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Tech Set 5: Storm-Resistant Roofing

     

A strong, durable roof is a home's first line of protection against a major storm. Most storm related damage is caused not by outright structural failure, but by water infiltration - which can lead to mold. This Tech Set shows you how to protect against both structural failure and water infiltration.

Taken as a whole, these recommendations will help builders build storm-resistant roofs in areas that are at a high-risk for being hit by strong winds. Most of them add minimal additional costs to the finished cost of a roof. Many of these recommendations should even be used on homes outside of the hurricane region.

1 Moderately pitched hips rather than gables
2 OVE structural members for roof truss components or engineered wood girders
3 Install hurricane straps to load bearing components
4 Use baffled ridge and soffit vents to minimize the number of roof penetrations
5 Extend fascia board so it terminates below the underside of the soffit
6 Securely attach grid-marked sheathing of appropriate thickness
7 Tape decking seams with a self-adhering bitumen tape or peel and stick roof membrane
8 Install underlayment correctly
9 Use proper flashing and starter strips techniques
10 Install a light-colored wind and impact resistant roof covering
11 Use proper fastener/nailing schedules
 



Tech Set Details

Building a storm-resistant roof is the most important way to protect a home from damaged incurred by strong winds. Consider the following 11 recommendations, especially if you are building in an area that is likely to be hit by strong winds.

  1. Moderately pitched (4/12 to 6/12) hips rather than gables

    Flatter roofs withstand strong winds better than steep roofs. Strong winds are less likely to catch the corner of a hipped roof than a gabled roof. In fact, winds will typically force hip roofs down into the house-a force that the roof is built to withstand, while exerting a much more damaging upward force on a gabled roof.

    Roofs with a slope that is too low (3/12 or less) can increase the potential for uplift from a high wind event much like the uplift encountered by an airplane wing. Roofs that are too steep increase the later wind loads along the entire roofing system.

  2. Advanced framing structural members for roof truss components or engineered wood girders

    Advanced framing roof trusses are engineered structural components that save lumber resources because they are typically spaced on 24-inch centers and fabricated with small dimension lumber -- 2 inches by 4 inches and 2 inches by 6 inches. What's more, the shape is more conducive than solid sawn lumber rafters to accept continuous insulation for building envelope energy efficiency. If roof trusses are not available or appropriate for your project, use lumber made from engineered wood.

  3. Install hurricane straps tied to load bearing components

    If you install hurricane straps on non-load-bearing lumber, you'll do nothing to increase the disaster resistance of the house. Tie them to load-bearing walls or bracing instead. Use hurricane straps that wrap over the top of the roof truss or rafter per Fortified...for Safer Living® standards (www.IBHS.org). Each part of a wall assembly should be strapped together or anchored 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 to minimize the number of roof penetrations

    Through-the-roof penetrations, like attic vents, can blow off in high winds, providing a hole for water entry. Baffled ridge vents are preferred over other types of vents because they prevent airflow and wind driven rain from entering the attic through the ridge vent.

    If added daylighting is required, specify tubular skylights with polycarbonate domes. Their compact profile will minimize through-the-roof penetration size compared to skylights.

  5. Extend fascia board to terminate below the 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 by 4 to a 2 by 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.

    If rain does penetrate, it should be stopped by plastic baffles installed between the trusses at the top of the wall that extend up the rafter about 18 inches. The baffles hold the attic insulation away from the vented soffit and roof decking, while providing a barrier and drainage plane for water that might enter during severe conditions.

  6. Securely attach grid-marked sheathing of appropriate thickness

    Grid-marked sheathing in 4-foot by 8-foot sheets is installed like any other roof sheathing; perpendicular to the framing components, gapped, and supported with "H" clips at mid-span between trusses spaced at 24 inches on center. The grid marks facilitate cutting the sheets, using the correct fastening schedule, and the layout of shingle underlayment. See The APA's Builder Tips: Proper Installation of APA Rated Sheathing for Roof Applications, to foster quality applications for more information.

    Use roof sheathing with a minimum thickness of 19/32 of an inch for added strength in high-risk high wind zones. Thicker sheathing can also offset sensitivity to overdriving. Use a nailing pattern of 8d ring shank nails at 6 feet on center. Do not attach with staples.

  7. Tape decking seams

    Roof coverings and underlayment can blow off in high winds, in which case the roof decking can act as a water and wind barrier if the seams have been taped with a self-adhering asphalt/rubber (modified bitumen) tape that is at least 4 inches wide.

    As an alternative, consider applying a peel and stick roof membrane product over the entire roof deck. These products are widely used in roof valleys, and provide excellent protection from water intrusion if the primary roof covering is damaged. 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. This underlayment costs about $700 to $1,000 more than standard roofing felt for a 2,400-square-foot house.

  8. Install underlayment correctly

    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. All underlayment rows should overlap 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 serves the dual purpose of channeling water runoff during storm events and providing 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-inch to ¾-inch. Meticulously follow building code and roof cladding manufacturers instructions.

  10. Install a light-colored wind- and impact-resistant roof covering

    In warm climates, consider the solar reflectance of the roof cladding and select light colors. The higher the reflectance factor, the less heat absorbed by the roof from the sun. Refer to the ENERGY STAR® website for the reflectance rating of roof products. Cladding products that carry reflectance ratings can be researched at the website for reflective roof products.

    Rated wind- and impact-resistant asphalt shingles, and standing-seam metal roofs are good choices for storm resistance. Make sure that they 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. Consult the manufacturer's specifications so that fasteners are installed to the proper embedment depth into framing members or sheathing. Be aware of the depth that you set your automatic tools. In high-wind zones, increasing the frequency of fasteners from four to six per shingle will also increase the wind-resistance rating.

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Code Considerations

Prescriptive specifications for structural member connections based on regionalized wind speeds are contained in the IRC. Both the IRC and roofing product manufacturers prescribe minimum fastener specifications.

Underlayment should be tested to ASTM D-226, D-2626, D-4869, or D-1970 and installed per manufacturer's specifications. In climates where the average daily temperature is 25°F or less in January, a 24-inch modified bitumen sheet (self-adhering ice and water barrier) is required around the roof perimeter to preclude water infiltration from melting ice dams.

The IRC is very specific about flashing size, material, and location.

The IRC prescribes tests that asphalt, tile (clay and concrete), wood, and metal shingles, and mineral-surface rolled roofing should conform to assure weatherization durability, as well as appropriate roof pitches for the product types.

Certain coastal tiers of Texas and Florida have specific, prescriptive codes covering products and installation in high wind areas. In Texas, asphalt shingles must conform to ASTM D-3161 testing. Florida maintains lists of approved roofing materials.

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Resources

American Society for Testing and Materials (ASTM)

Asphalt Roofing Manufacturers Association (ARMA)

Federal Alliance for Safe Homes (FLASH)

Federal Emergency Management Association's Roof Underlayment for Asphalt Shingle RoofsPDF file

Florida Building Code

Florida Roofing, Sheet Metal and Air Conditioning Association's Concrete and Clay Tile Installation Manual

International Residential Code, IRC 2003

Lowes' Flashing For a New Roof

National Roofing Contractor's Association (NRCA)

Texas Department of Insurance's Windstorm Inspections Program

Tile Roofing Institute

Underwriter’s Laboratory (UL)

The Institute of Business and Home Safety's UL Approved Class 4 Asphalt ShinglesPDF file

Wood Truss Council's Web Member Permanent Bracing/Web ReinforcementPDF file

Building Component Safety Information from the Wood Truss Council

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