Over the past six months, we have seen more than one severe wind event. These events have caused damage to residential and commercial properties and roofs are no exception. Missing or damaged shingles are sometime expected – but commercial roofs? That only happens during a hurricane – right?
Wrong, Colorado Springs lost a half dozen or more commercial, low sloped roofs in late 2005 due to wind. Most of these were not poorly constructed or poorly designed roof system. All were designed by an architect or a roof consultant but due to the conditions of the day these roofs failed.
I encountered three misconceptions about roof losses. The first is the term “blow off”. Low sloped roofs (asphalt-built up, EPDM, TPO systems) typically do not blow off but fail from negative pressure at the perimeter. Similar to the lift created by an airplane wing, negative pressure forms as the wind meets a building its velocity increases as it flows over the vertical surface, and rolls over the parapet. Directly behind the parapet wall (edge), the negative pressure pulls on the roof membrane and flashing.
The second misconception: Isn’t this covered by the roof warranty. Not typically. Most manufacturers’ warranties exclude winds over 54 miles per hour (or gale force winds). While the roof may be designed for heavy winds, your roof warranty does not cover “acts of God” meaning natural disaster. I tell most of my customers “always remember; their lawyers wrote the warranty”. This loss should be covered by your insurance provider.
Over the past decade several manufacturers have developed Increased Wind Speed Warranties. Prior to installation, the roof system must be designed to specific specifications approved by the roof membrane manufacturer. [Note: All four of the roof systems installed by Weathercraft Company after the wind failures were designed, installed, and warranted for 90 mph or more.] The design takes into consideration the building height, parapet height, deck type, area wind speeds and other design factors.
The last misconception is that concerning Factory Mutual FM I-90. Factory Mutual is an insurance company that tests building component for its insured customers. Many designers reference the FM I-90 in their specifications. Until hurricane Andrew, I-90 was the highest wind uplift design criteria at Factory Mutual; but the nomenclature has become standard in many specifications. While I-90 is an adequate design for most of this area of the country, the 90 does not stand for 90 mile per hour but for 90 lbs of uplift pressure (negative pressure).
Finally, we must remember that the roofs lost were only a small portion of the square footage installed in Colorado Springs. Most – performed as they were designed. My recommendation is to develop a relationship with your roofing contractor and set up a plan for disaster response plan. Whether the roof is “blowing off” or snow threatens to over load the structure, a good roofing contractor should be there when you need assistance.
John L. Fleming, Jr.