It only takes a moment for a hurricane or tornado to peel the roof off a house. And once the roof is incapacitated, the house is a sitting duck for further damage, if not total destruction.
Your best defense: fasten the roof securely to the house and include multiple layers of moisture protection.
Following are PATH's recommendations for achieving storm resistant roofing:
- Build a Moderately Pitched Hip Roof. Because mid-range slope roofs withstand strong winds better than steep roofs, keep the roof pitch below 6/12 and greater than 4/12. Flat roofs increase uplift forces on the overhang and are more prone to leaks.
"We encourage people to stick with hip roofs," says Dr. Claudette Reichel, a housing specialist for the LaHouse, a demonstration house showcasing energy-efficient and disaster-resistant construction in Baton Rouge, La.
"Hip roofs are inherently more aerodynamic and wind-resistant than gables. In addition, they are better because there will be an overhang to shed water away from the walls as well as shading windows."
Gable ends are particularly prone to storm damage, so it is better to avoid them altogether. But if you must have them, use a nailing pattern of 8d ring shank nails at 4 inches on center for any panel adjacent to a gable end and add diagonal bracing of the gable wall to the attic floor.
- Use OVE structural members for roof truss components or engineered wood girders. Pre-fabricated roof trusses allow for much faster installation than trusses built on site. Prefabricated trusses are just as strong as solid wood beams and they save lumber because they are typically spaced on 24-inch centers and fabricated with 2 × 4's and 2 × 6's. If roof trusses aren't available or appropriate for your project, use engineered wood.
- Tie hurricane straps 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.
- Add Baffled Ridge and Soffit Vents. Minimize the number of penetrations in the roof for added protection. Through-the-roof penetrations, like attic vents, can blow off in high winds, providing a hole for water entry. Baffled ridge vents prevent airflow and wind-driven rain from entering the attic through the ridge vent. Select soffit vents with perforations that are concentrated near the outer edge to minimize the area that wind-driven rain can enter. They will cost about the same as vents with uniform perforations. To withstand storm-force winds, it's best to use plywood or fiber-cement soffit materials and attach them securely to framing. A lumber baffle securely installed over the wall framing inside a soffit will greatly reduce water entry into the attic, but it must be able to withstand strong air pressure.
- Extend facia board to terminate below the underside of the soffit. An effective overhang design extends the fascia board below the underside of the soffit by building the fascia out of a 1×6, instead of the standard 1×4. This creates a drip edge so that buffeting winds do not drive the rain across the soffit surface and into the eaves. The added cost to implement this recommendation is the difference between the price of 1×4's and 1×6's.
- Securely attach roof decking. Use roof sheathing with a minimum thickness of 19/32 inches 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 inches on center. Do not attach with staples. Make sure that sheathing is properly spaced, allowing a 1/4-inch gap between sheets for thermal expansion. H-clips can provide this spacing, while adding strength to the horizontal connections.
Also consider using grid-marked panels, which make it quicker and easier to install sheathing. They cost about $0.85 more per 4-foot × 8-foot panel, but are likely to save more than that through quicker installation with fewer mistakes.
"Our crew said that the grids were helpful because they didn't have to measure and do chalk markings," said Reichel.
- Tape sheathing 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.
- 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. An uncovered underlayment by Grace Tri-flex 30 withstood both Hurricane Katrina and Rita on the LaHouse, which was still under construction when the storms hit. All other roofing felts in the area were shredded.
- 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). Ensure that asphalt and fiberglass shingles overlap the drip edge strip by at least 1/2 inches to 3/4 inches. Meticulously follow building code and roof cladding manufacturers instructions.
- Install a wind- and impact-resistant covering. Select appropriate wind- and impact-resistant roof coverings and install per the manufacturer's instructions. 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). In warm climates, specify light-colored or solar reflective roof coverings that absorb less heat from the sun and make the home more energy efficient.
- Use proper fastener/nailing schedules for your wind zone. 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.