Planning a roofing replacement incorporates several factors, some of them you’re probably already familiar with: slope, material and local climate. In this blog, the roofing contractors at Halo Roofing Inc. share a lesser-known but nevertheless important factor: roofing loads.
What Are Roofing Loads?
Roofing loads are forces that exert pressure on the roof. The higher the load limit, the better the roof can resist such forces. A good roof design incorporates anticipated roofing loads and balances other aspects such as roof design and material selection. If the roofing load is calculated correctly, the roof should be able to withstand most conditions throughout its life span.
Anticipated Roofing Loads
The following are some commonly anticipated roofing loads:
Dead Load — This is the downward force exerted by the weight of the roof, along with structures like chimneys, dormers and skylights. An average asphalt shingle roof dead load typically exerts 15 pounds per square foot (psf). Heavier roofing systems like clay tile could exert as much as 27 psf.
Live Loads — Live loads, on the other hand, are forces exerted by temporary objects on the roof. This includes rain and snow, as well as roofers if they happen to be working on the roof. The roof should be able to support the sum of live and dead loads. A typical anticipated live load is 20 psf. Using our earlier examples, the anticipated combined live and dead loads for asphalt shingle and clay tile roofs would be 35 and 47 psf, respectively.
Uplift Load — When high winds hit the side of the building, part of it is dispersed upward, exerting an uplift load. Miscalculating this can result in the roof getting torn off during a storm. The standard load limit is about 90 miles per hour, with an anticipated load of about 20 psf. The force exerted by the dead load provides resistance against uplift.
Slope Correction — Changing the roof slope can change the amount of downward force it would exert. A steeply-sloped roof would exert more downward force, compared to a gently-sloping roof with the same material over the same area. The slope can be adjusted in relation to anticipated loads.