135 freeze-thaw days a year: why Denver chimney crowns crack

Denver Fireplace & Chimney · October 15, 2026

Where the 135 comes from

NOAA's 1991–2020 climate normals for Denver-Stapleton, the city's long-running central weather station, show about 156 nights a year at or below 32°F. Only about 21 days a year stay below freezing all day. Subtract one from the other and roughly 135 days a year go below freezing at night and back above it during the day. That is about one day in three.

Denver International Airport works out almost the same, about 134, and we cross-checked both against NOAA's daily records. Boulder comes in a little lower at about 125, but it gets far more snow: 92.8 inches in a normal year against 49 inches at Denver-Stapleton.

The pattern is the problem. Many colder places freeze in December and stay frozen. Denver's bright sun warms masonry quickly, so the same chimney freezes and thaws over and over through the winter.

How the cycle breaks a crown

The crown is the concrete or mortar slab on top of a brick chimney, around the flue. It is the chimney's roof. It takes the snow, the hail and the full force of sun and cold.

Concrete and mortar soak up water. When that water freezes it expands, and it pushes on the material around it. A hairline crack takes in a little water, freezes, opens slightly wider, and takes in more water at the next melt. With about 135 cycles a year, a crack that looked harmless in October can be a gap by March, and the corners of the crown start to break off.

Once the crown leaks, the damage moves down. Water gets into the top courses of brick, which then spall, shedding their faces. It runs down the outside of the flue tiles and can show up as a stain on the ceiling or as water in the firebox.

Why so many crowns were built to fail

Many crowns aren't really crowns. They are a layer of leftover mortar troweled over the top bricks, sometimes called a mortar wash. Mortar mixed for laying brick is not made to be a weather cap, and it is often thin at the edges.

A crown built to last is thick enough not to crack under its own movement, slopes away from the flue, and overhangs the brick on every side with a drip edge, so water falls clear of the chimney face instead of running down it. It also has a gap or flexible seal around the flue tile, because the tile and the crown expand at different rates. A crown poured tight to the tile will crack along that line.

Signs your crown is failing

Some are visible from the yard with binoculars. Others only show inside.

• Cracks or missing chunks at the top corners of the chimney.
• Brick faces flaking or popping off near the top of the stack.
• White powdery staining on the brick, called efflorescence, from salts carried out by water.
• Water in the firebox or on the damper after rain or snowmelt.
• A stain on the ceiling or wall beside the chimney.

Repair, replace and protect

Small surface cracks in a sound crown can be cleaned out and sealed with a flexible crown sealer. A crown that is a thin mortar wash, broken at the corners or cracked through is better replaced with a properly formed one. Typical crown work runs $600–$2,200.

When the bricks below have started to break, the job grows: repointing typically runs $500–$2,500 and replacing spalled brick $1,000–$3,500. A water-repellent treatment that lets the brick breathe, typically $300–$800, slows the next round of damage. If the top few courses are too far gone, rebuilding them runs about $1,000–$3,500.

The cheapest moment to deal with a crown is before winter, when the problem is still a crack. Our inspection is free. We go up, photograph the crown and the top of the stack, and give you a written price before any work.