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What is the best way to waterproof a flat roof?

Answered by Brendan ~ April 12, 2012 ~ No Comments

We just put a new flat roof on a portion of our house that has very little slope (about a 2" over entire length). The new roof was covered with Ice and Water Shield and asphalt shingles, but now I am unsure about the long term water tightness of this. Was this the best way to waterproof the roof?

Tess ~ Columbus, OH

Brendan Fowler

Flat roofs always make me a bit uneasy even when done with every precaution taken. You say your roof has 2 inches of slope, for an absolute minimum slope of ¼ inch per foot, means your roof is less than 8 feet top to bottom. Under no circumstances should asphalt shingles be used on a slope less than 2 inches per foot. I am hoping that you have more than 2 inches of slope in any circumstances. A roof as flat as the one you have described should be covered with a membrane type roof. Even torch down asphalt will have a hard time draining at a ¼ inch per foot, because it does have slight surface variations due to overlap, but it could work. Ice and water shield covered by a metal roof could possibly work. The problem is that you will not find any roofers who will warrant any roof on that pitch other than membrane type of roof. Your asphalt shingles probably match other parts of the roof so if you are determined to change out the roof you will have a portion of the roof that will not match the rest. Call a local roofer that has experience installing membrane style roofs, and see if they have any ideas on how to make a membrane roof blend in with your other roofing. This type of roof is most commonly used in commercial and industrial applications but it can certainly be used on a private residence. I cannot say for sure that your existing is going to fail, but can say that it makes me very nervous. I hope this helped, good luck!

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