What Does a Roof Wind Rating Actually Mean?

A wind rating describes a product, and a design wind speed describes a place. The rating says how a shingle or panel did in a standardized test, or what a warranty covers. The design wind speed comes from the building code's wind map and is the engineering wind that a house at that location must be built to resist.

The two are not measured the same way, which is why a direct comparison can mislead. A fan test blowing air across shingles for two hours is not the same thing as a hurricane gust hitting a roof corner. Engineers turn the design wind speed into pressures for each part of the roof, with the corners and edges getting the highest numbers, and then check that the roof covering, its fasteners and the deck can resist those pressures with a safety margin.

In North Carolina, design wind speeds are higher near the coast, and the North Carolina Residential Code adds state-specific wind provisions to the model code. Our article on NC coastal wind zones explains how to find the number for your address. Once you know it, you can ask whether a proposed roof was tested or engineered to meet it, which is a far better question than which product has the bigger number on the brochure.

Asphalt shingles are rated under two ASTM test methods, and the class printed on the bundle tells you which test and level the shingle passed.

• ASTM D3161 blows air across a test deck with a fan for two hours. Shingles are classified Class A, D or F, tested at 60, 90 and 110 mph respectively.
• ASTM D7158 measures the uplift resistance of the sealed shingle and calculates performance from it. Shingles are classified Class D, G or H.

NRCA's Professional Roofing magazine points out two things homeowners rarely hear. First, D3161 results do not directly correlate to wind speeds experienced in service. Second, the D7158 class limits changed: under the 2005 version, Classes D, G and H corresponded to 90, 120 and 150 mph basic wind speeds, while the 2016 version maps them to ultimate design wind speeds of up to 115, 150 and 190 mph. The same class letter can be described with different mph figures depending on which version is being quoted.

Shingle ratings also assume the seal strips have bonded, which takes warm sun after installation, and that the shingles were nailed in the right zone. A high class does little if the nails are placed too high or the starter course is missing at the eaves.

Metal roofs are usually rated by how much uplift pressure an assembly resisted in testing, expressed in pounds per square foot, rather than by a single mph class. The tested pressure, reduced by a safety factor, is compared with the design pressure calculated for the roof.

Common test standards for metal roofing include UL 580, which subjects a roof assembly to cycling uplift pressures and assigns classes; UL 1897, which pushes a roof covering system until it fails and reports the highest pressure reached; and ASTM E1592, which tests sheet metal panels under uniform static pressure and is used to set clip or fastener spacing. Product approvals and evaluation reports summarize these results for code officials.

Because the results are tied to a tested assembly, the details matter: deck type and thickness, fastener type and spacing, clip spacing and edge trim. Change those and the rating may no longer apply. IBHS's FORTIFIED Roof program reflects this, requiring metal roof panels to be tested for site-specific design pressures rather than accepting a generic claim. When comparing metal roofs, ask for the test report or evaluation report and confirm the installation on your house will match the tested assembly.

A wind warranty is a manufacturer's written promise about its product, with conditions. It typically covers blow-off or wind damage up to a stated speed, provided the roof was installed to the manufacturer's instructions, and it carries exclusions and limits that are worth reading before you rely on the number.

Things to check in any wind warranty:

• Whether the stated speed refers to sustained wind or gusts, and how the speed will be determined after a storm.
• Whether special installation, extra fasteners or specific accessories are required to get the higher figure.
• Whether workmanship is covered, which is usually the installer's responsibility rather than the manufacturer's.
• What is excluded, such as damage from flying debris, falling trees or wind above the stated speed.
• How claims are filed and who pays for labor.

For Interlock aluminum roofing, sold and installed in North Carolina by Metal Roof NC as an authorized Interlock dealer, wind resistance is warranted to 120 mph when installed to manufacturer specification. The material and finish are covered by the Guardian Lifetime Limited Material Warranty from Interlock Roofing, and workmanship is covered separately by the Metal Roof NC 5-Year Labor Warranty. See metal roof warranties for the full terms.

Installation matters because almost every wind rating and warranty assumes the product was put down exactly as tested, and real-world wind failures usually start at an installation weak point. Edges, corners and ridges see the highest uplift, and that is where shortcuts show up first.

Common installation problems that undercut a good rating include shingle nails placed above the nailing strip, missing or loose starter strips at the eaves and rakes, drip edge nailed too sparsely, ridge caps that are not rated for the same wind class as the field shingles, and metal panels fastened at wider spacing than the tested assembly. Deck attachment matters too: a roof covering is only as secure as the sheathing beneath it.

IBHS built its FORTIFIED Roof standard around these weak points. It calls for a sealed roof deck so water stays out if the covering is damaged, ring-shank nails in an enhanced pattern for the deck, a wider drip edge and a fully adhered starter strip, with the work verified. Its research found that switching from smooth nails to 8d ring-shank nails at tighter spacing can double the deck's uplift resistance. Even if you are not pursuing a FORTIFIED designation, those items belong on a coastal reroof checklist.

Use them as one input, in this order: find the design wind requirement for your house, confirm the proposed product and assembly meet it, then compare warranties and installation quality among the options that qualify.

• Get the requirement. Ask the roofer which design wind speed and code edition the permit will use, and confirm with the inspections office.
• Match the assembly. Ask for the test report, approval or evaluation report and check that the fastening on your bid matches it.
• Read the warranty. Note the conditions attached to any wind figure and who covers workmanship.
• Look at the details. Edges, ridge, underlayment and deck fastening should be spelled out in the contract.
• Think about insurance. Coastal insurers may give mitigation credits for certain roof features; ask your agent before work starts.

A roof that meets the requirement for your address and is installed carefully will do more for you than a higher number on a product that was installed loosely. Our pages on hurricane-resistant roofing, aluminum roofing and choosing a metal roofing contractor help with the rest of the decision.