Replacing Plywood Roof Sheathing

House Doctor | Gary Dymski

February 15, 2001

Q. I live in a colonial home that is about 75 years old. There are currently three roofs on the house. I am in the process of hiring a contractor to put on a new roof. However, I am very confused. Half the contractors say that I need to strip the present plywood and start all over. The other half say that the existing wood should be fine because I don't have any leaks. Who do I believe? -Sam, Hewlett

A. You'll never really know the condition of the sheathing (the plywood layer underneath the shingles) until you strip away the old layers of roofing. I would expect that after 75 years and three layers of roofing material, however, some of the sheathing has rotted or become soft.

The contractors who expect to replace some or all of your sheathing probably have come to this conclusion from walking the roof. They can feel soft spots or areas that sag, and from this they determine the condition of the sheathing. Again, because of the years and layers, I would expect you'll have to replace quite a bit of sheathing.

Since icy weather has a tendency to bring out the worst in neglected roofs, many readers might be facing a decision on a roofing job. I have one rule when it comes to roofing jobs: One roof, one layer. Avoid urges to cover an existing roof with another layer, no matter what the local building code allows and no matter how much money you think you will save. Over the long run, multiple layers can mean multiple problems.

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Homeowners often fail to understand that a roof is not just a layer of shingles, but rather a system that includes the wood (both the framing and sheathing), ventilation and shingles (usually a combination of asphalt and fiberglass). A roof needs to breathe. With all those old layers, the system can develop problems, because it can't dry quickly or thoroughly. Also, three layers of shingles means a lot of extra weight.