Breathable Waterproofing for Chicago Brick Chimneys

A chimney above a Chicago roof is wet more often than it is dry. O'Hare averages 38.4 inches of snow a year (NOAA 1991–2020 normals), and the stack collects drifts on its windward side, then sits in melt on every sunny afternoon. By our reading of the same NOAA normals there are about 78 days a year at O'Hare when the night drops below freezing and the day climbs back above it. Water that soaked into the brick in the afternoon turns to ice that night.

A breathable repellent lowers how much water the brick takes in. It sinks into the pores of the brick and mortar rather than forming a skin on top, so rain beads and runs off while vapor already in the masonry can still escape. We only put it on after the crown, flashing and joints are in good shape. No coating closes a crack or fills a joint that has eroded back.

Signs you need this

• Brick that stays dark for days after the snow is gone
• White salt bloom (efflorescence) streaking down the stack
• Faces of bricks flaking off each spring
• A damp spot or peeling paint on the chimney breast indoors
• Sand from the joints collecting in the gutter below the chimney

How we do it

1. Free inspection of the stackWe look at the crown, cap, flashing, joints and brick faces and photograph every place water can get in.
2. Repair the openings firstOpen joints, a cracked crown or loose flashing get fixed and priced on their own, so the repellent never goes over an active leak.
3. Wait for dry, mild weatherRepellent only soaks into dry masonry above freezing, so we book the coat for a dry stretch and brush off loose material first.
4. Saturate each faceWe work from the top down until each side stops absorbing, with the roof, siding and windows covered.

Why breathable, not paint or a glossy sealer

Water reaches a chimney from inside as well as outside. A gas furnace or water heater sends vapor up the flue, and the base can wick water from the roof deck. A film coating traps that moisture in the brick. When it freezes behind the film, the face of the brick pops off.

The National Park Service's Preservation Brief 2 says new mortar must be softer and more vapor-permeable than the brick it sits in. A repellent follows the same idea: it cuts the water coming in without sealing the surface shut. On Chicago common brick, which was made from local clay and hasn't been produced since 1981, that matters, because a spalled common has to be replaced with a salvaged one.

Where it pays off most

Tall stacks on two-flats and bungalows, chimneys that face the lake, and anything north of the city that catches lake-effect bands. In the November 9–10, 2025 event, the National Weather Service in Chicago logged 10.0 inches at Winthrop Harbor, 5.7 inches in West Ridge and 5.2 inches near Evanston, while O'Hare got 1.7. Snow that heavy piles against one side of a stack and melts into it for days.

It pays off least on a chimney with a cracked crown or failed flashing. Those come first, and sometimes they are all the chimney needed.

Cost and how long it lasts

Breathable repellent typically runs $150 to $820+ in 2026, depending on the height of the stack, the roof pitch and how much brick is exposed. The exact number is written down at the free inspection. Repellents wear with sun and weather, so at later inspections we splash the brick to see whether water still beads or has started soaking in.