Wind-Driven Rain and Chimney Leaks

Why only some storms

A chimney stands above the roof, exposed on every side. The Mason Contractors Association of America (MCAA) describes masonry chimneys as facing months of exposure to wind-driven rain and melting snow each winter.

Wind changes how rain meets a vertical surface. In still air, little rain lands squarely on a wall. In a storm with strong wind, the face toward the wind is soaked continuously, and the pressure of the wind helps drive water into any gap. The Brick Industry Association (BIA) rates wind-driven rain exposure as severe on the Eastern Seaboard, where hours of rain can come with high winds, and states that under normal conditions it is nearly impossible to keep heavy wind-driven rain from getting through a single layer of brickwork, however good the materials and workmanship.

So a leak tied to one storm direction is not a mystery. It means the face exposed in those storms has a path that calmer rain does not reach.

Where wind pushes water in

• Between brick and mortar. BIA explains that water usually enters brick masonry through minute separations between the brick units and the mortar, and that very little passes through solid brick or solid mortar.
• Head joints. MCAA calls incompletely filled head joints, the short vertical joints between bricks, the most common source of leakage in brick masonry.
• Joint shape. MCAA recommends only concave, vee or compacted grapevine joints for chimneys, since these shed water. Raked joints leave a ledge where water sits against the brick.
• The crown edge. MCAA recommends a chimney cap (crown) that extends beyond the chimney walls with a drip at its edge. Without that overhang, water blown across the top runs straight down the face below.
• Flashing on the windward side. Wind can push water up and sideways under flashing laps that shed calm rain well.
• An open flue. Without a cap, slanted rain enters the flue directly. See chimney cap types.

Reading which face is involved

Where the water shows up inside often points to a side of the chimney:

• Water on the ceiling or wall on one side of the chimney breast suggests the flashing or masonry on that side.
• Damp in the firebox or rust on the damper during windy storms points to the top: the cap, crown or upper flue.
• Stains that appear a day or two after a storm and fade slowly suggest water soaked into the masonry rather than a direct path.

Compare that with the wind direction of the storms that caused trouble. If every leak followed a storm with wind from the east, the east face and the parts of the top exposed to the east are where to look first. These are clues for the inspection, not a diagnosis.

Keep a storm log

A few entries are more useful than a general sense that it leaks sometimes. Wind direction and speed are available after the fact from the National Weather Service or most weather apps.

How a leak is tested

Because the leak needs specific conditions, it may be dry on the day of an inspection. Visual checks of the crown, cap, joints and flashing come first, along with the attic side when it is safe and accessible. When the source is still unclear, we use controlled water testing.

The idea is to recreate the storm one area at a time. Water is applied to a single part of the chimney, such as one face of the flashing, for a set time while someone watches inside for water. Then the next area is tested. Testing lower areas before higher ones keeps water from above from confusing the result. It is slow by design, since a result is only useful when you know exactly which area was wet.

A RILEM tube test, a small graduated tube sealed to the brick face, is sometimes mentioned in this context. BIA explains that it measures how fast a tiny area of masonry absorbs water and that the results vary too much to describe a whole wall. It does not test flashing and is best used to compare surfaces, such as brick before and after a water repellent.

What fixes wind-driven leaks

The repair follows the path that the testing finds:

• Open or eroded joints. Repointing with a compatible mortar and a joint profile that sheds water. See repointing vs. tuckpointing.
• A flush or cracked crown. A crown that overhangs the brick with a drip edge, or repair of the existing one. See crown coating vs. crown rebuild.
• Flashing. Resetting counterflashing into the joints, or replacing step and counterflashing on the leaking side.
• An uncapped flue. A cap sized for the flue.
• Absorbent brick on an exposed face. Only after the repairs above, a breathable water repellent may help. Our guide on when to waterproof a chimney covers the timing.

Why the damage outlasts the storm

A wind-driven storm can push far more water into a chimney face in a few hours than calm rain does in a week. That water has to dry out through the same brick and joints it came in through, and on a shaded or north-facing chimney in cold weather, that can take a long time. While the masonry is wet, a hard freeze can damage the face. That is how a leak that seems minor inside the house can still be connected to flaking brick and crumbling joints outside. See why chimney brick flakes.

It is also why the inside stain sometimes shows up the day after the storm. Water soaked into the masonry keeps moving after the rain has stopped.

Chimneys that get the worst of it

Some chimneys see more wind-driven rain than others. A tall chimney that rises well above the ridge, an exterior chimney on the side of the house facing open ground or water, and a chimney on a house with no nearby trees or buildings to break the wind all take more exposure. BIA notes that older masonry buildings often relied on overhangs, cornices and other projections to keep water from sheeting down their faces; a chimney above the roof has none of that shelter. These chimneys benefit most from tight joints, a good crown and sound flashing.

Questions homeowners ask

Sources

1. Brick Industry Association, Technical Note 7: Water Penetration Resistance, Design and Detailing (2017)
2. Mason Contractors Association of America: Masonry Chimney Checklist for Weather Resistance
3. Brick Industry Association: RILEM Tube Testing and Brick Masonry (2023)

This guide is general information. It can't diagnose a specific chimney; an inspection can.