release film to meet the fastener seal requirements of ASTM D7349.