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The Weight of Roofing Materials

SOME QUESTIONS AND ANSWERS

We frequently receive questions regarding the weight of roofing materials and the ability of residential structures to accept the additional weight of various types of roofing materials when they are used to replace wood shake or shingle roofing materials. Some homeowners fear that the added weight of the replacement roof system could result in an increased structural failure risk during an earthquake. Unfortunately these fears are compounded by marketing techniques of some companies which provide inaccurate of incomplete information.

Some of the typical questions are:

Here are answers to those questions:

  1. In most cases reinforcement should not be required. Homes built to modern code standards(1950’s and later) are designed for roof loading of at least seven pounds per square foot (7-psf), also expressed as 700 pounds per 100 square feet (700 lbs/square)*.

Most building officials allow the installation of two to three layers of composition roofing (original plus one to two reroofs). The average weight of various composition roofing materials, according to a major California manufacturer, is approximately:

Therefore, the range of weight of three layers of composition roofing is 6 to 10.2 lbs./sq. (600 to 1,020 lbs./square)*.

Where trusses are used for residential construction they are generally designed for a dead-load of at least 14 lbs./sq.ft. (1,400 lbs./square)* according to a California roof truss manufacturer.

The experience of the roofing industry shows that only about three percent (3%) of homes, which use light-weight roofing materials (5 to 7 lbs./sq.ft.) require reinforcement. Usually, this is necessary only when the original construction did not comply with code requirements. When homes are constructed in compliance with the building code reinforcement is rarely required.

When roofing materials of 9.5 to 12.0 lbs./sq.ft. (900 to 1,200 lbs./square)* are used for existing structures, approximately thirty percent (30%) of the homes require reinforcement. An analysis should always be performed to confirm that the existing or modified roof framing system is adequate to support the reroof covering as required by the building code (Appendix Chapter 15 of the Uniform Building Code and Chapter 15 of the International Building Code, as adopted by local jurisdiction).

Many communities do not require roof material weight-loading calculations if the replacement roof covering has a weight of less than 6 to 7 lbs./sq.ft. For example, many cities, such as, Los Angeles, Buena Park, and Redwood City, only require calculations if the weight exceeds 6 lbs./sq.ft.; San Jose and Tustin, require calculations if the weight exceeds 7 lbs./sq.ft. Homeowners should check with their local building department to verify local requirements.

  1. Cost estimates for a roof weight-loading analysis, by a qualified structural engineer, is approximately $300.00 per average one-family residence. If a lateral loading (earthquake) analysis is required by the building department, the cost may be an additional $1,000.00 or more.
  1. In most cases where deficiencies are found, they usually require additional bracing. The cost for such reinforcement generally is $300.00 to $800.00. However, major deficiencies could be found in some homes if not constructed in compliance with the building code.

NOTE(*): A "square" is defined as 100 square feet of roof surface.

DISCLAIMER: The Committee for Firesafe Dwellings assumes no liability for the use or misuse of this information, which is intended to provide guidelines for consumers in their selection of building materials and fire protection systems for their homes.

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