1:5:10:365 EcoTip Blog

April 11, 2008

:102 Roof Slope

Filed under: :102 Roof Slope — Tags: , , , , , , , , , , , , — John Banta @ 12:01 am

Welcome to today’s 1:5:10:365 Tip for becoming a better steward for our home and planet.

1:5:10:102 EcoTip: Roof slope makes a great deal of difference in the uplift force exerted as wind flows over the top. The steeper the slope the less chance the roof will fly away.

Credit: Simpson-StrongTie Co., Inc.

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Additional Information

Suggested Review – :098, :099, :100, :101

 The following excerpt is from my book – Extreme Weather Hits Home:

The same forces that allow airplanes to fly are exerted on roofs exposed to strong winds. The angle of your roof can make a great difference to whether your roof will stay intact or sail away like the wings of a plane. Research conducted by Clemson University at their wind load test facility has demonstrated that a category 5 hurricane can develop uplift as much as 100 pounds per square foot. This is typically enough to lift a house right off its foundation. The lifting force is greatest for flat or low-slope roofs. As the angle of the roof increases the force drops. A roof slope between 4.5 and 6.5 seems to be the least aerodynamic. A gable roof will act more like the wings of a plane than a hip roof which slopes down on all sides, helping to cancel lift forces from all directions (Clemson).

The lifting force of wind is tremendous. When airplanes taxi for takeoff they achieve speeds in the 50 to 150 mph range, which is about the same as those speeds found in damaging winds. The roof of your home has a much greater lifting surface area than most airplane wings, allowing the entire roof to be lifted off the house. Once the roof is gone the walls will easily collapse. The lifting force for a roof — just like an airplane wing — is caused by the Bernoulli effect of air passing over the top of the roof creating lift. Some people have advised keeping your windows open to help neutralize the interior building pressure but this doesn’t really work and will let in lots of water causing additional damage to the interior. The best way to keep homes intact when exposed to strong winds is to keep them in one piece and securely fixed to the foundation.

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