The Answer to Blowing in the Wind
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. It is still the costliest natural disaster the country has ever experienced.
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 StudiesHurricane 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.”
SPF Stands AloneIn 1990 a tornado ripped through Plainfield, Ill., killing approximately 30 people. When Smith arrived on the scene to conduct his field study of the damage, he found a church left standing while all around it had been levelled. The concrete building had been completely covered with SPF.
“The SPF had been pretty beaten up by debris and was plastered with missiles, but there was no place where the foam had blown off,” he says. “There was a square foot gouge cut out of the foam in one spot, but it hadn’t gone all the way through to cause a leak, and the foam did not peel off.”
Failure of metal edge flashing is the most common failure mode for membrane roof systems. Wind blow-off is typically initiated by the metal edge flashings not being strong enough to resist the load, causing the membrane to lift and peel back. Smith reports that based on his investigations, SPF roofs are not as susceptible to this failure mode. He says he believes the foam acts as a stiffener along the horizontal edge of the flashing when the vertical flange starts to lift and roll up, offering less of a tendency to lift.
“I have never observed a situation where the edge flashing lifted and peeled and took the foam with it. All of the uplift failures I’ve observed with foam have been where we’ve had the underlying substrate go,” he says. “So I think if we did not have an existing membrane and were applying SPF to a deck with an edge flashing, we’d want to attach the flashing with great care, but I don’t think it’s as likely to have an edge-flashing-induced failure with foam as with other membrane types.”
Trouble in ParadiseTom Camp has been a roofing contractor serving the state of Florida for more than 20 years. He’s seen the damage a hurricane can do to a roof. After Hurricane Erin struck in 1995, his company, Tech Systems Inc., received a call from the Paradise Beach Club condominiums in Satellite Beach. The 75-mph winds of the Category 1 hurricane had nearly ripped the 20-year-old BUR off the 48-unit oceanfront complex.
Forced to choose a replacement, the club managers also sought to solve their history of leak problems and poor energy efficiency. The answer was SPF, with its high wind uplift capabilities, seamless surface, high insulation R-value and quick installation.
“With pressing concerns for the comfort and safety of the tenants, speed of installation was almost as important as water tightness and energy efficiency,” says Camp. “We were able to provide them with an initial leak-resistant roof stage in just a couple of days.” Camp reports that over the years, the SPF roof at Paradise Beach has survived at least three subsequent hurricanes with no damage and no leaks.
Urethane Adhesives with Single-plyOver the past eight years, Peter Monterose, owner of the architectural firm McDonald and Monterose, has specified half a million square feet of plural-component urethane adhesive with a single-ply membrane on roofs in the Hudson Valley, the Catskills and Mohawk Valley. “We get some pretty strong winds up here, but we’ve never had a problem,” he says.
Urethane adhesives with single-ply membrane cover systems have undergone FM Global and Underwriters Laboratory testing for wind uplift and exhibit similar uplift qualities to SPF. The urethane adhesive makes the system monolithic and seamless, reducing the chance of lifting and peeling during high wind events because there are no edges for the wind to grab. It also provides improved waterproofing.
Monterose says the SPF system allows for faster installation over more traditional systems like BUR. It is lightweight and eliminates the need for mechanical fasteners for insulation, resulting in improved puncture resistance.
“We had one place where the owner went up on the roof with snow blowers to remove some drifts and there was no damage to the membrane,” says Monterose. “The same owner went across the field to another school with a fully-adhered EPDM with fasteners holding the insulation down. The snow blower caught the fasteners and ripped a couple hundred holes in the roof.”
“I wouldn’t have any hesitation using it in a hurricane zone, provided it was suitable for the building and the budget,” he says.
Protecting Essential FacilitiesWith the financial toll of hurricanes being so high, not to mention the accompanying human suffering, Smith says he would like to see special effort applied to the design of roofs on essential facilities such as hospitals, police and fire stations and buildings used as evacuation shelters. Current model building codes in the United States don’t address the issue of wind born debris striking the roof membrane, although Dade County and the International Building Code now have a flying-missile requirement for protecting exterior glazing.
“Essential facilities are required to be designed for higher loads, but that’s just wind uplift,” he says. “There’s no recognition in the codes about wind-born debris. That’s something for designers to be aware of, because you can end up with a common situation where the roof is intact, it didn’t blow off, but it’s riddled with holes and it leaks. I’ve seen hospitals get enough water in that after the hurricane passed they had to evacuate the building, and that’s something you don’t want to have to do. For essential facilities in a hurricane prone region, spray foam is a very good option.”
SPF may also provide peace of mind for building owners, contractors and insurers after the storm. “Even after taking a lot of debris, the gouged foam can remain without repairs for months without causing serious problems,” says Smith. “The building owner doesn’t have to worry about getting someone up there to effect repairs immediately. After a major hurricane there are so many damaged roofs that getting someone to do repair and replacement is a real challenge. There are a limited number of workers and a great amount of work to be done. A roof that can just sit there in a somewhat damaged state but not leak and cause problems is a real attribute. “
One leading SPF supplier reports that when applied direct to decks, its formulation meets the FM Global Class 1-150 Windstorm classification and FM severe hail tests. These lab results, along with Smith’s field documentation, show SPF’s performance during severe wind events and the potential for damage reduction when the next big hurricane hits.
“Give me the right deck, with sprayed-in-place polyurethane foam and coating, and I can survive a typhoon,” says Fricklas.
Sidebar: Understanding Wind UpliftThe performance of any roofing system during a wind event such as a hurricane, tornado or tropical storm depends on its wind uplift resistance.
Richard Fricklas, founding father and former technical director of the Roofing Industry Educational Institute, stresses the importance to understanding the effects of wind on a roof.
“Mathematical models are used to convert wind speed into an uplift force. In high school physics, we learn that kinetic energy is equal to one half of mass times velocity squared. The energy is a function of the mass (density of air) and the square of the wind speed. So as the wind blows harder, the velocity gets greater, you’re squaring it, so the uplift force becomes greater very quickly,” he says. “When I’m teaching wind to my classes, I state that a Volkswagen going 30 mph will do less damage to a telephone pole than an SUV going 90 mph. It’s the velocity that makes high winds do so much damage because its energy is being squared.”
Wind maps project the highest wind speed a designer might expect in a given geographic area. For example, at an airport or adjacent to a large body of water, the design wind speed could be 90 mph. Designers and architects then pick a probability of getting a wind speed that high. For a very important building, such as a nuclear power plant, this expectancy factor or recurrence interval might be 1,000 years. For less important buildings, the design wind speed is adjusted downward.
The adjusted wind speed is then multiplied by a pressure coefficient for the specific portion of the roof: generally minus 3 at the corners where the uplift is greatest, minus 2 at the perimeter and minus 1 for the body of the roof. There are a number of variables, such as roof height and roof slope, that can affect these coefficients.
“We use a constant for air density. When you multiply the units out, density times the velocity, we now have the expected uplift forces in pounds per square foot. Once we have that number and we multiply it by 1, 2 or 3, the designer is ready to start looking at the best way to put the roof together,” says Fricklas.
Independent testing organizations, such as Underwriters Laboratories and
FM Global, test various roofing systems for wind uplift performance and then publish the results in their directories. Designers can then look for systems that meet the UL designations Class 30, Class 60 and Class 90 or FM1-60, FM1-90 or higher where needed.
Mason Knowles, technical director of the Spray Polyurethane Foam Alliance, reports that during laboratory testing of SPF systems, SPF’s wind uplift resistance exceeded the capacity of UL’s equipment. UL also observed that SPF roofs applied over BUR and metal increased the wind uplift resistance of those roof coverings. He says FM Global’s testing showed similar results over concrete, metal and wood.