Two types of loads affect roofs: dead loads and live loads. A dead load constitutes any load for which you know the exact weight. An air conditioning unit on a roof, for instance, qualifies as a dead load because its weight never changes. Snow, on the other hand, qualifies as a live load because you can't know how much snow will fall and how much weight stress it can place on a roof until it actually falls. Calculating dead loads requires little work because in all cases you already know the weight of these items. Pounds per square foot is the standard unit of measure for loads on roofs.
Live Load Calculations
Two types of formulas exist for calculating live load weight stress on a roof, one type for generic live loads and one designed specifically for snow. The snow load formula reads Pf = 0.7 Ce Ct I Pg, where Pf equals the snow load, Ce equals exposure factor, Ct equals thermal factor, I equals importance factor and Pg equals ground snow load. You can obtain values for these variables from a university extension or other building or engineering service in your area. Some of them might require a site inspection. Formulas for generic live loads can assume a variety of forms. They require complex math and a good deal of speculation. Contact a local building authority for the most appropriate formula, and information variables, in your area.
Roof Capacity Calculations
Roof capacity calculations help determine how much load a roof can support based on construction methods and materials. With lumber roofs, these calculations require considering the strength of the type of wood you use. Builders use charts such as standard load tables or economy tables provided by publications such as “Standard Specifications and Load Tables” to determine load capacity of specific materials and construction methods. Online calculators also exist to help determine basic load-bearing capacities of roofs. Flat roofs almost always employ joist systems, so you should always calculate flat roof load capacity based on joist capacity. Keep in mind that a flat roof support system must hold not only any live or dead loads on the roof, but also the weight of the roof itself.
Building codes can save you time when it comes to roof load calculations. Codes in areas with regular snowfall, for instance, contain minimum live load laws for flat roofs to accommodate for snowfall. The Connecticut Building Code requires load support of at least 30 PSF on all parts of a roof to accommodate for potential snow weight. Building codes may also contain information such as the type of joists, joist connectors, beams and timber you must use in a flat roof timber roof to meet minimum load requirements.