Asphalt Roofing Capacity

Loads

• The load capacity for asphalt roofs is generally determined based on the live load capacity and the dead load capacity. The dead load capacity is the actual weight of the building materials. This is normally an assumed weight that is based on industry standards. Builders are not required to calculate the dead load weight of each roof they build. The live load capacity, on the other hand, refers to the usage weight of the roof. Live loads are more important for floors, but still factor into a builder's calculations because workers must be present on top of the roof from time to time.

Dead Load

• The dead load capacity for an asphalt roof is assumed to be 10 pounds per square foot. This means that the maximum weight of the roofing materials, which includes the asphalt shingles, plywood and underlayment does not generally exceed this figure. In most cases, this assumed dead load is almost always more than the actual weight load of the roof. For this reason, builders do not typically have to make this calculation when starting on a roof.

Live Load

• The live load capacity of the roof has to be carefully considered and is affected by the spacing of ceiling joists. Each person thats working on the roof increases the amount of the load exerted. Snowfall and ice accumulation affect the live load significantly. Fresh snow exerts an additional pressure of 8 pounds per foot on a roof, while ice increases it dramatically to 56 pounds per foot.

Deflection

• The force of a load exerted on a roof is often deflected by the way that the roof is constructed. Roof loads are usually deflect to the outer walls of the house via the underlying rafters and trusses. This means that the buildup of snow and ice does not generally exert a direct downward perpendicular pressure on the roof. The pressure is actually exerted diagonally along the parallel path of the roof.