The Roofing System That Can Stand Up to High-Wind Disasters
"The application of Sprayed foam to steel deck and plywood deck demonstrated uplift load resistance up to the capacity of the test equipment to develop load (160-165 psf) without any sign of elimination or other damage to the foam." - Underwriters Laboratories, Inc.
Sprayed polyurethane foam roof resists wind uplift
These two buildings in Puerto Rico are located just a few yards apart, so they both received the same degree of wind force when Hurricane Hugo stormed over the island. The building on the left received major damage as wind uplift ripped off a portion of the roof. The building on the right has a sprayed polyurethane foam roof which held tight. There was no peeling, penetration or water leakage. Polyurethane foam is self-flashing--it forms a seal that grips the building wall and roof-mounted equipment so there are no edges for the wind to lift up and peel away.
This church survived the tornado that cut a path of destruction through Plainfield, Illinois. The building was completely covered in sprayed polyurethane foam. There was repairable missile damage to the exterior surface, yet the sealed polyurethane foam helped to strengthen the building and resist uplift from the high-force winds. Because polyurethane foam is spray-applied, it can conform to any shape and provide a sealed envelope of protection.
SPF vs Hurricane Andrew
SPF roofing holds it together during hurricane-force storms
Ten years ago, Hurricane Andrew wrought unprecedented economic devastation on the northwestern Bahamas, the southern Florida peninsula and south central Louisiana. According to the National Hurricane Center, the hurricane caused more than $26 billion damage in the United States. Before Sept. 11, Hurricane Andrew was the most expensive disaster in U.S. history.
The insurance industry identifies roofing as the primary contributor to disaster-related insured losses. The roof and exterior glass are the most vulnerable parts of the building envelope in any wind event. Because a damaged roof can expose the building’s interior — and its inhabitants — to the storm’s wrath, the total cost of a roof blow-off can rise as quickly as the storm’s own momentum.
Since Andrew’s devastation, many materials manufacturers and members of the roofing industry, along with members of the insurance industry, code officials, architects and consultants, have invested countless dollars and hours identifying ways to mitigate the damage caused by roof failure during wind events.
The reason for roof failure can often be found in the very design of membrane roof systems. Wind often grabs the edge flashing or coping and peels back portions of the membrane.
In comparison, spray polyurethane foam has gained recognition with industry experts for its ability to withstand high wind uplift and blow-off because its smooth, continuous surface grips the deck and walls. It offers superior adhesion with no need for fasteners and there are no joints or edges for the wind to grab onto. Lightweight yet rigid, it provides extra strength to help the roof stand up to the forces of nature.
“SPF has tenacious adhesion and there’s no way it will ever blow off,” says Richard Fricklas, founding father and former technical director of the Roofing Industry Educational Institute. “It sticks to anything with very high pull-off strength. The minimum strength of SPF would be about 10 pounds per square inch and most blow-offs concern pounds per square foot. Even if the blow-off strength was only 1 pound per square inch, by the time you multiply it by 144 square inches in a square foot, that would still be 144 pounds per square foot of wind uplift resistance.”
Field Studies
Hurricane Andrew struck southern Dade County, Fla., especially hard, with violent winds characteristic of a Category 4 hurricane on the Saffir/Simpson Hurricane Scale. In Dade County alone, the forces of Andrew resulted in 15 deaths and up to 250,000 people were left temporarily homeless. An additional 25 lives were lost in Dade County from the indirect effects of Andrew.
When it was over, Thomas L. Smith of TLSmith Consulting, and then research director for the National Roofing Contractors Association, went to Florida to see first-hand how SPF had weathered the storm.
“Two things stood out in my mind after my field studies — one was SPF’s adhesion and the other its ability to accommodate wind-born debris or ‘missiles,’” Smith reports. “A missile will tear into or gouge out the foam, but the roof will not leak. Typically there are a lot of missiles flying around during a hurricane, so that’s a significant advantage.”
In his published report of his findings post-Andrew, Smith provides a detailed assessment of the wind performance of 11 SPF roofs. Three of the buildings were in areas of very high winds, one in an area of high winds and seven in areas of moderately high winds.
Two of the three roofs in the very high wind zones were SPF over old BUR; the third was SPF over thin-shell concrete. Two of the three roofs sustained minor damage from missiles. One of the SPF-over-BUR roofs experienced peeling that did not progress beyond an area of missile impact.
“Often foam is applied over existing roof coverings and I did see a number of buildings in South Florida where this was the case,” says Smith. “If the underlying roof is weak and it lifts up, it will take the SPF with it. But it appeared to me, although it’s difficult to quantify, that the foam acted as a stiffener, so when the original roof lifted up the failure did not propagate as far into the field of the roof. You might have a corner peeled back, but the corner area would be limited because of the stiffening influence of the foam. Had the foam not been applied there, I think the roof failure would’ve been much larger. I feel very strongly about that, but it is very hard to quantify.”
Other buildings with traditional roofing systems in a 200-foot radius surrounding the SPF-over-BUR roof that peeled suffered significant damage, including gable-end wall failure and collapsed trusses, as well as blown-off sheathing panels and asphalt shingles. One building had reportedly had its BUR blown off.
Smith says that if SPF is going to fail during a wind event, it is because the surface it has been applied to has failed. “Typically, the foam is not going to lift unless whatever it is sprayed to lifts,” he says. “If the deck itself is not adequately attached, everything above it could just come off the supporting joists or beams, so that would be one failure mode. It could also fail if an element between the deck and the foam doesn’t have good adhesion or if the SPF is applied to an existing membrane that is not adequately attached.”
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