Calculating the load on a flat roof is a challenging task for the layperson that involves knowledge of building materials, their weights and local building codes. The standard guide in the U.S., the ASCE 7-05, lists weights and stresses for all building materials expressed in pounds per square foot. It also lists load factors for natural stressors, such as wind, rain and snow. Consult your local building codes for verifications on these values as they are highly regional, based on climate.
Determine the dead load of the roof. This includes the combined weights of all of the individual roof materials and anything attached to it permanently. This would be the ceiling materials under the roof and any permanent installations, such as HVAC components. These values are expressed in pounds per square foot (psf) and are found in the ASCE 7-05.
Calculate the live load of the roof. This is determined by adding the weights of any individuals who may work on the roof, and the combined weight of all tools and equipment used by those workers.
Determine the transient load of the roof. This is all of the natural stresses imposed upon a roof, such as snow loads, rain loads and wind loads. This is highly regional as weights of snow, for example, range from 10 to 300 pounds per square foot based on moisture and volume of the snow. Wind loads depend on wind speed and can vary as widely as the snow.
Add all three of these load values to calculate the load on a flat roof.