Best Practices: Attachment of Asphalt Shingles in High-Velocity Wind Zones
June 10, 2008
Mistake: Improper shingle nailing. Problem: Potential for roof blow-off. Proper design: Install the proper amount of shingles required by the manufacturer in the specific geographic wind zone. Most three-tab shingles require four nails in typical applications and six nails in high-velocity wind zones. The placement of the nails is as important as the number of nails. Place nails in the manufacturer’s required placement areas. Most dimensional shingle manufacturers now provide shingle placement zones. Nails should be driven straight into the deck; avoid nailing from an angle. Nails should penetrate through the deck a minimum of 3⁄4 of an inch. Standard roofing nails have barbed shanks and are typically 11- or 12-gauge nails with 3⁄8-inch to 7⁄16-inch diameter heads.
The recent increase in storm frequency and the number of high-velocity wind events throughout the United States is prompting changes in the roofing industry. While last year was relatively mild, attachment procedures for all roof systems - steep slope and low slope - are being scrutinized in the wake of the damaging hurricanes of the previous few years, and code changes are imminent. Stringent building codes and tightened regulations were enacted in South Florida after Hurricane Andrew in 1992, and the severe weather events of 2004 and 2005 are the driving force behind other proposed code changes.
Some codes have already been altered to reflect the need for increased attachment methods. In 2004, the International Building Code enacted new high-wind performance standards for asphalt shingles. The standards, published in the 2004 IBC Supplement, provide attachment criteria for asphalt shingles in high-velocity wind zones where wind speeds reach up to 150 mph. The new asphalt shingle standards are based ANSI/UL2390-04, “Test Method for the Wind Resistance of Asphalt Shingles with Sealed Tabs,” and ASTM D6381, “Measurement of Asphalt Shingle Mechanical Uplift Resistance.” Adoption of these standards was reached through the promotion of the Asphalt Roofing Manufacturers Association (ARMA), which spent nearly 15 years assisting code officials, wind engineers and insurance companies with research and development of new test methods for asphalt shingle performance in high-wind situations.
The primary advantage of the code adoption is that designers, contractors, building owners and homeowners now have standards for proper asphalt shingle selection in high-velocity wind zones. It also provides code enforcement officials with guidelines for enforcement of material selection and application methods. Until this time, many code officials would not allow the use of asphalt shingles in high-velocity wind zones due to the fact that there were no wind-uplift resistance standards.
The roof deck sheathing should be manufactured of structural plywood or OSB board in the nominal thickness range of 7⁄16 to ⅝ inches. Nails should be used to secure the decking to the structural trusses. Acceptable nails for this securement procedure are 8d common nails (0.131 inches by 2.5 inches long), 10d box nails (0.128 inches by 3 inches long) or power-driven nails (0.113 inches by 2⅜ inches long). If the trusses are spaced 16 inches on center or less, then the decking should be fastened at a minimum rate of 6 inches on center. If the trusses are spaced 24 inches on center, then the decking should be fastened at a minimum rate of 4 inches on center. Truss spacing should be in compliance with local code requirements.
On remedial applications, the existing decking should be inspected to verify that the proper nailing pattern of the deck to the trusses or rafters is followed. If the existing nailing pattern is inadequate, then the addition of nails to meet the current requirements should be completed. Proper attachment of deck panels near roof edges and ridges is critical, as these areas are the most vulnerable to damage because wind forces are the highest at these locations.
Most of the damage that occurs from high-velocity winds on steep-slope shingle systems comes in the form of the loss of shingles or other roof covering components. In a majority of cases, damage is localized and it rarely results in structural damage. The major losses typically occur from moisture infiltration into the interior spaces, which results in damage to drywall, carpeting and furnishings. The interior damage can be limited - or avoided altogether - if a secure underlayment is applied between the structural decking and shingle coverings.
Steep slope underlayments are typically 15-pound or 30-pound fiberglass felts. The underlayment should be secured to the decking with capped-head nails or with metal disks applied under the roofing nails to improve the water-resistance capabilities of the underlayment. The fastening pattern should be determined using Factory Mutual or ASCE Standard wind-uplift calculations, which determine the wind-uplift pressure rating on the specific building. The wind-uplift calculation takes into account the basic wind speed in the geographical area, the ground surrounding the building and the roof uplift pressure at the field of the roof.
The most prominent change that the code enacts is in the area of shingle application methods. The code addresses application methods at the most critical points of steep-slope applications: shingle attachment in the field of the roof and at areas that are most vulnerable to wind damage, such as at ridges, eaves, hips and rakes. Shingle application should be completed by applying a six-nail pattern - not the traditional four nails - at each shingle. On three-tab shingles, the nails should be placed just below the self-sealing tar line and within the nailing zone provided on dimensional shingles. The shingles must be secured with approved nails such as 11⁄4-inch No. 12 wire gauge shank nails with a 3⁄4-inch diameter head.
The nails applied at the starter strip should be a maximum of 1⁄2 inch from the edge of the sheathing. At the ridges, one nail should be placed on each side of the shingle between the sealant and the exposed part of the shingle, 1 inch from the edge.