Sep 2010

Hurricanes and Severe Windstorms

Hurricane season runs from June 1 to November 30 in the Northern Hemisphere. Windstorm is the second highest cause of property loss after fire. Recent severe windstorms have proven that buildings and roofs designed to proper standards are capable of withstanding these storms with normal damage. Loss experience has shown buildings that are not designed to these same standards sustained significant to heavy damage.

Hurricanes and cyclones are categorized based on the maximum wind speeds achieved by the given storm. The hurricane categories are as follows:


Wind Speed


74 to 95 mph

(120 to 153 km/h)


96 to 110 mph

(154 to 177 km/h)


111 to 130 mph

(179 to 209 km/h)


131 to 155 mph

(211 to 249 km/h)


> 155 mph

(249 km/h)

Tornadoes are similar to severe squalls but have a vortex that can have internal wind speeds of up to 300 mph. They can be extremely destructive due to the very high negative pressures created within the rotating vortex. The land area affected by a tornado which touches ground can range from several hundred feet to upwards of over 100 miles in length, but typically covers a width of 200 feet to a mile and a length of 2 to 20 miles.

The wind uplift forces on buildings and roofs vary, depending on many factors, including wind speed, building height, surrounding ground terrain, roof deck type, presence of a parapet, roof slope, and wall openings. All of these factors should be considered when choosing or designing a roofing system.

The majority of roof covering failures resulting from windstorm involves improperly designed or constructed perimeter flashing. Recent severe windstorms have confirmed that perimeter flashing is arguably the most important feature of a wind resistant roof. If flashing fails, a sizable amount of roof covering will likely be lost, regardless of the adequacy of securement of the roof cover. If roof cover securement is also inadequate, the expected loss is even greater. Fortunately, flashing securement is among the least costly of improvements when trying to upgrade the windstorm resistance of your facility.

For most windstorms, your country’s weather service (National Weather Service or NWS in the United States) provides an advanced warning to those areas likely to be in the path of the storm. The NWS issues a hurricane watch when winds of 74 mph or greater pose a possible threat within 36 hours. A hurricane warning means that hurricane conditions are expected in 24 hours. These warning states vary from country to country. This advance warning should be used to begin taking emergency actions in accordance with your site’s formal emergency plan.

If your facility does not have a formal windstorm emergency action plan or if you would like an analysis of the level of preparedness and protection at your site, you may contact Risk Logic Inc. for a professional consultation. Risk Logic can also review plans and drawings for new construction and roofing systems to ensure they are properly designed with regard to windstorm resistance. These are services offered to all locations under contract in Risk Logic‘s property loss control program.