Roof structural considerations (rafters and trusses)
The rafters and trusses are the main components that support the structure of a roof system. Different factors work against this structure and cause stress on the rafters and trusses. Roof structural failure often begins with rafter deflection or rafter spread. Deflection results when horizontal loads (such as snow and ice build-up) cause wood fibers to bend. Eventually, deflection causes rafters to rupture in the centre third of the span or at such weak points as notches or knots on the top or bottom edge of the rafter. Rafter spread results from the failure of mechanical ties, such as nails, to hold ceiling joists, top plates, and studs together or, occasionally, failure in the ceiling joist itself.
Estimating roof structural strength
An expert carpenter, contractor, or architect can help determine how much weight a roof can support. Characteristics to be considered include the rafter lumber (its size, strength, and stiffness), rafter span, slope of the rafters, distance on centre between rafters and any strength-reduction items such as notching, rotting, insect attack, grain slope or knots. With this information, you can refer to span tables, available at lumberyards or through engineers, building inspectors, or carpenters. Those tables enable you to estimate the snow load capacity of your roof, which is measured in pounds per square foot.
Measuring roof dead weight
Another variable affecting your roofs snow-bearing capacity is the roof's dead weight, or the weight of all the roofing materials (sheathing, roofing paper, felt, shingles, or built-up membrane.) Roof rafters or trusses in the majority of new homes are assumed to carry 10 pounds of roofing materials per square foot. This assumed weight is well within the realm of new construction practices, but application of additional layers of roofing over the lifetime of a dwelling may exceed dead weight assumptions and reduce snow load capabilities. To estimate your roof's dead weight, find how many layers of materials have been added to the original roof. If the combination of roofing materials on your roof exceeds the assumed 10 pounds of dead load, your roof's snow load capabilities are reduced by an equal number of pounds per square foot.
Measuring roof snow and ice weight
It is not the depth, but the weight of accumulated snow/ice moisture content that are critical in assessing your roof's vulnerability. Water content of snow may range from 3% for a very dry snow to 20% for compacted snow to nearly 100% for ice. Water per inch of depth weighs 5.2 pounds per square foot. Thus a modern roof designed to carry a snow load of 40 pounds per horizontal square foot is designed to support an equivalent of approximately 3' 6" of compacted snow.
