Homefix: Putting shingles over wet plywood likely to lead to problems
Q: Shortly after it was built, we noticed a slightly lifted-looking horizontal line (shingles not flat as the others) on the upper-left area of the roof when looking at the front of the house. The builder said they would "keep an eye on it." The roof got progressively worse over several years and we complained. They came back and did some work on it from inside the attic and said that should fix the problem. Well, it is worse today. We see the underlying plywood outlines when the sun is just shining over the roof in the morning. The builders put the roof on during rainy weather, so I suspect that is the root of the problem. The contractor will not answer my phone calls or registered letters and our home is now 8 years old. What might be going on? Thankfully, we have no leaks.
A: It is never a good idea to install a roof over wet plywood or any type of roof sheathing. Trapped water will boil when the sun heats the roof and the shingles will then have small pits where the steam finally escapes.
Installing shingles over wet decking also will void any warranty you might have from the shingle manufacturer. Most roofs' rafters are set 2 feet apart and the plywood decking should be a minimum of 7/16 of an inch thick. To reduce costs, some builders choose to use a thinner plywood deck that is only 5/16 of an inch thick. The thinner plywood is prone to cupping in between the rafters, but is approved by some building authorities.
The line you are seeing is the long edge of a 4- by 8-foot sheet of plywood that has cupped or curled because of moisture variations in the wood or improper fastening of the plywood to the rafters.
The plywood is nailed to the roof rafters, and unless the builder used a tongue-and-groove plywood or "H" clips, the long edge can warp in between each rafter. When the workers returned to the attic, they most likely added a 2- by 4-inch block between two or more of the rafters and then tried to secure the warped edge of the plywood using wood screws. What they were trying to do was to force the wood to bend back to normal.
Most of the time this type of repair is unsuccessful because the screws used to secure the plywood are too short. Longer screws would come through the surface of the shingles where the roof could then leak.
To properly repair the cupped plywood, some of the shingles need to be removed to provide direct access to the decking. Unless the roof is leaking, the line you see on the roof is an aesthetic problem, but the cupped plywood is putting an added strain on the shingles either side of the visible line. To avoid the possibility of a roof leak, have the shingles removed near the cupped plywood, make the repairs from the roof side and replace the shingles. Have the work done on a warm day when the shingles are pliable. By using care, the shingles that are removed can more than likely be salvaged and reinstalled.
Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at C. Dwight Barnett, Evansville Courier & Press, P.O. Box 268, Evansville, IN 47702 or email him at d.Barnett@insightbb.com. Distributed by Scripps Howard News Service.