Last week, a reader asked if we knew of a quick fix for a leak that showed up after her garage was expanded and a front porch was remodeled.

We offered a temporary solution, giving her a primer in the sometimes-confusing world of caulk. Today, we would like to tell her how to begin getting rid of the leak once and for all.

Water is the bane of all homebuilding, whether new construction or renovation. It will seek the weak point in the structure and find its way inside. What’s more, leaks can be tough to locate. Their source and the place they ultimately show up inside the building can be many feet apart.

Kevin had a persistent leak in his Mansard roof in Alameda, Calif. Mansard roofs, sometimes called French roofs, have steeply pitched, nearly vertical sides that connect with the building’s walls. The top of the roof is pitched only enough to shed water and is almost always invisible from the ground. This created more living space and, at one point in history, a tax dodge.

In 18th-century Paris, buildings were taxed by their height to the base of the roof. This height was measured only to the cornice line, so any living space contained in a Mansard roof was tax exempt.

Kevin started searching for the leak about two years into his seven-year stay in the house. Logically, the leak should have been a flaw in the flat roof, so he put on a new roof. The leak persisted and Kevin remained befuddled for years. But just before he put the house up for sale, he discovered water was infiltrating the steeply pitched sidewall shingles.

We’re willing to bet that our reader’s leak is in one of three places:

  • Where the porch was reconfigured to connect with the garage addition.
  • At the point where the new roof structure meets the old.
  • Where the new roof structure meets the siding.
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