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What pitch for a slate roof?

Posted By On 12 Oct 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)

Roof pitch diagram showing various slopes

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)

Steep slope slate roof/slope on church

Headlap:

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

Lower sloped slate roof pitch/slope with black slate tiles

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.

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