Why a Westchester roof leaks after heavy rain but not light rain: wind-driven rain, flashing, and dry-season sealant failure.
By Wrench Pulse Roofers · Roofing guides · Published · Last updated
Heavy rain tests a roof differently from light rain: wind drives water under flashing and into details that light rain never reaches, and a flashing or sealant failure that survived the dry season only shows itself under sustained, wind-driven rain. The usual entry points are valleys, kick-out flashing, chimney and wall flashing, skylight curbs, and tile underlayment that has already failed beneath intact tile. The fix is at the entry detail, not at the stain. Related: roof leak repair and skylight repair.
A heavy-rain leak is a different failure from a light-rain leak, and the difference explains why a roof can seem sound all summer and fail in the first sustained storm. Light rain falls vertically and sheds off the planes; heavy rain, driven by wind, is pushed sideways — under flashing, into laps, through gaps that vertical rain never reaches. A flashing or sealant failure that survived months of light rain, and the entire dry season, only shows itself when wind-driven rain arrives with enough force and duration to exploit it.
The entry points are the junctions, in a predictable order. Valleys collect the most water and fail where two planes meet; kick-out flashing, the small diverter that throws valley water into the gutter, fails by absence — where it was never installed, water runs down the wall instead; chimney and wall flashing fail where rigid masonry meets a flexing deck; skylight curbs fail at their corners; and tile underlayment, already failed beneath intact tile, admits water that the tile itself never showed. Each entry is a detail, not a plane — which is why the fix is always at the detail.
The stain on a ceiling is where water finished its journey, not where it began. Water enters at a junction, travels along decking and framing, and drips at the lowest convenient point — often rooms away from the entry in plan, and always below it. Repairing the stain, or sealing above it, treats the symptom while the entry keeps admitting the next storm. The durable sequence is the reverse: trace uphill from the stain, find the junction, rebuild the detail, and let the stain dry as evidence that the entry is closed.
For Westchester’s coastal postwar roofs, the seasonal pattern sharpens the story. Sealants crack through the dry summer, then the first sustained winter storms push wind-driven rain through the gaps — which is why a leak that appears “suddenly” in winter usually began months earlier, in silence. A scope that names the entry detail — the valley, the kick-out, the chimney flashing, the skylight curb, the underlayment — is describing a repair; a scope that only promises to stop the stain is describing a cover-up.
Because wind drives heavy rain sideways, under flashing and into laps that vertical light rain never reaches. A flashing or sealant failure that survived light rain and the dry season only shows itself under sustained, wind-driven rain.
The entry point. Water travels along decking and framing before it drips, so the stain marks where water finished, not where it entered. The durable repair rebuilds the failed junction — valley, flashing, curb, or underlayment.
Contact Wrench Pulse Roofers · Call (213) 301-0900 · Open 24 hours, 7 days a week. Google Maps profile.