Can new plywood be installed on a 40-year-old roof without removing the old?
Roy Gingrich of Ellicott City has a 40-year-old house with an asphalt shingle roof surface. The existing asphalt shingles need replacement, so he plans to do a tear-off and re-shingle.
Because the plywood roof sheathing under the shingles has become brittle and weak, he plans to install new plywood as well as new shingles. But can he simply install new plywood over the old plywood sheathing to avoid the extra time, expense and attic debris that he believes removing the old plywood would entail?
Although I don't think installing new plywood roof sheathing over the old would be a grave mistake, I would not recommend doing it, for several reasons.
The new plywood will have to be nailed to the rafters or roof trusses. But, with the old plywood left in place, it will be considerably more difficult to locate the rafters to ensure proper nail placement. The risk is that many of the nails could miss the rafters, and will simply fasten the new plywood to the old plywood.
The result could be a poorly attached roof surface, more vulnerable to wind damage. This problem could be minimized by snapping a chalk line on the sheathing to mark the rafter locations before nailing - a simple and effective precaution, if it actually gets done. Also, make sure that the roofing nails used when the shingles are installed are long enough to penetrate both layers of plywood.
Another concern is that the roof will have added weight if new plywood is installed without removing the old.
A square - the equivalent of 100 square feet - of three-eighths-inch plywood, the minimum thickness I would expect to be used, weighs about 110 pounds. Since a standard three-tab asphalt shingle roof typically weights between 225 and 300 pounds per square, the extra plywood will significantly increase the weight of the roof you install.
That increase in weight should not be a problem with the single layer of asphalt shingles you will be installing, because it is normally safe to assume the roof structure is built to support the weight of two layers of asphalt shingle roofing.
However, when the shingles reach the end of their life, it might be inadvisable to install another layer of shingles at that time because of the weight already on the roof. Thus, a tear-off would be needed when re-roofing, an added expense.
Dean Uhler has been a home inspector for more than 12 years and is president of Baltimore-based Boswell Building Surveys Inc. Uhler is a member of the American Society of Home Inspectors (ASHI) and is the treasurer of the Greater Baltimore Chapter of ASHI.
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