The maximum slope ARMA considers suitable for normal asphalt shingle application is 21:12, or 21” per foot. The use of asphalt shingles on slopes greater than 21:12 (e.g., mansard roofs) requires special application methods because such very steep slopes reduce the effectiveness of factory-applied self-sealing adhesives, especially in colder climates and shaded areas. If normal application methods are used for asphalt shingles on very steep slopes, problems can occur that are not a result of any weakness or defect in the product but rather because special application methods for very steep slopes were not employed.
If a roof slope exceeds 21:12, the shingles should be applied by the following recommended method:
- Secure the shingle to the roof deck with fasteners as directed by the roofing manufacturer. Manufacturer directions may call for more than the normal number of fasteners per shingle and may provide specific fastener locations. On very steep slope roofs, correct placement of nails on laminated (two-piece) shingles is even more critical as the nails will penetrate both shingle layers when placed correctly. If the nails are placed too high on the shingle, it is possible that, over time, the bottom layer, which will be held in place only by the factory-applied adhesive, may delaminate and slide off the roof.
- Apply a manufacturer-recommended asphalt roof cement complying with ASTM D4586, Standard Specification for Asphalt Roof Cement, Asbestos-Free, or other cements approved by the roofing manufacturer under all shingle tabs in spots equivalent to the size of a quarter (about 1” or 25mm diameter), immediately upon installation.
- For standard laminated shingles (i.e., ‘no cut-out’ shingles), apply four spots of cement under each shingle near the lowermost edge, with two near the corners and two equally spaced between the corners.
- For ‘three-tab’ shingles, apply two spots of cement under each tab near the lowermost corners.
- For other specialty shingles, refer to the manufacturer’s recommendations.
- Over-application of adhesives is not recommended; follow the manufacturer’s recommendation for cement type and minimum and maximum quantities.
- An unvented space may be behind a mansard roof. It is important to provide a properly ventilated flow-through air space behind the roof sheathing to prevent the entrapment of moisture-laden air. Additional information is available in ARMA’s technical bulletins “Considerations in Attic Ventilation System Selection” and “Why Ventilation is Important.”
- See also ARMA’s Residential Asphalt Roofing Manual.
DISCLAIMER OF LIABILITY: This document was prepared by the Asphalt Roofing Manufacturers Association and is disseminated for informational purposes only. Nothing contained herein is intended to revoke or change the requirements or specifications of the individual roofing material manufacturers or local, state and federal building officials that have jurisdiction in your area. Any question, or inquiry, as to the requirements or specifications of a manufacturer, should be directed to the roofing manufacturer concerned. THE USER IS RESPONSIBLE FOR ASSURING COMPLIANCE WITH ALL APPLICABLE LAWS AND REGULATIONS.
Nothing contained herein shall be interpreted as a warranty by ARMA, either express or implied, including but not limited to the implied warranties of merchantability, fitness for a particular purpose or non-infringement. IN NO EVENT SHALL ARMA BE LIABLE FOR ANY DAMAGES WHATSOEVER, including special, indirect, consequential or incidental damages or damages for loss of profits, revenue, use or data, whether claimed in contract, tort or otherwise. Where exclusion of implied warranties is not allowed, ARMA’s liability shall be limited to the minimum scope and period permitted by law.
