Why Seattle chimneys leak: moss, flashing and crowns

Seattle Fireplace & Chimney · October 14, 2026

It is the number of wet days, not the inches

Seattle's rain totals are not extreme. NOAA's 1991 to 2020 climate normals put Sea-Tac at about 39 inches a year, which is less than many East Coast cities. What stands out is how often it falls: measurable rain on about 156 days a year at Sea-Tac and about 178 days at Everett's Paine Field, roughly every other day. Tacoma Narrows sees 42 inches over 168 days.

A chimney rarely gets to dry out from October to spring. Water that gets into masonry stays there, and Sea-Tac still averages about 25 nights a year at or below freezing, which is enough to work on wet brick. Snow, at about 6 inches a year, is not the problem. Constant wetness is.

Flashing: where most leaks start

Flashing is the metal that seals the joint between the chimney and the roof. It is usually two layers: step flashing woven into the shingles and counter-flashing cut into the mortar joints of the chimney. When the counter-flashing pulls out of the joint, or roofing cement is used as a substitute, water runs straight down the side of the chimney into the attic or ceiling.

Leaks that show up as stains on the ceiling beside the chimney, or that only appear in wind-driven rain, usually point to flashing. Flashing repair typically runs $300 to $1,800, depending on whether it can be resealed or has to be replaced.

The crown and the cap

The crown is the concrete or mortar slab on top of the chimney around the flue. On older Seattle chimneys it was often just a thin layer of mortar, which cracks and lets water into the top of the brick. A rebuilt crown typically runs $600 to $2,200.

The cap sits over the flue and keeps rain from falling straight down it. A missing cap lets water into the flue, the damper and the smoke chamber, where it rusts metal and wears out mortar from the inside. Caps run $250 to $850. Factory-built chimneys in a framed chase have a metal chase cover instead; when it rusts or pools water, replacement runs $300 to $700.

Brick and mortar that soak up water

Brick and mortar are porous. The National Park Service's Preservation Brief 2 on repointing notes that masonry is subject to deterioration, especially at the mortar joints, and that salts carried by moisture can make the surface spall off. The white powder you may see on a chimney is efflorescence, those same salts left behind as water dries.

Two cautions follow from the NPS guidance. Repair mortar should not be harder than the brick, and anything applied to the outside should let moisture escape rather than trap it. That is why we use breathable water repellents rather than paint-like coatings. Waterproofing typically runs $300 to $800 and only makes sense after cracks and open joints are fixed.

What moss does around a chimney

Oregon State University Extension says moss traps soil and debris and holds moisture against the roof, and on asphalt and metal roofs it can lift shingles and let water reach the layers underneath. The uphill side of a chimney is a natural moss bed: shaded, slow to drain and full of needles from the trees our neighborhoods are known for.

OSU's advice is worth following. Trimming trees for more sun is the most effective long-term fix. Pressure washing can damage shingles. And zinc sulfate, a common moss treatment, corrodes copper gutters and flashing, so keep it off copper around the chimney.

• Clear needles and debris from the uphill side of the chimney each fall.
• Remove moss gently with a soft brush rather than a pressure washer.
• Keep zinc products away from copper flashing.
• Check the flashing whenever moss has been thick against it.

How we track down a leak

A ceiling stain rarely sits right under the source. Our inspection is free: we look at the chimney top to bottom, from the roof and from inside, take photos and give you a written price for the repair before any work. Typical 2026 leak repair runs $300 to $2,500, and general chimney repair $300 to $3,500. We book Sunday to Friday in 3-hour windows and call to confirm.