Staying on Top of Roofing Problems

Is it okay to build a roof right over the existing one?
By: Dwight Barnett
Related To:
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Q: We recently had a roof put on our house. When the contractors got started, one of them fell through the roof and into our kitchen. The contractor said our roof was too far gone to save, so he suggested the best way to fix the problem was to build another roof right over the top of the existing one. Is it OK to do that? We wanted him to just tear the whole thing off and put in new trusses, but he said this way would do the same thing and it would be cheaper. Then he put in a ridge vent instead of gable vents with that old roof up there. Is this ridge vent doing us any good and should we go ahead and have someone install gable vents with an attic fan?

A: As long as there is room for ventilation between the old and new roofs, the reroof should be OK. I sometimes see homes with more than one roof, but it's usually where a flat roof has been changed to a pitched roof for aesthetic purposes. There should be at least 2 inches of clearance between the roofs. If a ridge vent has been installed, you must utilize soffit (overhang) vents for proper operation and venting. A continuous soffit vent is preferable. Do not use gable vents, turbines, fans or can-style vents in conjunction with a ridge vent.

Q: I installed a garage about 10 to 12 years ago. I live in the Detroit area. The shingles are standard 3-tab, 25-year, and I believe they are Owens Corning that I purchased from The Home Depot. I cannot find my receipt. The garage is an A-frame construction, 26 feet by 24 feet, with the A facing east and west. There are two roof vents, one on each end of the roof. On the south side of the roof or A-frame, the shingles are curling and disintegrating. The pebble material is actually coming off and winding up in my gutter. There was tarpaper applied, and I did the installation myself. By the way, the interior is open to the rafters. There is no ceiling inside. Can you advise me on what has gone wrong (and) if it is my fault or if there is a problem with the product?

A: It is typical for the south, or sunny, side of the roofing material to wear faster than the north, or shaded, side of the roof due to ultraviolet (UV) radiation. Curling shingles indicate poor ventilation, but I seldom see this on a garage, which is neither heated nor cooled.

One thing you failed to mention is the angle of the roof. Under similar conditions, shingles on a steep-pitched roof last much longer than those on a low-pitched roof. If you used felt paper under the shingles, then I assume we are talking about a single layer of shingles.

If you have more than one layer, the life expectancy of the additional layer is reduced by about 20 percent. That equates to about five years off the normal life expectancy. Add another 40 percent for a third layer, but do not add a fourth layer.

In Detroit, two layers may be too much for the roof structure when you have to consider the weight of snow on an unheated garage. A single layer of asphalt/fiberglass shingles on a medium-pitched roof that has proper ventilation should last more than 25 years.

You really need that receipt. According to Owens Corning's website, you would have used its Supreme AR shingles.

(Dwight Barnett is a certified master inspector with the American Society of Home Inspectors.)

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