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.

Sheena Owen

Sheena Owen began working for North Country Slate on October 12, 1999. As the Sales Coordinator, Sheena’s day to day duties include providing customer service, new business development, inside sales, handling all cross-border customs issues/documentation, arranging shipments, health and safety representation and social media marketing.

Sheena graduated from the University of Guelph in 1992 with a BA in English and Drama and in 1995 graduated with a Diploma in Radio and Television Broadcasting from Centennial College. Sheena’s first job was the Producer’s Assistant for TV shows including Robocop, F/X The Series and Nikita. She went on to becoming an Assistant Manager at Eddie Bauer. In her spare time, Sheena enjoys traveling, gardening, volunteering at her local soccer club and cheering on her kids at their sporting events.

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