Reducing Wind Damage Vulnerability When You Re-Roof
How Weak Roof Connections Increase Wind Damage
Most existing houses contain a weak link in the connection of the roof sheathing to the rafters or roof trusses. This makes them vulnerable to loss of roof sheathing in a severe windstorm such as a hurricane, downburst, microburst or tornado.
The reason is that nail sizes and spacing used to attach the sheathing to the roof’s structural members (rafters or trusses) do not provide enough strength to keep the sheathing on during an intense windstorm. Before Hurricane Andrew devastated South Florida in 1992, roof sheathing was generally attached using 6d nails spaced at 6 inches along the edges of the sheathing and at 12 inches along interior structural members.
Tests at Clemson University’s Wind Load Test Facility show that the sheathing can be pulled off the rafters or trusses with a 40 pound per square foot uplift pressure when it is attached using these older nailing patterns. Use of 8d nails has become more common in recent years but this only increases the typical failure pressure to about 70 pounds per square foot. In contrast, a strong hurricane such as Hugo could exert uplift pressures as high as 100 pounds per square foot in critical areas of the roof if your house is in an exposed location.
Stricter requirements were adopted by most building codes in hurricane prone regions after Hurricane Andrew. However, most existing houses have been built using the older standards and even the new requirements do not provide a very good margin of safety (extra strength beyond the bare minimum).
There are several ways to improve the roof sheathing attachment. The cheapest and easiest is to re-nail or better yet, screw down the sheathing when you replace your roof covering. Re-fastening your sheathing and undertaking the extra measures outlined in this article will provide a significant reduction in the vulnerability of your roof to wind damage.
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Step 1: Remove All Roof Covering
Have your roofer remove all of the roof covering materials down to the sheathing. In strong wind areas you should never add a second layer of shingles over an existing layer.
Step 2: Inspect for Damage
Have the roof sheathing inspected for damage or decay. Replace any weak material with sheathing rated for your truss or rafter spacing (16 or 24 inch).
Step 3: Refasten Properly
Have the sheathing refastened using either 8d ringshank nails or #8 screws 2 inches long (see listing of auto-feed screw guns). The nails or screws should be installed between the existing fasteners and at a spacing of not more than 6 inches between the old and new fasteners. A smaller spacing (4 to 5 inches) is recommended if the ring-shank nails are used.
Also, if your roof has gable end overhangs, you should have nails or screws installed at 4 inches on center on the last truss or rafter.
Step 4: Sealing
Provide a backup level of water protection by installing self-adhesive roof or window flashing tape over the joints between the sheathing. Four- or six-inch wide strips are readily available in 100-foot rolls.
Have your roofer seal around roof deck penetrations with roof tape, sealant, or asphalt roof cement. Replace boots around pipes and flashing at wall or chimney intersections as needed.
Step 5: Underlayerment
Specify as a minimum, ASTM D 226 Type I (15#) asphalt impregnated felt paper underlayment with a 19 inch overlap. The felt paper should be attached using low profile capped head nails or thin metal tins and roofing nails. The tins or caps should have a minimum diameter of 1 inch. Fasteners should be installed at no more than 6 inch intervals along all seams or laps and at 12 inch spacing in the field.
Offset any vertical laps 12 inches. Have metal drip flashing installed along roof edges, nailed at a minimum spacing of 10 inches on center. If your house is within 3,000 feet of salt water, use hotdipped galvanized fasteners for the underlayment
attachment.
This underlayment specification corresponds to IBHS recommendations and the middle level option suggested in the FEMA Coastal Construction Manual. The FEMA recommendation for the highest level of protection uses a single layer of ASTM D 226 Type II (30#) felt paper nailed as above and covered by a single layer of self-adhered modified bitumen sheet complying with ASTM D 1970.
Step 6: Install Shingles
Install shingles with six nails per shingle or as recommended by the manufacturer. Consider specifying a Dade County, Florida approved shingle.
Use a starter course at the bottom of each slope and consider specifying hand tabbing shingles within 3 feet of roof edges. Shingles should not extend more than 1/4 to 1/2 inch beyond the edge of the roof deck.