What Is the Difference Between a Chimney Crown and a Cap?

The crown: the roof of the chimney

Look at the top of a masonry chimney and you will see clay flue tiles sticking up out of a flat or sloped slab. That slab is the crown. It covers the last course of brick so rain and snowmelt cannot run straight down into the masonry and the mortar joints below.

A well-built crown slopes away from the flue tiles so water runs off, overhangs the brick faces so it drips clear of the wall, and leaves a small flexible joint around each tile. That joint matters because the tile heats up and expands faster than the crown around it; packed tight, the tile cracks the crown from the inside. On a tall stack, the crown is also the first thing hit by wind-driven rain and the last surface to dry, so small defects there grow faster than anywhere else on the chimney.

Mortar wash: the crown that is not really a crown

Many chimneys on brownstones, rowhouses and pre-war brick homes were finished with a mortar wash instead: a thin layer of ordinary mortar troweled over the top course and sloped roughly toward the edges. It looks like a crown from the street, but it is too thin and too brittle for a surface that takes direct weather.

Freeze-thaw winters find it quickly. Hairline cracks open, water gets in, freezes and widens them, and pieces lift away from the tiles. A mortar wash with cracks is usually replaced with a proper crown rather than patched, because patching a thin layer adds more of the same weak material.

The cap: a hood over the flue

The cap is a separate metal piece, usually stainless steel, galvanized steel or copper. A solid top stops rain falling into the flue, and mesh sides let smoke and exhaust out while keeping birds, squirrels, raccoons and leaves from getting in. The mesh also stops larger embers from landing on the roof.

A cap can cover a single flue, clamped around the tile or with legs screwed into the crown, or it can cover the entire top of the chimney when several flues sit close together. Because it is fastened on rather than built in, a cap can be removed for cleaning access and replaced when it rusts or blows off. The fastening is the weak point: galvanized caps rust at the base first, and caps pressed onto a tile with no clamp are the ones found in the yard after a windstorm.

Side by side

The two parts sit within inches of each other, which is why they get confused, but they solve different problems and are repaired with different skills.

What each failure looks like from inside

A crown problem shows up in the masonry. Water that gets past a cracked crown travels down through the brick, so you may see white mineral deposits (efflorescence) on the chimney breast in the attic or top floor, brick faces flaking off outside (spalling), or a damp, stained patch on the wall beside the fireplace that comes and goes with the weather.

A cap problem shows up in the flue. Rain falling straight down lands on the damper and the smoke shelf, so you see rust flakes or puddles in the firebox, hear dripping after storms, or smell damp soot. Twigs, leaves and nesting material in the firebox, or animal noise in the flue, point to a missing or broken cap.

What happens when one is missing

With a good cap but a cracked crown, the flue stays dry while water seeps into the brick around it. Over several winters that water freezes, loosens the top courses and stains the walls inside. With a good crown and no cap, the brick stays sound but rain falls straight down the flue, rusting the damper, and the open hole invites animals to nest.

That is why a chimney inspection covers both. Fixing one and ignoring the other leaves half the top exposed, and the repair you paid for gets blamed for a leak it was not designed to stop.

Crown caps and full-width caps

People sometimes search for a crown cap. The term usually means one of two things: a full-width cap that covers the entire top of the chimney and sits on the crown, or a cast concrete crown made in one piece and set on top of the brick.

A full-width cap is useful on multi-flue stacks, common on rowhouses and older homes with several fireplaces, because one hood replaces several small caps. It still needs a sound crown underneath, because its base is fastened to the crown and wind-driven rain that gets under the hood still lands on the crown. A crown in poor shape is repaired before a full-width cap goes on.

What you can check without going up

A few observations from the yard, the attic and the fireplace tell you whether the crown, the cap or both deserve a closer look. None of them require a ladder.

Report what you see when you book an inspection. Knowing that water appears in the firebox after storms, or that the attic chimney wall is white and damp, points the technician to the right part of the top before anyone climbs up.

• A cap visible on every flue tile, sitting straight rather than tilted
• Rust streaks running down the brick from the top of the chimney
• Flaking brick faces or open joints near the top of the stack
• White deposits on the chimney where it passes through the attic
• Water, rust flakes or twigs on the damper or the firebox floor

Which one to fix first

When both need work, the crown usually comes first, because a new cap fastened to a crumbling crown loosens with it, and crown replacement means removing the caps anyway. If only one is failing, fix whichever is letting water in now and plan for the other. A homeowner who is replacing the roof should also have both checked beforehand, since crown work is easier before new roofing goes on below it.

Both are looked at during a yearly chimney inspection, which NFPA 211 calls for on chimneys, fireplaces and vents. Catching a crown while the cracks are still fine, and a cap before its base rusts through, keeps both repairs small.