Wind-resistant Roof Design

Shandy Heil, Skinner, Patricia, Reichel, Claudette H.

Roof design is important when building a house to withstand the forces of hurricanes. Before construction or significant roof repairs begin, determine the wind exposure at the property.

The Basic Wind Speed map used in the International Residential Code and other I-Codes is a good guide to wind risk. Keep in mind, however, that wind speeds are higher close to open water, and that all of south Louisiana is getting closer to open water as the coastal land is disappearing. Building to the wind speed one or two zones closer to the coast will provide greater protection.

These are the elements of wind resistant roof design, construction and, in some cases, retrofit.

Design considerations

Hip roof style: Hip roofs (four-sided roof design) are more aerodynamic and inherently more resistant to uplift forces of high wind than gable end and other roof designs.

Gable end walls braced: If your home has a gable roof (two-sided pitched roof), the end wall takes a beating during a hurricane, and typical framing methods create a vulnerable “hinge” line at the joint between levels. If not properly braced, it can collapse. Properly bracing gable end walls can prevent wind from pushing or pulling the gable end at that weak point.

In new construction, this can be provided with code-specified bracing details – which may include lateral bracing and blocking of ceiling joists, extra metal strapping, structural panels under the joists and sometimes cross bracing of ceiling to roof framing. A simpler alternative to bracing is to use a “balloon framing” method for gable end walls; this refers to using continuous studs or columns from the foundation to the roof without a break. This method is less labor intensive, but tall spans may need larger lumber or closer spacing than for other walls.

Moderate roof pitch: A roof pitch of 3:12 to 6:12 is preferred since it results in lower loads on the house than with steeper or flatter roofs. In a one-story house, a roof pitch between 6:12 and 12:12 can be treated like a two-story house. A roof pitch greater than 12:12 will require engineering and stronger connections along the full load path. So it is more economical to design your home with a moderate roof pitch.

Modest overhangs: Roof overhangs should be no more than 2 feet wide. Those larger than 2 feet wide have to withstand very high uplift, so they should be engineered.

Roof construction and materials considerations

Roof decking: For high wind resistance, use 7/16-inch or 19/32-inch plywood or OSB roof decking attached with nails, not staples. Required nail spacing varies from 3 inches to 6 inches apart, depending upon type of nail and rafter spacing. Ring shank or specialized hurricane nails that have greater resistance to pullout are highly recommended.

In an existing home, you can apply wood adhesive with a caulking gun on both sides of the intersection of rafters and the decking to increase uplift resistance. Look for adhesives that are certified as APA AFG-01 or ASTM D 3498.

Roof to wall connections: Hurricane straps, not clips, should connect wall framing to each roof rafter or truss. Other building systems may use different connectors (rebar, long screws, interlocked steel mesh, etc.) to achieve needed uplift loads. Connectors must be installed according to the manufacturer’s recommendations.

Use corrosion-resistant connectors in coastal areas. When wood is copper treated, use stainless or double-hot-dipped galvanized nails and either shield the connectors from the wood with a product designed for that purpose or use connectors with high corrosion resistance.

Roof coverings that add wind and rain resistance

The most common and extensive hurricane damage is to roof coverings. When roofing blows off, water damage often results, even when the roof decking remains in place. Stronger roofing material (shingles, metal, and others), stronger underlayment and proper installation of all components can prevent much storm damage.

Sealed seams: Roof decking is installed with gaps that allow expansion and contractor with temperature. As a secondary protection from water leaks, it’s highly recommended to seal all decking joints with an adhesive flexible flashing (roofing tape) at least 6 inches wide. An alternative is to use an adhesive peel-and-stick membrane roofing underlayment over the entire roof.

Underlayment: Synthetic felts that are extremely tear resistant are available and a good investment in hurricane zones. The most leak- and wind-resistant underlayment option is a peel-and-stick membrane – particularly needed under more leak-prone roofing (like tile, shakes or metal roofing with exposed fasteners) and long-life roofs (tile, slate). If installing a membrane underlayment on OSB decking, apply the membrane manufacturer’s primer to the OSB before the membrane; the resins and waxes in OSB can reduce its adhesion. This is not generally needed with plywood. If using felt, use a minimum #30 felt.

Roofing material:
Choose a roofing that has been tested to UL 2218 with a Class 4 rating or FM 4473 (for rigid roofing) for impact resistance. In addition, choose roofing rated for high wind resistance by ASTM D 3161 or the new ARMA wind classifications for shingles. Class D is for wind zones up to 90 mph, Class G up to 120 mph, and Class H to 150 mph. Be sure to specify these standards and look for labels on the products confirming these standards because ordinary roofing materials may not look any different from the wind-resistant versions.

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1/6/2007 3:15:56 AM
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