Hurricane-Proof Your Roof
By BOM Editorial Staff September 2007 - Roofing
The way a facilities executive protects a building from the weather depends on where it is located. In Arizona, owners worry about reflectance ratings for a roof or about optimizing HVAC loads. And at the eastern end of Lake Erie, in Buffalo, massive snow loads can cause concern.
But in other parts of the country — the Gulf States or southeastern seaboard in particular — the eyes of facilities executives are focused on the ocean. This year’s hurricane season is in full swing, and the impact of Ivan, Andrew, Katrina and Rita is not easily forgotten.
Walt Rossiter, director of technical services for RCI, Inc., describes a school building he investigated in southern Florida following hurricane Andrew. At the time, Rossiter was part of a team sent by the National Institute of Standards and Technology.
“The school was of masonry construction and built like a fortress,” he said. “It was built around an interior courtyard, and most of the building’s windows faced this courtyard — there were very few on the exterior walls. But the roof ended up in the parking lot. I believe that, if the roof had stayed on the building, the school would have passed the storm largely unscathed.”
Rossiter’s story highlights a crucial fact about roofing performance during big storms: A roof system’s ability to withstand gale-force winds often determines how well the rest of the facility survives.
What Can Go Wrong?
Imagine a hurricane has hit and now the facility’s roof is in the parking lot. What caused it to fail?
Typically, there is no quick answer. There are as many ways for roofs to fail as there are roofing systems. But when roofing systems fail, they usually do so at areas most susceptible to uplift: the perimeter, corners, along seams, and at rooftop accessories, such as vents, satellite dishes, or chiller units.
David Hawn, president of Dedicated Roof and Hydro-Solutions, says that weak areas of attachment are the leading cause of roof damage. “Recently, a high wind event we investigated revealed damage and failure due to a gutter edge,” he says. “The gutter and components above it were blown back onto the roof along with four feet of roofing at the edge.”
Even though the gutter was attached properly to the wood blocking, the blocking attached to the structure itself had failed. Further investigation revealed that the blocking had been attached at spacing greater than 12 inches using nails.
“The blocking was most likely installed by another trade and not confirmed to be attached to meet the same uplift resistance as the roof, roof edge and gutter,” Hawn says. The good news for the affected facility was that a perimeter attachment strip of the roofing system stopped the wind damage from migrating further than four feet from the gutter edge.
Hawn says he believes that most wind damage can be avoided given proper care and diligence during the design process. Most roofing systems, when properly designed, can withstand high-wind events and even hurricane-force storms.
Jon Macias, president of Construction Moisture Consulting, agrees. “Such mishaps can often be avoided if an experienced design professional, such as a licensed structural engineer, provides the necessary wind uplift design pressures for all roof levels on the building.”
Macias cites minimum standards from which to work, such as the American Society of Civil Engineers (ASCE) 7-02. For more stringent requirements than ASCE 7, both Macias and Rossiter suggest using the Miami-Dade County building codes as a starting point in design for facilities in hurricane-prone or high-wind areas. If local codes aren’t adequate, other sources should be consulted.
“If there is any question, err on the side of conservative design and construction,” says Rossiter.
Hawn says designers have to be diligent when accounting for all the ways a roof can fail. “There seems to be a lack of understanding concerning how to calculate and know the forces that should be expected based upon published history of wind,” he says. “The force of wind can be calculated, provisions can be included to resist those wind forces, and most wind damage can be avoided with reasonable care in the design process.”
For example, designers need to account for the influence of openings in a building. During hurricane conditions, if winds breach doors and windows, it can create tremendous positive pressure from within the building itself.
Roof consultants also warn against careless placement or maintenance of rooftop mechanical equipment. A great deal of damage is done — particularly to membrane roofing — when filters, screens, or other objects with sharp metal edges are blown free from rooftop units and cartwheel across a rooftop, puncturing the roof system as they go.
Hawn suggests offering protection from wind for rooftop units, or removing them from the rooftop and placing them on the ground or inside the facility mechanical room or penthouse.
Re-roofing projects can also cause grief. “Inadequate due diligence by a designer on re-roofing projects can result in unanticipated problems, such as deterioration of existing roof substrates, wood nailers or blocking, and wet roof insulation,” says Macias. “If not properly dealt with during the design and renovation process, any of these will have a detrimental effect on the overall integrity of the roof replacement system.”
Roof consultants suggest using an engineer to help with the design of a roofing system and warn against simply trusting various wind-rating standards. Furthermore, facilities executives should consider the entire attachment path.
“If the roof or edge is attached well, but to an item that is not attached to the same or greater resistance for force, failure simply occurs at a lower level of construction,” Hawn says. “Not including a requirement for a review of roof and accessory installation is another mistake.”
Macias also recommends rooftop designs that can help break the effect of wind, such as parapet walls or use of air barriers within the roofing system to avoid excessive air leakage.
Costly Installation Mistakes
Just as important as proper design is adequate preparation of the substrate to which the assembly is attached.
Installation practices that disregard the project specification and established best practices for avoiding wind damage are a major problem, says Hawn. “Every building and site is unique. Simply doing the same thing done on the last job may not be sufficient.”
Improper installation of fasteners for perimeter wood nailers or blocking as well as inadequate attachment of insulation, roof membrane and sheet metal flashing can all cause system failure.
Macias cites a list of improper installation conditions: fasteners improperly spaced, particularly if too far apart; adhesives used for securement which have been allowed to prematurely dry or harden before materials, such as roof insulation, are set into place; installing roofs under wet conditions, which weakens the insulation boards; improper mixing of lightweight insulating concrete; and inadequate attachment of the roof deck itself.
Deciphering wind ratings and deciding how to apply them can be difficult. Because of the variables involved, roof consultants say that the entire design of the roofing system should be addressed via either ASCE 7 or a local code if it is more stringent. Alternately, facilities executives can rely upon Factory Mutual (FM) standards, if the building is insured by a carrier underwritten by FM.
Hawn says he often sees the only reference to roof attachment as FM-1-60 or FM-1-90. But he cautions against relying on these standards without thoroughly understanding them. Some specifiers mistakenly believe that FM-1-60 or FM-1-90 refer to the ability of roofing system components to withstand wind speeds. “This is a major shortcoming and indicates a total lack of understanding concerning roof attachment,” says Hawn.
Here’s why: The FM-1-60 designation of FM indicates a pressure of 60 pounds per square foot. With safety factor removed (safety factor is an engineer’s metric for additional design strength above the theoretical limit), that is actually 30 pounds per square foot. While wind speed is a basis of calculation of rooftop uplift pressure, other considerations need to be applied prior to calculating the actual uplift forces resulting from a published wind speed for a specific building and its location. The “60” or “90” in the designation does not indicate wind speed.
Macias agrees investigation is called for when facilities executives select new roofing. “Facilities executives should be cautious if they are simply told a roof has a ‘wind resistance’ rating without specific data,” he says. “Pressures on buildings vary based on the building’s exposure category, topographic effects of surrounding terrain, for example.”
Design ratings also vary depending upon whether or not the building is enclosed, partially enclosed, or open, as defined by ASCE 7.
If A Hurricane Hits
The perfect storm is coming. Now what? “You pray,” says Macias. All joking aside, a roof should be adequately secured well ahead of a hurricane’s arrival. Nevertheless, some last-minute checks should be made on the rooftop. Act before hurricane season, because vital resources often become scarce. Wind uplift from within a building can cause blow-off, so make sure supplies, such as plastic or plywood, are on hand to cover openings before the hurricane hits.
Also before hurricane season, make a thorough review of the roof. Roof defects should be repaired, and all rooftop equipment should be reviewed to ensure there are no loose parts. Make sure that the equipment is functioning properly and well attached to the structure.
In particular, facilities staff should physically check equipment screens, access doors, exhaust fans and roof drainage components. Make sure that removable panels are tightly secured to equipment. Clear paths so as not to impede water flow. Loose or compromised material should be taken off the roof.
As much as possible, facilities executives should review the area surrounding the building. “If you see a dead tree limb or a loose sign near your roof or on a neighboring building,” says Hawn, “do whatever you can to remove those windblown hazards before the storm season approaches.”
Taking corrective action before the season can reduce a facility’s susceptibility to storms — and help ensure supplies are on hand and experts available to assist organizations. Finally, consult a professional if a facility has sustained wind damage, as there might be more damage than initially appears.
In addition, consultants suggest that facilities executives should avoid holding down a wind-damaged roof membrane with a few bags of sand or old tires. Hawn says he remembers one case in which temporary fixes like those caused even more damage during a subsequent storm. The reason: One 40-pound sand bag placed in 5-foot intervals translates to 1.6 pounds per square foot (psf) of uplift resistance. At 1.6 psf, a high wind could move those sand bags or could even flip them over the edge of the roof.
And use common sense. Never go on a roof during a high wind event or hurricane even if preventing further roof damage is possible.
Construction is a human process prone to errors and shortcomings. Designers can miscalculate, and workers sometimes use the wrong fasteners or space them less frequently than required. Additional layers cover substrates or attachment devices and hide them from view. When a roof fails — whatever the reason — facilities executives usually discover mistakes after the failure. To help protect an organization against roof failures in a hurricane, learn more about local building codes, read the Miami-Dade County standards, consult with structural engineers, and have knowledgeable roofing consultants review the project. In short, Macias says, requiring a quality assurance review in the specification can help avoid failures.
Significant advances concerning wind and roofing have been made in the last few years. Several new standards are being formulated to address wind and storm damage issues. For example, codes have been upgraded with new standards. But it is important to remember that codes state the minimum acceptable standard of construction to protect the public. Because unique sites or buildings might require more than the minimum acceptable approach to roof attachment, get as much help as possible.
Robert Elsdon, with the Inter-Provincial Roof Consultants in British Columbia, doesn’t see hurricanes in northwest Canada. But he suggests that facilities executives tap the technical expertise available from industry organizations, such as the Roofing Industry Committee on Weather Issues.
He also suggests studying the building standards published by Miami-Dade.
The Federal Emergency Management Agency provides disaster recovery information.
And finally, the strong hurricanes of the last several seasons have led to more wind-related education. Classes are available through RCI, Inc.
About RCI, Inc.
RCI, Inc. is an international association of professionals who specialize in the evaluation, specification and design of roofing, waterproofing and building envelope systems.
Since 1983, RCI members have offered unbiased design, repair planning, quality observance, legal testimony, and general roof management services. In March 2006, RCI (then known as Roof Consultants Institute) expanded its mission and service scope to include waterproofing and other aspects of the building envelope. This action reflected the understanding that roofing issues often influence the entire building envelope and in increasing frequency, RCI members are called upon to address building-wide concerns from below-grade to rooftop.
RCI Professional members adhere to a strict ethical code that promotes unprejudiced service without affiliation with any product or manufacturer.
The organization’s current membership numbers more than 2,500 and includes an international constituency residing in all 50 U.S. States, Canada, Mexico, Africa, Europe, Australia, the Middle East and the Philippines.
The professional registrations of Registered Roof Consultant (RRC), Registered Waterproofing Consultant (RWC) and Registered Roof Observer (RRO) distinguish professionals with the proven standards of education, experience and ethics.
RCI regularly hosts educational programs designed to demystify and explain the practical application of roofing technology, waterproofing methods and sound building envelope theory.
A monthly technical journal (Interface), two online newsletters, and international, regional and chapter events provide interactive forums for learning, information exchange and networking.
The organization’s mission is realized through a professional staff headquartered in Raleigh, N.C., through the efforts of a volunteer Board of Directors, by numerous committee members, and by its individual members across the globe.
For more information about RCI and its members’ services, visit: www.rci-online.org or call 800-828-1902.
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