Freeze-Thaw Damage to Chimney Masonry in Redmond: How Much Frost Is There?

On WeatherSpark's averages, Redmond's cool season begins November 13 and ends March 1, some 3.6 months. December is the coldest month, with an average low of 37°F and an average high of 46°F, and the temperature rarely drops below 26°F. Snow is light: a snowy period of about 1.6 months, with January averaging around 1.6 inches.

For the wider lowlands, the Western Regional Climate Center gives January lows that average 28°F to 32°F and January highs that average 41°F to 45°F. Read those two ranges together and the point is clear. On a typical January day the temperature crosses the freezing line overnight and climbs back over it by afternoon. It is that crossing, repeated, that matters to masonry, far more than a single deep freeze.

We could not find a published count of freeze-thaw cycles per year for any of the four towns, so we will not invent one. The nearest long-record measurement is from the Monroe cooperative station, down the Snoqualmie Valley, north of Duvall, where the daily minimum reached 32°F or lower on an average of 58 days a year in a summary covering 1929 to 2012. That is a count of frost days at one station, not of cycles in Redmond, but it shows that frost nights are a routine part of winter in the valley, not a rarity.

Averages hide the bad years. Redmond's hazard plan records the 2019 event, when snow fell over nine days and totaled more than 20 inches, while noting that severe winter weather is not typical for the city. Sammamish's plan states that the plateau's altitude brings heavier snow accumulation than surrounding communities and that ice closes many of its steep roads. Go back far enough in Duvall and HistoryLink describes three feet of snow collapsing barns and houses in 1915, followed in 1916 by 40 inches of snow and six inches of ice.

A week of snow sitting on a chimney crown, melting by day and refreezing at night, does more to masonry than a whole ordinary winter. That kind of week is when marginal masonry gives way.

Water expands by roughly nine percent when it turns to ice. Inside a dry brick with a little moisture in its pores, that expansion has room. Inside a saturated brick it has none, and the pressure breaks the pore walls. The fired face separates from the body of the brick and falls away; that is spalling. In mortar the same pressure turns the joint sandy and lets it wash out.

Redmond's cool season sits entirely inside its wet season, which by the same WeatherSpark averages runs from early October to early May, with November the rainiest month. By the time frost nights arrive, an unprotected chimney has had six or seven weeks of regular rain and very little sun to dry it. A mild frost on soaked masonry does more harm than a hard frost on dry masonry. That is the whole local story in one sentence.

Age is part of it. Census ACS estimates for 2020 to 2024 show that about 23 percent of Redmond's housing units were built between 1960 and 1979, and roughly a quarter of all units date from before 1980. Carnation has the oldest stock of the four towns, with about 14.5 percent of its homes built before 1960. A chimney from those decades has been through forty-five or more wet winters, and unless it has been repointed, its joints are the original mortar.

Exposure is the rest. A chimney on an outside wall is cold on three sides. A tall stack on a ridge or hilltop gets more wind-driven rain. The crown matters most of all: Washington's residential code requires the top of a masonry chimney to be sloped so water sheds, with a drip edge that throws it clear of the brick. Many older crowns are flat mortar washes with no overhang, so every rain runs straight down the top courses.

Walk around the house on the first dry day and look at the chimney with binoculars. Check the ground and the roof near its base for thin, curved flakes of brick, which are spalled faces. Look for joints that have receded behind the brick edges or show gaps. Look at the crown for cracks or missing chunks, and at the top three or four courses for bricks that seem to have shifted. White powder on the brick shows that water has been moving through the wall.

Inside, a gritty layer of sand or mortar on the smoke shelf or in the firebox after winter is a sign that joints in the flue or smoke chamber are shedding as well.

The sequence is always top-down and structure-first: rebuild or resurface the crown, replace spalled brick, repoint open joints, and only after the new mortar has cured apply a breathable water repellent to slow the next cycle. Doing these out of order wastes money, because a repellent cannot close a crack.

Our masonry repair work, which covers crowns and repointing, starts at $475. General chimney repair on brick and mortar starts at $425, and waterproofing is $285 to $790. The scope decides the figure, so each job is quoted in writing after an on-site inspection billed at $99 plus tax; that $99 comes off the total if you go ahead. Masonry is best done in the dry months, roughly mid-June to early October here, so a chimney that showed damage this winter should be on the calendar by early summer.