Chimney Waterproofing in Seattle's Wet Climate: What a Repellent Can and Cannot Do

The Western Regional Climate Center puts yearly precipitation at 32 to 35 inches for the lowlands running north to Seattle, close to the edge of the rain shadow cast by the Olympics. Local gauges agree. Sand Point averaged 35.50 inches a year from 1986 to 2016, and the Portage Bay station, on the Lake Union side, averaged 37.92 inches across roughly 151 days a year between 1972 and 1998. Both gauges sit toward the east and southeast of our area; we found no station record for Ballard, Phinney Ridge or Greenwood and do not quote one.

Direction counts as much as quantity. The same climate narrative has wet-season winds over the Puget Sound lowlands blowing from the south or southwest. That points to the south and southwest faces of a stack as the ones rain is pushed against, and they are where we look first for dark, saturated brick.

Brick and mortar are porous. They draw water in during a wet spell and give it back when the air dries. Trouble starts when the masonry never gets its chance to dry: mortar softens and washes out of the joints, white salts bloom on the surface, and water that freezes inside a soaked brick can break off its face. Sand Point logged an average of 24 days a year with a low of 32°F or colder, most of them in December, January and February, so the cold arrives in the same months as the rain.

Shade slows the drying further. Oregon State University Extension, writing about roofs in the Pacific Northwest, ties moss to moisture plus shade and notes that moss holds soil and debris, which keep a surface wet longer. A stack under a tall conifer, or on the north slope of a steep roof, lives in those conditions.

The product we apply is a breathable water repellent made for masonry. It soaks into the outer layer of brick and mortar and lines the pores, so rain forms beads and sheds from the face, yet vapor inside the wall keeps a way out. That second property is why paint and film-forming sealers are the wrong choice on a chimney: they hold moisture behind them.

A repellent is a maintenance coat with a limited life. One Seattle chimney company states on its site that the treatment lasts 5 to 10 years and suggests renewing it every 5 to 7 years on chimneys in this area. Read that as one company's published guidance. Exposure and the state of the brick set the real interval for a given stack.

A repellent sheds rain from sound masonry. It does not bridge a crack, refill a joint or seal metal to brick. Sprayed over those defects, it hides them for a season while water keeps coming in.

Two code points are worth knowing. Seattle's list of permit-exempt work names chimney repointing, although the work still has to meet code. And section R1003.20 of the Seattle Residential Code requires a cricket once a chimney measures more than 30 inches across, taken parallel with the ridge. A cricket is a little peaked saddle built on the uphill side. A wide stack lacking one collects water and debris behind it.

If the flashing, gutters or downspouts near the chimney are copper, tell whoever treats your roof for moss. OSU Extension warns that zinc sulfate moss treatments attack copper.

The masonry has to be dry before a repellent is applied, and the weather has to stay dry until it has cured. WeatherSpark's figures for Seattle give July about 0.7 inch of rain against roughly 7.5 inches in November, and they place the clearer part of the year between mid-June and about October 8. Summer and early fall are when this work is done well. Mortar repairs come first and need curing time of their own, so a stack repointed in June can be treated later the same summer.

We charge $320 to $795 to waterproof a chimney. Height, the amount of brick, roof access and the cleaning needed beforehand place a job inside that span. Repairs found during the inspection are quoted separately, line by line.

In print elsewhere: a Seattle company, The Mad Hatter Chimney Sweep, posts $200 to $500 per chimney, and Fixr's national guide gives $150 to $500. We opened both pages on October 4, 2026. Not our quote.

The starting point is the inspection, billed per chimney at $99 plus tax. Approval of the quoted work takes that amount off the invoice.