Mastering Roof Inspections: Wind Damage, Part 1

by Kenton Shepard and Nick Gromicko

 

 

The purpose of the series “Mastering Roof Inspections” is to teach home inspectors, as well as insurance and roofing professionals, how to recognize proper and improper conditions while inspecting steep-slope, residential roofs. This series covers roof framing, roofing materials, the attic, and the conditions that affect the roofing materials and components, including wind and hail.

 

 
 
Courtesy of Mike Hollingshead
 
 
We know that hail is associated with storms. Usually, storms that drop hail also bring wind, as low-pressure fronts move in and out of the area. You may see damage from either one during an inspection.
 

WHAT CAUSES WIND?

In talking about wind damage, we should first cover some basics.

Wind is air moving from areas of high air pressure to areas of low air pressure. The greater the difference in air pressure between two points on Earth, the faster the air will move between them.
 
 

 
These differences in atmospheric pressure are created by uneven heating of the earth’s surface. Because of the earth’s shape and orientation to the sun, more warming takes place along the equator than at the poles. The different rates at which heat is absorbed and released by land, water and the atmosphere itself also contribute to thermal differences.
 
 

As air in warmer areas rises, cooler air moves in to replace it. It’s this replacement air that we experience as wind.  The force that pulls the replacement air is called “convection.” Convection can cause both updrafts and downdrafts, which create wind with different characteristics, including varying:

  • direction;
  • strength;
  • duration; and 
  • speed.

Hurricanes

This process happens on both large and small scales. At the large end of the scale are hurricanes in which the area of strong winds might be between 25 and 150 miles wide. Hurricanes are classified according to wind speed:

  • Category 1 = winds 74 to 95 miles per hour;
  • Category 2 = winds 96 to 110 miles per hour;
  • Category 3 = winds 111 to 130 miles per hour;
  • Category 4 = winds 131 to 155 miles per hour; and 
  • Category 5 = winds greater than 155 miles per hour.

Tornadoes

Courtesy of Mike Hollingshead
 
An example of much smaller but equally intense storms is tornadoes, which can have wind speeds of over 200 miles per hour, although wind speeds of less than 90 miles per hour are much more common.  Tornadoes are also classified according to wind speed, as follows:
  • F0 = winds 65 to 85 miles per hour      (53.5% of all tornadoes);
  • F1 = winds 86 to 110 miles per hour    (31.6% of all tornadoes);
  • F2 = winds 111 to 135 miles per hour  (10.7% of all tornadoes);
  • F3 = winds 136 to 165 miles per hour  (3.4% of all tornadoes);
  • F4 = winds 166 to 200 miles per hour  (0.7% of all tornadoes); and 
  • F5 = winds greater than 200 miles per hour  (less than 0.1% of all tornadoes).

Many other convection-related wind events occur both with and without storms. Another condition that creates winds which can damage roofs is large-scale weather patterns that produce a strong, deep flow of air which passes over a mountain chain.

Forced up out of its natural path by the peaks, the wind seeks to return to its original level.  But before settling back into a horizontal flow across the plains, it oscillates through several up-and-down cycles. As you can see in this illustration, these winds will actually bounce several times, and they can cause damage in certain areas with some predictability. If you’re inspecting in an area that fits this description, be looking for wind damage.  These winds can slam into the ground at over 100 miles per hour.

You can sometimes spot these areas even when the wind is not blowing.

Over many thousands of years, the area near the base of the foothills where the wind first bounced has developed a trough, with soil scooped out by the wind.

This is often more pronounced below geological features which channel wind, such as canyons.

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Learn how to master a roof inspection from beginning to end by reading the entire InterNACHI series: Mastering Roof Inspections.

 

 InspectorSeek.com


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