Well, I was up on the roof of that 1960’s contemporary in Chevy Chase, MD again yesterday, and the problem is MUCH worse than I imagined. In addition to water vapor questions, we now know that the roof is actively leaking at every skylight (there are ten) and at the chimney.

In every case, the problem is faulty “flashing” – the material (aluminum, copper or bituminous membrane) used to protect joints between the main roofing material and the various items that pass through the roof. Now, I’ve seen a lot of roof problems over the years, but this beats them all, hands down.

We cut test holes into the flashings and water just poured out. Not a little trickle, but like when someone cuts an artery; the water spurted-out in streams. In twenty-five years as a residential architect, I’ve never seen such a total failure of roof flashing.

This bears a quick note about flashing; in all roofing systems, flashing is inherently the most vulnerable link because it manages the joint where the main roofing material is interrupted by something else — like skylights, pipes, chimneys, etc. Roofs rarely leak in the main body of the roof, but more often leak at these joints, where flashing is the primary defense. Faulty flashing = leaky roof.

And a word in defense of skylights: to paraphrase the NRA, “Skylights don’t leak on people – people (installing bad flashing) leak on people”. Most modern skylights are so water-tight you could use one for a boat. And properly designed and installed flashing WILL NOT LEAK. So don’t shy away from skylights, but DO invest in quality and make sure a true professional is in charge of specifying and supervising the installation.

Final thoughts… Do not hire a roofing contractor just because a friend had a good experience with them. That’s just putting your fate in the law of averages – and sometimes you’ll wind up on the wrong side of that average.