How to build a roof that is storm-resistant
A strong, durable roof is a home’s first line of protection against a major storm. Most storm-related damage is caused by water filtration (which can lead to mold), not by outright structural failure. Building a roof that withstands storms requires careful attention to these key areas. Here’s how to build a roof that is storm resistant:
1. How to build a roof with a moderately pitched hip
Because moderately pitched roofs withstand strong winds better than steep roofs, keep the roof pitch between 4/12 and 6/12. Strong winds are less likely to catch the corner of a hipped roof than a gabled roof.
2. Use advanced framing structural members for roof-truss components or engineered wood girders
Pre-fabricated trusses are faster to install than trusses built on site, and are just as strong as solid wood beams. They save lumber resources because they are typically spaced on 24″ centers and fabricated with 2′ x 4′ and 2′ x 6′ lumber.
3. Tie hurricane straps to load-bearing components
Tie the straps to load-bearing walls or bracing.. Use hurricane straps that wrap over the top of the roof truss or rafter. Strap each part of the wall assembly together or anchor each part from the foundation to the roof to provide a continuous load path, effectively connecting the roof to the foundation.
4. Use baffled ridge and soffit vents
Through-the-roof penetrations, such as attic vents, can blow off in high winds, providing a hole for water entry. Baffled ridge vents prevent air and wind-driven rain from entering the attic through the ridge vent.
5. Extend fascia board to underside of the soffit
An improved overhang design extends the fascia board below the underside of the soffit by increasing the size from a 2′ x 4′ to a 2′ x 6′. This creates a drip edge so that buffeting winds do not drive the rain across the soffit surface and into the eaves. Limit vent perforations to the outer edge of the soffit, reducing the area where moisture can enter. Install plastic baffles between the trusses at the top of the wall that extend up the rafter about 18 inches. These baffles will hold the attic insulation away from the vented soffit and roof decking, while providing a barrier and drainage plane for rain.
6. Securely attach roof decking
Use grid-marked sheathing in 4′ x 8′ sheets. Install it as you would any other roof sheathing: perpendicular to the framing components, gapped, and supported with “H” clips at mid-span between trusses spaced at 24″ on center. Use roof sheathing with a minimum thickness of 19/32″ for added strength in high-risk high wind zones. Use a nailing pattern of 8d ring-shank nails at 6″ on center. Do not attach with staples.
7. Tape decking seams
Tape roof-decking seams with a self-adhering asphalt/rubber (modified bitumen) tape that is at least 4″ wide, so the roof decking can act as a water and wind barrier in case the roof coverings and underlayment blow off in high winds. Or, apply peel-and-stick roof membrane over the entire roof deck. If you use the membrane, other underlayment is not required, but you must vent the attic or add rigid insulation over the roof decking in most climate zones to prevent winter moisture problems.
8. Properly install underlayment
To build a roof that is storm resistant is important to install underlayment per manufacturers’ specifications with approved fasteners. Roofs with a slope of less than 4/12 require two layers of approved #15 felt underlayment or a continuous peel-and -stick membrane underlayment. Overlap all underlayment rows by at least 4 inches. A single layer of #30 felt may be used on higher sloped roofs. For optimum protection, use a high-performance, tear-resistant synthetic felt.
9. Use proper flashing and starter strip techniques
Install flashing wherever there is a change in surface plane or cladding material. It will channel water runoff during storms and provide a second level of water-intrusion protection. Flash roof edges with a shingle starter strip (with tabs cut off or special adhesive starter strips) or a self-adhering ice and water barrier membrane. Do not use an upside-down shingle as the starter strip. Apply a drip edge to all roof edges (including rake). Insure that asphalt and fiberglass shingles overlap the drip edge strip by at least ½” to ¾”. Meticulously follow building code and roof cladding manufacturers’ instructions.
10. Install light-colored wind- and impact-resistant roof covering
In warm climates, select light colors for the roof cladding to increase its solar reflectance. Choose rated wind- and impact-resistant asphalt shingles and standing-seam metal roofs that meet the standards for wind resistance (ASTM D 3161 enhanced or UL 2390) and impact resistance (UL 2218).
11. Use proper fastener/nailing schedules
Fastener/nailing schedules tend to change with the product and wind zone, so pay specific attention to these details, and consult the manufacturer’s specifications. In high-wind zones, increasing the frequency of fasteners from four to six per shingle will also increase the wind-resistance rating.
Source: The Partnership for Advancing Technology in Housing (PATH)