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- Strengthen Roof Sheathing
Strengthen Roof Sheathing
Most houses contain a weak link in the connection of the roof sheathing to the rafters or roof trusses, making them vulnerable to loss of roof sheathing in severe winds, but solutions to this problem are available to the homeowner.
The problem exists because 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, like 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).
The attachment of roof sheathing can be improved in one of two ways. The cheapest and easiest is to re-nail or, better yet, screw down the sheathing when you replace your roof covering. The other approach is applying an AFG-01 rated adhesive to enable the roof sheathing to withstand pressure to 250 pounds per square foot or greater.
Before beginning such a project make a survey of your attic to get a sense of working conditions and the feasibility of the project.
- Is there a floor in the attic?
- Is there adequate lighting and ventilation?
- Do you have enough access to get adhesive out to the edges of the roof?
- Does attic insulation obscure the roof sheathing connection near the eaves or is there insulation between the rafters or trusses?
Next you’ll need to assemble the tools and materials that you will need to complete the job.
- Boards for the floor
- Caulking Gun for 30-ounce tubes of adhesive
- AFG-01 rated adhesive in 30-ounce tubes
- Cleaning supplies, such as rags and paint thinner
- Fan and/or light
- Utility knife and long screwdriver to open tubes
- Safety items, including glasses or goggles, and a bicycle helmet or hardhat for protection from roofing nails
- Caulk gun extension if you have a shallow pitch roof or problems with access to the eaves
Pick a cool day and preferably start in the morning when the attic will be cooler. Prepare the work area with boards, lights, and ventilation.
Install adhesive with strips of wood imbedded in the glue along the last rafter or truss at any gable end. Joints made using quarter round strips were about 50% stronger than those formed using only a bead of adhesive.
Apply adhesive along the connection between the roof sheathing and rafters in a continuous bead, much like caulk around a bathtub or apply adhesive to two adjacent sides of 1 by 2 blocks 6 inches long and space them with a 6-inch gap between the blocks. Apply beads or blocks to both sides of the rafters or trusses.