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What pitch for a slate roof

What pitch for a slate roof?

By     Oct 12th, 2017

The slope of a roof is often referred to as the roof’s pitch. Generally a slate roof is not installed on a pitch less than 4:12.

As per the NRCA Roofing and Waterproofing Manual (Fifth Edition), they recommend that “roof deck slopes intended for the application of steep-slope materials at 4:12 (18 degrees) or greater”. (Page 405)

Steep slope slate roof/slope on church

Roof Slope:

As defined in the National Slate Association’s Slate Roofs design and Installation Manual (2010 Edition)  roof slope is “The angle of incline a roof section surface sometimes expressed as a ratio of the units of vertical height to the units of horizontal length (referred to as run).  Slope may be expressed as a ratio of rise to run, such as 8:12, or as an angle.  Sometimes called the pitch of a roof.” (Page 250)

Lower sloped slate roof pitch/slope with black slate tiles


The headlap of the slate is adjusted according to the roof pitch. Headlap as defined in the National Slate Association’s Slate Roofs design and Installation Manual (2010 Edition) is “The length (in inches) of the upper portion of a roofing slate that is covered by the butt of the slate two course above.  The part of the lapping system where there are three layers of slate.  Headlap is typically 3 inches, but could be 2 inches on a very steep slopes or 4 inches on very shallow slopes.” (page 247)

  • Slate pitch greater than 20:12 = 2” headlap / 20” rise per 12” run / 59.04° Angle of Incline
  • Slate pitch 8:12 – 20:12 = 3” headlap
  • Slate pitch less than 8:12 = 4” headlap / 8” rise per 12” run / 33.69° Angle of Incline

Formula to adjust pieces per square based on headlap requirements (1 square covers 100 square feet)

Example: 20” length x 10” width

3” headlap formula 20 – 3 (headlap) / 2 = 8.5 x 10 (width) = 85.   14400/85 = 170 pieces per square

4” headlap formula 20 – 4 (headlap) / 2 = 8 x 10 (width) = 80.       14400/80 = 180 pieces per square

2” headlap formula 20 – 2 (headlap) / 2 = 9 x 10 (width) = 90.       14400/80 = 160 pieces per square

For more technical information, please call our office at Toll Free 1-800-975-2835.

By     Oct 12th, 2017

Sheena Owen

Sheena Owen began working for North Country Slate on October 12, 1999. Started as the Sales Coordinator and transitioned to a Sales Representative. Daily 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 Executive 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 and hikes with family and friends.
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1 Comment

  • Posted on 25-07-2018 by Mark Calloway

    Nice Blog! thanks for sharing. Slate roofing provides an opportunity to contribute to nature as it is the most eco-friendly material available. Though asphalt shingles and metal roofing can be recycled, slate roofing is more environment-compliant.


  • What minimum roof pitch is required in order to utilize slate?

    With special consideration for slate size, headlap and underlayment – natural roofing slate can be applied to roofs with a slope of four inches per foot.