I’ve installed and repaired a lot of siding in the Des Moines, Iowa, area over the last 30 years — enough that I now specialize in troubleshooting and consulting on exterior renovations. When a builder or homeowner calls me about a water intrusion problem, I usually find that shoddy workmanship or poor detailing led to a materials failure. However, in the case of the nine-month-old house shown on these pages, the source of the problem was less obvious.

The original builder had responded to homeowner complaints about dampness and water stains around a window — a double unit centered in a facade clad with manufactured-stone veneer — by reflashing it. But his repair only seemed to make the problem worse, so he asked me to help find the source of the leakage and suggest a way to fix it. When I...

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