Wind-Driven Rain: How a Derecho Gets Water Into a House

Iowa gets storms that behave less like rain and more like pressure. Sustained straight-line wind drives water horizontally into wall and roof assemblies designed on the assumption that water falls downward, and it finds gaps that never leak in an ordinary downpour. Call Des Moines Water Damage Restoration at (515) 400-0302 if water came in during a storm.

Why a building leaks in wind and not in rain

Siding, shingles, and window flashing all work by overlap. Each course sheds water onto the one below, and gravity does the rest. That system has no answer for water arriving from the side under pressure, because pressure pushes water up and under the laps rather than across them.

Wind also creates a pressure difference between outside and inside. Air pushing against a wall drives water into any opening it finds, and it keeps pushing for as long as the wind holds. A gap that would take a teaspoon of water in a normal storm takes gallons in a derecho.

Where it actually enters

Window and door heads are the leading candidate. Flashing at the top of an opening is the detail most often done poorly, and it is invisible once the siding is on. Water entering there runs down inside the wall and appears at the bottom of the opening or lower, which sends people looking at the wrong place.

Wall penetrations come next: hose bibs, dryer vents, electrical service entries, and the holes cut for air conditioning line sets. Each has a sealant joint that has aged, and each faces whatever direction the storm came from.

Roofs leak at the edges and at anything that interrupts the field. Wind lifts shingles at rakes and eaves and pushes water beneath them, and ridge vents that perform perfectly in rain can admit water when the wind drives directly at them.

Attics take water through soffit vents in the strongest events, which is why insulation can be wet with no roof damage at all.

The first day matters

Temporary protection is worth more than a permanent repair scheduled for next week. A tarp properly secured over a compromised area, or boarding over a broken window, stops the next round of weather from doubling the loss. Storms in this region arrive in series.

Photograph the exterior before anything is covered and the interior before anything is moved. Storm claims are settled on evidence of cause, and a tarp installed before any photographs makes the origin harder to establish.

Get water out of the building while the exterior is being addressed. Those are parallel jobs, not sequential ones, and every hour that saturated insulation sits against framing is working against the drying that follows.

Drying a wall that was wet from outside

This differs from a plumbing loss because the water entered high and traveled through the cavity. The wet area is frequently larger than the stain suggests and extends upward toward the entry point rather than only downward.

Insulation in an affected bay comes out where it has been saturated. Fiberglass holds water against sheathing and framing, and a wall closed up over wet insulation grows mold in a cavity nobody will open again for years.

Sheathing gets checked for moisture from inside, because the exterior may look sound while the panel behind the siding is soaked.

Before the next one

Walk the exterior on a dry day and look at the sealant around every penetration. Renewing failed caulk is inexpensive and it addresses the most common entry point directly.

Have flashing inspected at windows and at any roof-to-wall junction. Those details are the ones that decide how a house performs in the worst hour rather than the average day.

Insurance after a wind event

Wind-driven rain sits in an awkward place in most policies. Damage from wind creating an opening, and rain then entering through that opening, is typically covered. Rain entering through a pre-existing gap that wind merely exploited is frequently not, and the distinction turns on evidence rather than argument.

That makes same-day photographs of the exterior genuinely valuable, particularly of any lifted shingle, displaced siding, or broken seal. Note the storm date and keep local weather records, which are public and establish wind speeds at your location.

Expect an adjuster to look for prior maintenance. Records of roof or sealant work strengthen the claim considerably.

We cover Des Moines and the metro around the clock. Call (515) 400-0302.