What underlayment for slate roof?

Posted By On 29 Jan 2018

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

Slate roofers working on The Cathedral of Saint Paul in Birmingham, AL

Underlayment considerations:

  • 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.

Other underlayment options

  • 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.

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