Aurora Roofing: Article About Understanding Roof Load
Roofs are under a great deal of pressure at all times and are designed to support and distribute that weight in a way that allows it to remain stable. This is a delicate balance, and if that balance is disturbed, then major damage that requires the attention of an Aurora roofing professional can occur. In order to remain in place and intact, a roof must support permanent load, but it must also be able to account for temporary loads, such as accumulating snow or contractors walking on it.
The dead load is the permanent load on a roof. The dead load not only includes the weight of the framing and the pressure generated by the design, but also any permanently attached items, including sheathing, underlayment, shingles and roof protrusions. Load on a roof is measured in pounds per square foot or psf. A typical residential roof framed with wood and covered by standard asphalt shingles has a dead load of approximately 15 psf. A roof with clay tiles can have a dead load twice that amount.
The live load is the temporary load on a roof. Temporary loads are based on reasonable expectations and are usually defined in local building codes. Snow and workers are the two most important factors that live load account for, and therefore, roofs in the south tend to be designed for a lower live load than those in the north.
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When a roof is designed, it is planned based on the dead load plus the live load plus any necessary slope correction due to the angle of the roof.
There is also a kind of load called uplift load, which is generated when wind energy disperses. Wind with a speed of 90 mph can exert upward forces on a roof as high as 20 psf. Generally, the dead load of a roof, which pushes downward, is enough to counteract this. However, in areas that regularly experience high winds, the local building code may call for added support or for specific roof features that better displace the wind energy.
Roof load matters because it determines what homeowners can and cannot do with their roofs without a major structural renovation. If a homeowner wants to install solar panels, then load must be considered because the solar panels will permanently increase dead load and permanently decrease live load. Likewise, slate roofs often have a psf that is more than twice as much as an asphalt roof, which is why slate roofs often require structural upgrades prior to installation.