What Should You Do About a Cracked Chimney Crown?

What the crown does

The crown is the slab across the top of the chimney, around the flue tiles. Its job is to shed water. Rain and snow that land on top of the chimney should run off the crown, drip clear of the brick face and fall away, instead of soaking down into the masonry below.

A true crown is a cast slab of concrete, sloped away from the flues and wider than the chimney so it overhangs the brick on every side, with a groove underneath called a drip edge. Plenty of older chimneys on brownstones, Jersey colonials and Philadelphia twins do not have that. They have a mortar wash: a layer of ordinary mortar troweled over the top bricks, often thin, flat and flush with the brick face. A wash can work for a while, but it was never as durable as a formed crown.

Why crowns crack

The top of the chimney takes rain, snow, sun and frost head on, and it is the first part of the masonry to suffer. On older homes in this region, the usual causes stack up in roughly this order:

• Freeze-thaw: water gets into fine surface cracks, freezes on a cold night, expands and pries the crack wider
• A thin mortar wash instead of a crown, which cracks and erodes far sooner than a cast slab
• A crown poured tight against the flue tile, so the tile cracks it from inside as it heats and grows
• No overhang or drip edge, so water runs down the brick face rather than dripping clear
• A poor mix or a crown that cured too fast, leaving it weak and prone to shrinkage cracks
• Settling of the chimney, which cracks the top slab along with the brick

What you can see from the ground

You do not need to climb to get a first read. From the sidewalk, the backyard or an upstairs window with binoculars, study the chimney top on a dry day. Chunks of concrete or mortar in the gutter or on the roof below the chimney are a strong sign the crown is breaking up. So are plants or moss growing on top, dark damp patches on the upper brick that stay after the rest has dried, and white powder or flaking faces on the top few courses.

Inside, rust on the damper, water in the firebox after a storm and a musty smell from the fireplace can point to water entering at the top, though a missing cap or failed flashing produce similar signs. Leave the roof itself to a crew with the right gear; chimney tops are high, steep and often slick.

Hairline crack or failed crown?

The right fix depends on which you have, and that is judged up close on the roof. These are the lines a technician draws:

• Fine surface cracks, crown still solid and firmly attached: seal
• Cracks wide enough to see into, or running all the way across: usually replace
• Pieces loose, lifting, hollow-sounding or missing: replace
• A thin, flat mortar wash that is breaking up: replace with a proper crown
• Damp, stained or spalling brick just below the top: water is already getting through, so the brick is part of the job

How crack sealing works

For a solid crown with fine cracks, the surface is cleaned of moss, dirt and loose material and allowed to dry. The cracks are opened slightly so the sealer can grip, then a flexible crown sealant is brushed over the cracks and the whole top surface. That coating stays elastic and moves as the crown heats and cools, which a rigid mortar patch or a bead of caulk does not do well.

The joint around each flue tile gets the same attention. If the crown was poured hard against the tile, a flexible seal there lets the tile expand without cracking the crown again. Sealing is a modest job, and on a crown caught early it can add many seasons of life.

When the crown has to be replaced

If the crown is broken through, crumbling or loose, sealer will not hold it together. The old crown or wash is broken out down to sound brick, and the top courses are checked. Bricks loosened by years of water are reset or replaced. Then a new crown is formed and cast, or a precast crown is set, depending on the chimney.

A well-built replacement slopes away from the flues, extends past the brick on every side with a drip edge underneath, and has a bond break and a flexible joint where it meets the flue tile, so the tile can move without cracking it. A cap usually goes on at the same time, since an open flue lets water straight into the chimney no matter how good the crown is.

Why another winter of waiting costs more

A cracked crown rarely causes trouble the day it cracks. The damage comes later and lower down. Water that gets past the crown soaks the top courses of brick, freezes, and pops the faces off the brick, which is spalling. It washes out mortar joints until bricks loosen. It runs down inside the chimney to rust the damper and stain the chimney breast, and it can wet the roof framing and attic around the chimney.

Each winter makes the crack, and the damage behind it, bigger. Sealing a crown is a small job. Rebuilding the top of a chimney, or the whole stack above the roofline, is not. Crown work also needs dry weather and temperatures above freezing for the materials to cure, so late summer and fall are the natural window before the freeze-thaw season starts.

What the visit looks like

A crew sets ladders or roof anchors, looks at the crown, the flue tiles, the cap and the top brick, and taps the crown to find hollow or loose areas. The flashing is checked while they are up there, since a leak blamed on the crown sometimes comes from the flashing instead. You see what they found and hear whether the crown can be sealed or needs replacing, with the reason.

Our own crew does the work, and you get the choice in writing: seal now with the expected limits, or replace with a crown built to shed water properly. Either way, the answer comes with what you saw, not just what you were told.