In California, contractors have been heard to grumble that there are only two types of French doors: the kind that leak and the kind that are going to leak. When they have the first kind, they call my company, TLS Laboratories in Laguna Niguel. We’ve investigated plenty of leaking French doors over the years, and in our experience it’s usually the installation that’s faulty, rather than the door itself.

A hinged exterior French door is something like a very large jigsaw puzzle with a small number of pieces. If the pieces are assembled correctly, the pair of sash should fully compress the weatherstripping against the jambs and one another. If the fit isn’t quite right, the weatherstripping won’t compress and water will seep in around the sash....

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