PATH - A Public Private Partnership for Advancing Housing Technology
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.
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.
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" centers and fabricated with small dimension lumber -- 2 x 4 and 2 x 6. 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.
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.
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.
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 1×4 to a 1×6. 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.
Securely attach grid-marked sheathing of appropriate thickness
Grid-marked sheathing in 4' x 8' 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" 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" 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" on center. Do not attach with staples.
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" 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.
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". A single layer of #30 felt may be used on higher sloped roofs. For optimum protection, use a high-performance, tear-resistant synthetic felt.
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" to ¾". Meticulously follow building code and roof cladding manufacturers instructions.
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).
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.
Content updated on 4/20/2007
| | | | |
Affordable Housing Providers