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We have a lead lined, concave roof on a bay window. The roof itself has a small drain.

Inside the house, below the bay window we have a small leak that occasionaly appears when it rains. I have attempted to seal up the roof using bitumen tape and most recently painted with elastomer based roof seal paint (really thick rubbery paint).

The leak is much reduced. However there is still a small one, sometimes.

Any tips on finding the source of the leak in such a situation ? My current plan is to go up with bucket of water and assess possible holes by pouring water in them and waiting for any leak (there was nothing particularly obvious last time I was up).

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The best way to water test a roof is to have one person on the roof with a hose and another inside watching for water to begin entering. The idea behind a water test is a process of elimination. You want to start the water test at the lowest point of the roof closest to the interior leak. Turn the water on, wait 15 minutes, then start moving uphill and in a progressively wider arc every 15 minutes.

Considering that you've already coated the roof with elastomeric paint, it's likely that the leak could be coming from the edge detail (coping metal, drip edge, fascia, etc), a flashing, or even through the frame of the bay window itself.

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You could have someone stand on the roof with a hose and attempt to find the leak that way. It would simulate rain falling on the roof and falling to that one location. Just don't shoot the water under the shingles... You could then be inside looking for the leak.

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