Wind-resistant Roofs and Attatchments

Claudette Reichel  |  7/23/2008 1:14:26 AM

Figure 1, Wall Framing and Hurricane Connectors

Figure 2, Hip and Gable Roofs

Figure 3, Balloon Framing of Gable End Wall

Figure 4a, Roof Deck Nailing

Figure 4b, Ring Shank Nail

Figure 5, Taped Roof Deck Seams

Figure 6, Wind Pressure on Roofs

Roof-to-wall connection: 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.  (Figure 1, Wall Framing and Hurricane Connectors)

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.

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. (Figure 2, Hip and Gable Roofs)

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. (Figure 3, Balloon Framing of Gable End Wall)

Overhangs: Roof overhangs larger than 2 feet wide (or less in very high wind zones) have to withstand very high uplift, so should be engineered.

Roof decking: For high wind resistance, use 7/16 or 19/32-inch plywood or OSB roof decking attached with nails, not staples. Required nail spacing varies from 3 in. to 6 in. apart, depending upon type of nail and rafter spacing. Ring shank, or specialized hurricane nails that have greater resistance to pull-out are highly recommended.   (Figure 4a, Roof Deck Nailing Pattern and Figure 4b, Ring Shank Nail)

In existing homes, 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 coverings: The most common and extensive hurricane damage is to roof coverings. When roofing blows off, water damage often results even when the decking remains in place.

Sealed seams: 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. This may be labeled as a secondary water resistance (SWR) system. (Figure 5, Taped Roof Deck Seams)

Min. #30 roofing felt (UL rated) should be used as underlayment for the roof covering. 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.

Roofing material: Choose a roofing that has been tested to UL 2218 with a Class 4 rating or FM 4473 (for rigid roofings) 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.

Equally important is installation according to manufacturer’s recommendations. Many roofing failures occur in storms because of faulty installation, such as insufficient nails or the vulnerable practice of using a backward shingle instead of a starter strip..

Attic vents: Choose hurricane-resistant attic ventilation products and systems. The most damage resistant option is a properly detailed unvented attic (see Energy Efficiency section), since it has no openings that could allow wind driven rain to enter.

For vented attics, it is recommended to use a ridge vent with water excluding components and high wind installation details in combination with a well secured, wind resistant soffit vent system. If there will be a gable end vent, the vent should be shuttered or otherwise protected from blowing rain. Typical turbine vents are vulnerable to wind damage; power vents are not a recommended attic ventilation system (see Healthy section).

Soffits have been a common failure point in recent storms that lead to substantial water damage. Particularly vulnerable are standard suspended vinyl and aluminum soffits. Soffits and soffit vents should be a rigid material such as perforated fiber cement or vent products that are securely attached to plywood or framing members. 

Porches, decks and carports are subject to very high forces in a hurricane, so such large “overhangs” and their foundations should be designed and securely anchored to resist high uplift loads. The roof structures may need stronger beams, and supporting columns or posts must be anchored to the foundation and roof beams with connectors rated for the uplift load.  (Figure 6, Wind Pressure on Roofs)

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