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 Hurricane 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.  Read the rest of this brochure for more details.