Chimney leak repair for Jersey City rowhouses and frame houses

A chimney is a brick box pushed up through the roof, exposed on four sides and across the top, and the joint where it meets the roof is one of the hardest spots on a building to keep dry. The roof and the masonry move differently with temperature, the brick absorbs water, and the chimney blocks the flow of rain running across the roof. Add freeze-thaw cycles and snow drifting against the uphill side, and it's easy to see why chimney leaks are among the calls we take most in winter.

In Jersey City we see two main settings. Frame two- and three-family houses in the Heights and Greenville often have a brick chimney rising through a pitched front roof, sealed with step flashing and counterflashing. Brick rowhouses downtown usually have chimneys on a flat roof, often several in a row along the party wall, sealed with membrane base flashing and metal counterflashing. Both fail in predictable ways, and both are fixable when the brick is still sound.

On a flat roof, membrane base flashing runs up each face of the chimney and is covered by counterflashing. The base flashing needs enough height that ponding water or drifted snow can't reach its top, and the corners need reinforcement because that's where cracks start. On a pitched roof, the sides get step flashing woven in course by course, with counterflashing over it, an apron at the front and a back pan or cricket at the rear. When one piece is missing or wrong, water finds the gap.

The most common shortcut we find is counterflashing that was never cut into the brick. Instead, it's surface-mounted and sealed with a bead of caulk, or the whole joint is smeared with roof cement. That holds for a season or two, then cracks. A proper repair grinds a clean slot into the mortar joint, sets the counterflashing into it and seals it with a compatible sealant or mortar. The methods are the same ones we use in roof flashing repair elsewhere on the roof.

Water trapped behind a wide chimney is its own problem. On a pitched roof a cricket, a small peaked ridge behind the chimney, splits the flow around both sides. On a flat roof, water dams against the uphill face and sits on the flashing for days, and a membrane cricket built during ponding water repair sends it toward the drains instead.

Not every chimney leak starts at the roof line. The crown, the concrete or mortar slab on top, cracks with freeze-thaw and lets water straight into the core of the chimney. A missing cap lets rain fall down the flue. Both produce leaks that look exactly like flashing failure. We seal small crown cracks with a flexible crown sealer, fit caps where they're missing, and check that the crown overhangs and sheds water past the brick instead of onto it.

Then there's the brick itself. Old soft brick and washed-out lime mortar absorb rain through every face, and that water travels down inside the chimney until it reaches the roof line or a ceiling below. Where the joints are open, they get repointed with a compatible mortar. Where the brick and joints are sound but thirsty, we apply a breathable silane or siloxane repellent so the chimney sheds liquid water while vapor from the flue can still escape. Paint and film sealers trap that vapor, and we don't use them on chimneys.

Many rowhouse and walk-up roofs carry old chimneys that no longer serve anything. Nobody maintains them, so the crown cracks, the cap rusts away, the mortar washes out and the base flashing gets buried under decades of tar. These abandoned chimneys are one of the most common leak sources we find on flat roofs downtown and around Journal Square.

Options depend on the chimney's condition. A sound one can be capped with a solid cover, its crown sealed, its brick treated and its base reflashed so it stops taking water. A deteriorating one can be taken down below the roof line by a mason, after which we close and waterproof the opening so it disappears into the roof surface and stops feeding water into the top floor. When the brick is spalling, the chimney leans or the mortar is mostly gone, flashing won't hold, and we say so plainly rather than sealing a wall that needs rebuilding first.