Use Wind Resistant Roofing and Enhanced Underlayment in High Wind Areas as Required by the IRC

Standard asphalt composition roof shingles, the most common roof covering for residential buildings, vary in their capability to resist wind loads. Using better performing shingles can help prevent roofing damage leading to potential moisture intrusion and more serious consequences to building contents. In hurricane-prone and high wind areas, the IRC requires that shingles must be rated to meet hurricane level wind speeds according to ASTM Standard D 7158 classes G or H or ASTM D 3161 class F. Shingles meeting ASTM D 7158 Class H are rated for wind speeds as high as a Category 4 hurricane (or very severe thunderstorm downburst). Selecting higher performing shingles is advisable in all wind regions for durable roof construction.

Using a shingle rating greater than that required by code, if available, will also provide some relative benefit in limiting damage. Further, designers and builders should also be careful to cross-reference shingle ratings with the locally applicable, code-mandated wind speeds. As a matter of practical importance, following the shingle manufacturer’s installation instructions for high wind conditions and also ensuring a good adhesive tab seal between shingle courses are crucial to durable asphalt shingle roofs. Recent insurance industry research indicates that “SBS polymer modified shingles” are most likely to provide a durable roof installation.

Underlayment and Flashing—The IRC requires a minimum of a single layer of 15# felt lapped 2” for composition shingle roofing on slopes ≥ 4:12; a double layer of underlayment is required for shallower pitches. Flashing is required in valleys, at sidewalls, and wherever there is a roof penetration such as chimneys and vents. In high wind areas, a 4” overlap, reduced fastener spacing (12” grid pattern with 6”oc at lap joints), and plastic or metal cap nails are required.

Because non-adhered membrane-type underlayment (e.g., 15# felt) isn’t intended to be an exposed water-resistant barrier under high wind conditions, it does not provide a durable back-up should roof shingles become damaged and torn off the roof from wind. Therefore, in high wind regions it’s good practice to seal the roof deck below. There are several options including taping of the seams of the sheathing with a bituminous tape (Figure 9–5). For even further protection in the event of shingle damage or loss, the entire roof can be covered with a self-adhering membrane. As another alternative, a recent test at the IBHS Research Center on full-scale roofs demonstrated that two layers of ASTM D226 Type II or ASTM D4869 Type IV underlayment can provide excellent water intrusion resistance without requiring application of tape or a self-adhering membrane.