How freeze and thaw works
Brick, mortar and concrete are porous. Water soaks in through cracks and the surface itself, then freezes. Water expands as it freezes, and that pressure pushes the masonry apart a little. When it thaws, more water gets into the slightly bigger gap. Minnesota goes through this cycle again and again every fall and spring, so damage that starts small grows every year.
Where it starts
Almost always at the top. The crown takes direct rain and snow, the cap keeps water out of the flue, and the flashing seals the roof line. When any of those fails, water gets into the brick right below, and that is where the damage shows first.
Signs you can see from the yard
Cracks in the crown, pieces of concrete or mortar in the gutters, brick faces flaking off, white staining on the brick, rust streaks from the cap, and loose or missing bricks near the top. Inside, look for water stains near the chimney or rust on the damper.
Fix it early or pay for it later
Early repairs are small: crown repair from $500, flashing repair from $250, cap replacement from $350, and brick replacement at $25 to $50 per brick. Wait a few winters and the same chimney can need a crown rebuild from $1,200 or a partial rebuild. Full prices are in our repair cost guide.
Prevention
A good cap, a sound crown with a proper overhang, flashing set into the mortar joints, and a breathable water repellent on the brick. A yearly inspection catches the small cracks before winter makes them big.