One of the most common issues a roof experiences after a storm is shingles that get blown off. While no roofing material is guaranteed to withstand the brunt of Mother Nature, picking materials with the proper wind rating will reduce the likelihood of roof damage due to high winds.
What is a wind rating, and how do you pick the right one?
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Wind Rating - What Is It?
All roofing manufacturers label their products with a wind classification, identifying the maximum wind speed the material can handle without damage.
These ratings come from standards set by independent organizations, which outsource testing to reliable third-party companies. Each set of standards ensures that all materials are tested equally and receive fair, unbiased ratings.
Roofing materials can have standards listed from one to three unaffiliated organizations that set testing standards for many products besides roofing. Different types of roofing materials can have ratings from one or more of the three organizations. The three possible standards to be aware of are:
- ASTM (American Society for Testing and Materials) International
- UL (Underwriters Laboratories)
- FM Approvals
ASTM International sets two standards for wind resistance classifications. Understanding these two different standards will help you make a more informed decision on what roofing materials would be best for your needs.
The first standard to be aware of is the ASTM D3161. ASTM D3161 rating is the Standard Test Method for Wind Resistance of Steep Slope Roofing and measures a roofing material's ability to withstand wind speeds, simulated using an industrial fan. A product can receive one of three classifications:
The second standard - D7158 - is the Standard Test Method for Wind Resistance of Sealed Asphalt Shingles - uplift resistance/uplift force. Individual products get a classification based on how well they resist uplift forces at specific wind speeds. The five classifications are:
Uplift Wind Speed
Wind uplift occurs when there's a higher air pressure under the roof than above it. When the wind blows over a roof, it causes the air to decrease - negative pressure. However, it also causes air to infiltrate under the roof through cracks and openings - positive pressure.
These differing pressures create a push-pull force that can cause the separation of the roofing from the deck. The most vulnerable areas are ridges, rakes, eaves, perimeter overhangs, and windward corners. Roofs with steep slopes are at a higher risk of wind uplifts.
ASTM D7158 uses two steps to determine a shingle's uplift resistance. The first step requires the surface to get exposed to blowing wind and decides if there's an uplift on the shingle's edges. Next, testers measure the uplift resistance of the interlocked shingles. Calculations then determine the uplift strength needed to lift the shingle edges, so it becomes unconnected to the shingle beneath it.
Another independent organization that sets the rating for metal roofing materials is Underwriters Laboratories. Like the ASTM, UL uses two separate standard tests for uplift resistance of roofing.
Both sequences of testing go through five phases.
- Phase 1, the materials undergo five minutes of negative pressure.
- Phase 2, positive pressure mixes with negative pressure for five minutes.
- Phase 3, takes one hour (60 minutes), during which the material gets exposed to cyclic positive and negative pressure.
- Phase 4, there are five more minutes of negative pressure.
- Phase 5, the process from step two repeats - five minutes of positive and negative pressure.
The UL580 test determines a metal roof's uplift wind resistance. This standard test focuses on the metal panels and included accessories such as fasteners, underlayment, clips, and the support frame's structural integrity.
A small sample (usually 10'x10') goes onto the testing platform for a metal product to receive a UL wind rating. Closely spaced fasteners seal the edges.
The sample product will undergo three sequences of testing. If the material exceeds a Class 90 rating, the product moves on to the UL 1897 test.
Results for the UL580 testing can be one of three ratings. The lowest rating is Class 30, whereas Class 60 is in the middle. The best rating for a UL580 test is Class 90.
If the material sample receives a Class 90 rating, it moves onto testing using UL1897 standards. During this phase, testing continues until the product fails.
This test consists of applying increasing negative pressure while maintaining positive pressure. Typically, the negative pressure goes up in increments of 15 PSF.
Wind Resistance Ratings by Materials
When shopping for new roofing, you'll see that wind resistance rating vary by brand and type of roofing material. Asphalt shingles have a lower wind resistance than metal roofing.
However, 3-tab asphalt shingles have the lowest wind rating of Class A or Class D. A more wind resistant type of asphalt shingle is an architectural style, which is heavier and thicker than 3-tab. Architectural shingles can have a Class F wind rating.
Pro Tip: You can increase your shingle's wind rating by using additional fasteners. While most manufacturers require a minimum of four fasteners, increasing the number to six can give you a higher wind rating of 130 mph for architectural shingles or 90 mph for some 3-tab options.
If you live in an area prone to hurricanes, you'll want a roof with superior wind resistance. Some architectural shingles can withstand Category 3 hurricane winds (up to 110 mph) using the proper six-nails application. But you may experience some tear-offs due to the lightweight.
Metal roofs are the best option for locations that risk experiencing significant wind damage from hurricanes. Because most metal roofs come as panels, there's no worry of losing pieces, as with shingles. And they can handle winds up to 160 mph (Category 4).
Choosing a roofing material with the correct wind rating can save you money on repairs due to wind damage. If you live in an area with the potential for damaging winds, you'll want a higher wind-rated material. But before you pick a roof, it's essential to know about other significant features that may affect the durability and lifespan of your roof. Be sure to check out our guides on ratings for fire, impact, and algae.