Doors to roof decks are notorious for leaking. The issue is there's typically nothing protecting the door from the elements and since they are above ground level, when they leak you know about it. Also, since most doors to decks swing in, it's easier for water to get into the cracks, pool on the threshold and drip in. Another issue is the brick molding around it, if not treated properly, can get saturated and allow water penetration. Remedies are: replace with out swing door, add storm door, replace brick mold, and make sure all gaskets are new and in good condition forming a tight seal.
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