Chimney crown vs cap: two parts with two jobs

Where each part sits at the top of the stack

The difference between a chimney crown and cap is clearest from the roof. At the top of a brick chimney, three things are stacked in order. The last course of brick forms the outside walls of the stack. Across those walls lies the crown, a slab that closes the top of the masonry and slopes down to its edges.

Standing up through the crown are the clay flue tiles, one for each fireplace or appliance the chimney serves. The Brick Industry Association's chimney design note wants the tiles to stand proud of the crown by 2 inches or more, which keeps water on the slab from running into the flue.

The cap sits on the tile, or over all the tiles at once. It is a metal hood held above the flue opening on legs or a frame, with screen mesh around its sides. So the crown covers the masonry, the tile carries the smoke, and the cap covers the tile. Each part keeps water out of a different place.

The code calls the crown a cap

Some of the confusion between the two words starts in the codes. Section R1003.9.1 of the 8th-edition Florida residential code is headed chimney caps, yet what it describes is a slab: every masonry chimney gets a top of concrete, metal or stone that sheds water by its slope, has a drip edge, and is separated from the flue liners by a caulked joint laid out under ASTM C1283. Masons call that slab the crown. BIA's chimney note uses the same word, cap, for the slab and calls the metal hood a rain cap.

The metal hood shows up in the code under a different name. Section R1003.9.2 covers spark arrestors: the screen that surrounds a hood. So when a home inspection report, a code section or a contractor mentions the cap, it is worth asking which one is meant: the slab across the brick, or the hood over the flue.

What a crown has to get right

What is a chimney crown for? It has one job, keeping water off the top of the masonry, and a few details decide whether it does it.

• Slope: the code requires it to shed water, so the top falls from the flue tiles toward the edges.
• Drip edge: water has to leave the crown clear of the brick face below. BIA's note shows the crown overhanging the brick and adds that where there is no overhang, the top two courses of brick should be corbeled out to form a drip.
• Bond break: the same code section wants a caulked, flexible joint where the slab meets each flue liner, so tile and slab can move separately as they heat, cool, wet and dry.
• Thickness at the edge: BIA advises against feathering a cast crown to a thin edge, because the thin edge breaks up.
• Crack control: BIA treats shrinkage cracking of cast concrete as a certainty, and calls for reinforcement and for flashing under a cast-in-place crown, which can also serve as the bond break with the brick.

What a cap has to get right

A cap's job is the flue opening. Its hood keeps rain from dropping into the tile, its screen keeps birds and small animals out, and on a wood-burning fireplace it holds back embers. It has to do those things without slowing the draft, and BIA's note suggests asking the cap's maker how a given design will change the flow of smoke before one is picked.

Where a screen is fitted to a masonry chimney, the Florida code sets its rules: open area of the screen no less than four times the flue outlet's own open area, mesh no weaker against heat and corrosion than 19-gage galvanized sheet or 24-gage stainless, openings that let no sphere bigger than 1/2 inch through and stop none smaller than 3/8 inch, and a screen or cap that can be taken off so the flue can be cleaned. BIA adds that a metal cap should be made of a corrosion-resistant metal.

What happens when one of them is missing or failing

The two failures look different, even though both end in water where it should not be.

With no cap, or a cap that is bent, torn or undersized, rain falls straight down the flue. It lands on the damper and the ledge behind it, rusts the damper plate, and can puddle in the firebox. Leaves collect above the throat and animals move in. The guide to water in fireplace after rain follows that path from top to bottom.

With a failing crown, the flue can stay dry while the masonry soaks. Water gets into cracks in the slab or around a failed bond break at the tile, runs inside the chimney's walls, and comes out as damp patches, white salt deposits on the brick, flaking brick faces and soft mortar joints lower down. A cap does nothing to stop that, however good it is. Our note on cracked chimney crown explains which cracks matter.

How the two depend on each other

A cap that fits over a single tile grips the tile, and the tile is held by the crown. A cover that spans the whole crown is fastened into the crown itself, with masonry anchors. So a crown that is cracked or crumbling makes any cap less secure, and the order of work matters: the crown is repaired or recast first, and the cap goes on afterward. When a crown is recast, a full-crown cap comes off and goes back on once the new crown has cured.

The pair also share the water load. A full-crown cap shades part of the crown top from rain, and a crown with a proper drip keeps water off the brick below the cap's legs. Neither replaces the other. A top-sealing damper, which mounts on the tile and closes the flue at the top, can stand in for a cap when it carries a lid and screen, but it does nothing for the crown either.

Factory-built chimneys have neither

Everything above applies to masonry chimneys. A fireplace built in a factory sends its smoke up a metal pipe from its own maker, and when that pipe is boxed in with framing and siding, a sheet-metal chase cover closes the top of the box, and there is no crown. The pipe itself ends in a termination cap that is part of the listed chimney system and is replaced only with the maker's part. If your chimney is a framed box with a metal pipe on top, the chase cover vs chimney cap comparison is the one that applies to you, and the two parts are described in detail there.