Waterproofing a Brick Chimney in Renton: Three Conditions and a Calendar

Brick looks solid and behaves like a very slow sponge. How much it drinks depends on how it was fired and how weathered its face is. Some chimneys shed rain for decades. Others darken within a minute of a shower and stay dark long after the driveway has dried.

The local record explains why absorption matters here more than in a drier region. Renton's hazard mitigation annex puts the city at 38 inches of rain a year, falling on 154 days. November, December and January each bring about 6 inches. And it is not only the rain: in January, the Western Regional Climate Center reports, relative humidity across western Washington hovers near 87 percent in the small hours and has dropped only to about 78 percent by late afternoon, while a typical winter month sees the sky clear, fully or in part, on no more than four to eight days. Wet brick in that air does not dry. It waits for the next front.

Shade makes it worse. The same annex mentions Renton neighborhoods that stand wholly inside tall timber, and the one for Newcastle describes many homes backing onto dense native growth. A chimney under a fir canopy gets little sun and less wind. Oregon State University Extension points out that moss depends on shade as well as moisture, which is why a green north face is a fair sign that the masonry behind it rarely dries.

A repellent lines the pores of brick and mortar so that liquid water beads on the surface. It does not fill a crack, bridge a missing joint or seal metal to masonry. Three parts of the chimney have to be right before it goes on.

The crown. Under Washington's 2021 residential code a masonry chimney must finish in a cap of concrete, metal or stone that is pitched to shed rain, carries a drip edge and is sealed, with a bond break, where the flue liner comes through. Many older stacks have only a thin wash of mortar up there, and once it cracks, rain goes straight into the core.

The joints. Receded or sandy mortar is repointed first. New mortar then needs time to cure before any treatment is applied over it.

The flashing. At the roof line, metal flashing and a bead of sealant are what stop rain from running into the attic. No product brushed onto brick will fix a lifted flashing.

Treating a chimney with any of these open is not harmful, but the good brick ends up protected while the leak carries on. We would sooner tell you that before the job than after.

Masonry has to be dry through its depth to accept a repellent, and it needs dry weather for the hours that follow. In this climate that turns the job into a question of dates. The table follows the year, using the rainfall figures of the Renton and Snoqualmie hazard annexes where they give one.

The practical consequence: a call in spring gets you a summer slot, with room for a crown or joint repair to cure first. A call in October usually means the treatment waits for the following summer, and only the repairs that cannot wait are done before winter.

Our service area climbs from Lake Washington toward the Cascades, and rainfall climbs with it. Renton's annex gives 38 inches a year. Snoqualmie's 2020 annex reports 61.25 inches a year since 1931 at an elevation of 410 feet, with about two-thirds of it arriving between mid-October and late February, and 11.5 inches of snowfall between November and April.

So a brick chimney in Snoqualmie or on the wooded lots around Maple Valley faces a longer soak than one on the flat ground near the lake, and its dry window is shorter. The treatment is the same; the scheduling margin is not.

A chimney takes in moisture from inside as well as outside: flue gases carry water vapor, and so does the air of the house. That moisture has to leave through the masonry. Repellents in the silane and siloxane family let vapor pass outward while stopping liquid water from entering, which is the whole point.

Film-forming coatings do the opposite. Paint, clear acrylic sealers and roofing compounds close the surface, moisture gathers behind them, and when it freezes the brick face comes away with the coating attached. Renton's annex notes that several days of freezing temperatures are not uncommon in winter, so this is not a theoretical risk here.

Waterproofing a chimney with us runs $320 to $795. How tall the stack is, how many of its four sides rise above the shingles, and the amount of washing the brick calls for beforehand decide where a given chimney lands. Crown, joint or flashing repairs are quoted on separate lines so you can see what each part of the job is. As a countrywide reference only, Fixr reports $150 to $500 nationally for sealing a chimney; that figure is not a local one and not our quote.

It starts with a visit to look at the chimney from the roof, $99 plus tax. Approve the quote and the $99 is deducted from the work.