Roof leaks

Published on January 26th, 2014 | by Henri

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Determining source of leak in garage roof

Q.    Today we noticed some water dripping down the inside back wall of the garage during a steady light rain. I went in the garage attic and noticed at the seam of the sheathing (½” plywood) four feet from the end of the roof, it was wet. It appeared to be running down the inside of the roof, hitting the wall and then dripping down (conveniently into an empty bucket we use for bird food).

I went up on the roof during the rain and didn’t notice anything anywhere near that location. There is a valley  a couple of feet away (to the side, not above). The wood on which the dripping occurred did not appear stained so this may have been the first time it leaked. How do I figure out where the leak is coming from and how do I get it fixed? The trusses are 2 feet on center. I am somewhat handy and might be able to take care of things myself with guidance.

A.    You haven’t told me how old the roof is and what type of roofing it is. I will assume that the roof is covered with asphalt or fiberglass shingles. Since you are handy, I will also assume that you would have noticed if any of the shingles had cracks or missing pieces, which can happen with fiberglass shingles that are 15 years old or more.

At this point, my guess is that the leak is occurring at the valley. If it is made of coil-stock aluminum, it is probable that it was nailed all along both its sides and that it has buckled. This is caused by temperature changes and, eventually, the thin metal develops cracks, which will admit rain.

A valley gets water from two roof planes so quite a bit of water runs down the valley, even in a gentle rain. Once the water has penetrated through the cracks, it can travel sideways following the rows of shingles until it finds a joint in the sheathing where it seeps through into the building. If that is the case, roofing cement applied over the cracks will stop the leaking for a while, but you will have to re-apply it every couple of years as the sun destroys it.

When a new roof is put on, have the roofers use heavier gauge metal with no run longer than eight feet. Each run should only be nailed at the top and held in place on its sides with clips so expansion and contraction are not impaired.

An ice and water protecting membrane applied on dry sheathing and extending a couple of feet on each side of the valley is also a wise thing to do.

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