Q: Our house was built in 1949 and has a split-level layout with a flat tar-and-gravel roof. Shortly after we moved in, a leak was discovered coming from the lower, smaller portion of the roof. It was replaced with a multilayer tar-paper version. The roof is fine except for a fairly large low spot. The water sits about an inch deep and remains four or five days before evaporating.
What is the easiest, most economical way to get rid of the standing water? I’m thinking of placing gravel in the low spot – but wondered if you had another suggestion.
A: Gravel is a bad idea. You may fill the puddle, but the water will remain on the roof, hidden by the gravel. Your goal should be to build the low spot of the roof up so that water runs off the roof and does not puddle.
Flat roofs are not really flat. They are built with a slight pitch (usually one to two inches of fall for every foot) to allow water to run off. Sometimes, when a tar-and-gravel roof is installed or when repairs are made, the surface is irregular and low spots develop. Usually they aren’t a problem, but a 1-inch-deep puddle that lasts for four or five days should be repaired. If you don’t fix it, you run the risk of that portion of your roof becoming saturated and a leak developing.
Several years ago Kevin had a similar problem on the mansard roof of a Victorian he owned in Alameda, Calif. He solved it by applying several layers of asphalt rolled roofing to the low spot to build it up and then applying two layers of nylon fabric saturated with an elastomeric coating. Elastomeric is just a fancy word that means the coating is rubberized and, when dry, moves with the roof.
These coatings have the look and consistency of marshmallow creme, although we wouldn’t advise eating it. They are available from roofing supply dealers and all home centers. We have had success with Snow Roof products.
To apply the rolled roofing, cut increasingly large circles of asphalt roofing material and cement them over the low area. Use a sharp utility knife and cut through the bottom of the material, not the pebbly surface.
Start with a 12-inch diameter circle and increase each subsequent circle by 4 inches – the next circle is 16 inches, then 20 inches and so on.
Make sure to cover the entire surface of each piece with roofing cement to ensure that each one sticks to the next. Pay particular attention to the edges.
It should take about 10 layers to fill the 1-inch dip. Once the low spot is built up, apply two layers of nylon fabric over the patch and saturate it with the rubberized coating. Thoroughly apply the coating to the patch, spreading it out several feet beyond the patched area.
After this dries thoroughly, spray the roof with water. Your puddle will be a thing of the past.
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