How to Calculate a Roof's Weight Support

Calculating a roof's weight support is challenging for the lay person. It requires knowledge of all of the components that are fixed parts of the roof, known as the dead load. This includes the weight of the shingles, plywood, insulation, rafters and ceiling materials directly attached. It also includes permanent add-ons such as HVAC systems, satellite dishes and solar panels. The live load and transient must then be calculated. The live and and transient loads refer to any weight or stress placed on the roof that is transitional. This includes people walking on the roof, and forces of natures that exert weight upon the roof. The standard guide for international building codes to calculate various values is the ASCE 7-05.

Things You'll Need

• ASCE 7-05
• Local building codes to calculate live loads

Show More

Instructions

1. • 1

     Calculate the dead weight of the roof. Multiply the volume of each component by the unit weight of the materials. These figures can be found in ASCE 7-05. Glass, for example, is about 55 lbs. per square yard, and hardwood is about 25 lbs. per square yard.

   • 2

     Calculate the live and transient loads that the roof will encounter. These values range widely across the country. It includes natural forces such as snow loads, rain loads and wind loads. It also includes uplift loads, which is the amount of distance a structure moves in the wind. It also includes weights of people who will be walking on the roof, and temporary structures such as planters and furniture. Consult local building codes to determine what values are required in your particular region. Snow loads for example range between zero lbs. per square foot up to 300 lbs. per square foot. Wind loads vary depending on wind speed. For instance, windloads are 12.6 lbs. per square feet for a 70 mph wind and 43.3 lbs. per square foot at 130 mph.

   • 3

     Add the values to determine the total roof's weight support and how much weight it is rated to support.