High Winds and Hail: The Roof Damage They Cause

April 28, 2020

According to the United States Federal Emergency Management Agency (FEMA), approximately 10,000 severe thunderstorms occur in the United States each year. Aside from lightning and heavy rain, many also produce high winds and hail. These can result in the destruction of commercial/industrial roof systems and components.

Let’s equip you with the knowledge you need to weather these storms.

Roof Damage From Wind

Powerful gusts and sustained periods of strong winds abuse the roof. What follows are just some of the types of damage you can expect.

It’s common for wind scour to redistribute and even blow aggregate off a roof. Once the underlying membrane is exposed, it’s more vulnerable to the elements.

Punctures, tears, and abrasions are common and result when rooftop unit hoods, tree limbs, debris, obsolete equipment, and dislodged roof elements (roof vents and roof hoods) become airborne. As they are pushed across the roof membrane, they leave a trail of damage. Leaks often follow.

Wind uplift pressure is greatest along the edges and corners, often resulting in edge metal damage. Gutters or metal and membrane at the roof’s edge take the brunt of the force and, once damaged, can compromise the whole roof system.

What starts as a small opening allowing wind to penetrate, can have catastrophic results. The entire roof, including the roof deck, can blow off during tropical storms, tornadoes, or anytime the wind uplift is strong enough.

Roof Damage From Hail

The area between the Rockies and the Appalachian Mountains is thought to be the most hail-prevalent in the United States. According to the National Severe Storms Laboratory (NSSL), Nebraska, Colorado, and Wyoming have the most frequent hailstorms. You don’t have to be located inside “hail alley” to experience damage from hail though.

Unlike the havoc caused by windborne debris, hail damage is more widespread. Unfortunately, the destruction may not be visible, so don’t be fooled by a quick visual walk-over.

How badly the roof is damaged is dependent on a variety of factors (e.g. size of hail, force of its impact, duration of the storm, and the age and condition of the roof system). More than 20 years ago, William Cullen presented on the topic of hail damage at the NRCA’s Fourth International Symposium on Roofing and provided the following observations about damages by roof type.

BUR: membrane fracture, flashing puncture and cracking, interior ply fracture or separation, interface separation between membrane and substrate, and substrate damage.

Modified BUR: weakening of granular adhesion, granule loss, cracking, puncture, insulation facer separation, substrate delamination, and substrate damage.

Single-ply membrane: indentation, puncture, cracking, shatter, substrate damage, fracture at fasteners, and membrane/substrate delamination.

Metal roofing: indentation, creasing, coating adhesion loss, and puncture.

SPF: indentation, fracture, surface cracking, and coating damage.

We’d be remiss if we didn’t also add broken skylights to Cullen’s list of damages. Hail that exceeds the impact resistance of skylight structures will cause them to crack.

Guidelines, Codes, And Requirements

Wouldn’t it be great to construct a roof that was impervious to these forces? Engineers and manufacturers alike have tried. Building codes and roof system requirements from insurance providers are proof of their efforts.

Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-16) has been integrated into building codes (e.g. Intl. Building Code and NFPA 5000) throughout the United States and serves as the basis for determining wind loads. While there’s no requirement that buildings be built to withstand tornadoes, these codes provide requirements that address typical wind conditions and the consequential destruction.

Code officials aren’t the only ones working to make roofing assemblies more resistant to storms. “FM Global is influencing our industry tremendously by codifying or standardizing roof designs, mandating safer, fire-, and wind-rated roofs. The industry needed that,” explains Jack Kenney, Vice President National Account Manager, D. C. Taylor Co., Cedar Rapids, IA. FM Approvals, the insurance company’s independent testing division, evaluates commercial and industrial building products and systems to ensure they meet strict loss prevention standards. Even companies who aren’t insured with FM Global look to the company for roofing design specifications.

In late 2016, the organization revised FM Approval Standard 4470, Approval Standard for Single-Ply, Polymer-Modified Bitumen Sheet, Built-Up Roof (BUR), and Liquid-Applied Roof Assemblies for Use in Class 1 and Noncombustible Roof Deck Construction. This standard is used to evaluate the performance of roofing products and assemblies for fire, wind uplift, metal corrosion, hail damage, foot traffic, roof leaks, puncture resistance, and solar reflectivity. A roof system must meet the requirements of FM 4470 to receive certification from FM Approvals.

Recent revisions added a higher hail impact-resistance classification and updated the map in FM Loss Prevention Data Sheet 1-34 (FM 1-34), which designated areas as prone to moderate hail, severe hail, and (now) very severe hail. Depending on the location of a building, the roof assembly installed must meet the appropriate classification rating.

What To Do If You Suspect You Have Roof Damage

A visual inspection is the easiest way to check for storm damage. If possible, look at the edge from the ground. Only go onto the roof if you can do so safely. Take photos. Call your roofing contractor and share the images; this will prepare them for the repair and ensure they bring appropriate materials when they arrive. They can also instruct you on notifying the manufacturer of a leak, in accordance with your roof system warranty. Don’t risk injury and perform the repair yourself; improper repairs could void your warranty and exacerbate the roof problem.

Contact the D. C. Taylor Co. office nearest you with your roofing questions or to schedule service.


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