Analysis After Disaster

After a wind event has occurred, it’s important to analyze what went wrong with the roofing design and installation as valuable lessons can be gleaned for the future.

The first step is gathering information about the roofing system. This includes the roof deck type; structural dead load and live load capacity; roof length, width, and slope; fastener pull-out and metal pull-over results.

Experts are then brought in to help determine which aspect/aspects of the roof design contributed to the damage/failure.

“Typically, what I do right after a hurricane is I’ll rent a helicopter to get up there and do an aerial survey. You can really get a good lay of the land and see what worked and what didn’t,” explained Sean Reid, managing director, Precision Industries, Cayman Islands.

As borne out by testing, the metal itself very often survives the weather event, and the failure is ultimately attributed to a problem with the installation.

Photos courtesy of ATAS International

This roof experienced edge failure creating a “domino effect” where the failure propagates the continued peeling back of the roof panels.

“In many cases, you can pretty much pinpoint a deficiency in the as-installed portion where an attention to detail wasn’t done,” said MCA’s Bush. “Or 90% of the roof was properly done, but somewhere along the line during the course of installation, somebody got lazy and a portion of that roof gets breached. When that happens, it often propagates failure in surrounding areas as well.”

In a similar vein, Roger Bressi, managing director, Rooftops Silicone Distributors, Inc., U.S. Virgin Islands, said, “If the installers do the job properly, there should be no reason that the roof should come off. Most all failures where we live are due to improper installations and lack of inspecting via the authorities.”

To better understand common causes of failure, the following is a list of roofing problems which can occur from a high wind event:

• Poor edge securement identified as a flashing failure – This is why the ANSI/MCA FTS-1-2019 test protocol was created
• Edge failure creating a “domino effect” where the failure propagates the continued peeling back of the roof panels
• Open framing failure – With a metal roof panel being applied to an open frame assembly in lieu of solid decking, the internal positive pressures along with the external negative pressures create a mode of failure as the system cannot handle the combined loads
• Load path deck failure
• Fasteners pull out failure into the metal decking supporting the roof panels – Tighter clip spacing may prevent this
• Pull-over values are not strong enough to support wind loads
• Failure at the first course of panels causes a progression of panels blowing off
• Panel disengagement
• Door or window failure causes building pressurization
• The anchor system at the eave fails. This reveals how the gutter is an integral component of the roof assembly
• The door or window on the windward side of the enclosed structure fails. Pressurization of the building can also cause the roof and walls to blow out

Bressi added to the list improper screw lengths and/or sizes, using nails instead of screws, and improper clip spacing.

Best Practices

Working on everything from high-scale industrial to multifamily housing throughout the Caribbean over the past few decades, Precision Industries’ Reid has seen it all. His firm is often called in, post-hurricane, to assess roofing damage and assist with rebuilding efforts.

With a firm handle on what works and what doesn’t, he’s developed a number of his own best practices, above and beyond the manufacturer’s installation guides.

“One of the major changes we made in the region, even before we even started having these back-to-back hurricanes, was closing soffits,” he explained. “Everyone likes to have circulation in the attics, but here in the Cayman Islands—and in the Caribbean as a whole—the buildings were so damaged. Even if the roof stayed on, the wind and water came in through the soffits. So we’re now eliminating soffit vents and closing in ridges as well.”

Bush added that contractors have started closing attic spaces in Florida, not only for wind resistance, but to cash in on energy savings. “By conditioning the attic spaces and keeping the ductwork cooler, it’s more energy efficient and it keeps that building tighter where it minimizes that internal positive pressure.”

Photos courtesy of ATAS International

As opposed to more secure solid decking, this metal roof panel was installed in an open frame assembly. In the presence of high winds, the internal positive pressures combined with the external negative pressures to create a mode of failure.

Granted, through the process of completely sealing the enclosure, this alters the pressures in the structure, as noted, but if the envelope is not breached, a well-designed metal roof should stay on.

Another best practice Reid has adopted is hooking all the drip edges. “We created an offset to cleat so that we can still get gutters up underneath the drip. We like to have that drip all the way around and completely cleated.”

If the drip edge system is not properly attached and secured, which is the primary start on a standing seam roof, the gutter will typically fail if it’s peeling on the leading edge.

The contractor also puts in additional clips, thereby reducing the clip spacings, and has increased the number of rivets on all the hips and ridges of the roof.

Bush adds that the perimeter and corner areas of a building are subject to the greatest pressures created by the wind. Consequently, these zones often employ additional clips and fasteners to resist these greater forces.

These strategies seem to be working, as Reid reported not losing a single roof to Hurricane Ivan, which was the worst to hit the Cayman Islands. “Even the roofs that were in process remained on,” he said. The hurricane peaked at 165 mph as it passed within 30 miles of Grand Cayman and ultimately caused $2.86 billion in property damage.

Photos courtesy of ATAS International

At the Lime Tree Bay Condominiums in Grand Cayman, Cayman Islands, Precision Industries installed a metal tile roof for enhanced durability and high wind protection.