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by CareyBot

Q: I enjoyed an article written a couple of years ago about puddling on flat roofs. I have a slope problem on a 12-by-10-foot one-story addition attached at one edge to a higher wall of my split-level house.

It is a tarpaper roof, applied by professionals over a flat wood surface. They had no idea how to add a slope after I said the roof was gathering several inches of water after heavy rainfalls.

A different roofer managed to lower the escape outlet an inch or so, but we still have too much accumulation. There is no leak, and the roofer tells me not to worry. But I don’t like all that water sitting there.

A person at a roofing yard suggested a peel-and-stick roofing material that I could layer, as you suggested, beginning at the overflow spout edge and building up to the outer edge.

Also, is there any reason not to use something called Low Slope Peel and Stick, using the cement at the edges, rather than asphalt rolled roofing, cementing each layer? I would still add the saturated fabric on top.

A: The problem you have is the reason we despise flat roofs. No matter how well the roof is installed, low spots that gather water always seem to spring up. But take heart, there are worse problems.

In San Leandro, Calif., where we grew up, there are many homes built in the 1930s and ’40s that were built not only with flat roofs but also with parapet walls to hide the rooflines.

The parapets were built with scuppers to remove the water from the roof, but when the scuppers become plugged, as they inevitably do, a dam is created that traps water on the roof.

We have seen houses where the water slowly seeps down the walls and over time rots the wood framing from the foundation sill to the roof. When discovered, usually at a termite inspection when a house is being sold, the cost to repair can be huge.

In your case, there is no easy solution. Kevin faced a similar problem at his home in nearby Alameda and was able to solve it by filling in the low spots with built-up rolled roofing. This, however, did not stop water from puddling; it just minimized it.

If you want to keep the water off your roof for good, you need a more radical solution.

Kevin’s neighbor had a situation similar to yours. She had an addition constructed with a flat roof on the rear of her Victorian home. The solution was to frame a sloped roof over the existing flat roof.

Creating a sloped roof on your 10-by-12-foot addition isn’t as difficult as it might seem.

Six or seven 2-by-6 rafters, 4 to 5 sheets of 1/2-inch plywood or OSB (oriented strand board), some roofing felt and a couple of rolls of asphalt roofing material will solve your problem once and for all. If you frame the new roof with a slope of 2 inches for each foot, it will only add 10 to 12 inches to its height. Try to orient the change so that it does not affect the look of the house from the street.

If this method doesn’t work for you, and you must try to build up the existing roof, we’d be leery about using a peel-and-stick product over an existing roof. Generally, peel-and-stick products require a clean, dry surface to adhere properly. Unless your roof is pristine, the bottom layer could fail. Then all your work will be for naught.

Bill and Kevin Burnett will attempt to answer your questions although the volume of e-mail sometimes makes this impossible. Contact them at sweat-equity@comcast.net.

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