The Parts of a Chimney, Top to Bottom

A quick cutaway, from the sky down

Picture slicing a brick chimney in half from the top. Working downward, these are the parts you would see on most fireplace chimneys built in New Jersey over the last century:

1. Chimney cap: a metal hood or screen over the flue opening.
2. Crown: the cement or concrete slab that covers the top of the brickwork.
3. Flue liner: the clay tile or metal tube that forms the smoke passage inside the brick.
4. Chimney walls: the brick or block and the mortar joints between them.
5. Flashing: the sheet metal that seals the joint where the chimney passes through the roof.
6. Smoke chamber: the funnel-shaped space that narrows from the fireplace up to the flue.
7. Damper: the door that opens for a fire and closes when the fireplace is idle.
8. Firebox: the brick box where the fire actually burns.
9. Hearth: the noncombustible floor inside the firebox and the extension in front of it.

Furnace, boiler and water heater flues follow the same top-to-bottom idea, but they connect to the appliance through a metal connector pipe instead of a fireplace.

At the top: cap, crown and the exposed liner

The top of the chimney takes more weather than any other part of the house. Rain lands on it directly, and in winter it freezes and thaws over and over.

The crown is the slab that covers the brick around the flue. Its job is to throw water off and away from the chimney walls. The Brick Industry Association's guidance on residential chimneys describes a cap or crown that overhangs the brick, a separation (bond break) between the crown and the brickwork so they can move independently, and a sealed joint where the crown meets the flue liner. The liner itself should stick up a short distance above the crown.

The cap sits on top of the flue opening. A typical cap has a lid to shed rain and a mesh screen to keep birds and animals out and to catch larger embers. Caps come in single-flue models that clamp to the liner and larger models that cover several flues and fasten to the crown.

These two parts are confused so often that we gave them their own page: Crown vs. cap: what's the difference?

The chimney walls and flashing

Brick and mortar. The outer walls carry the weight of the chimney and protect the flue inside. Brick usually outlasts mortar. Mortar joints are meant to be the softer, replaceable part, so it is normal for joints to need repointing long before the brick needs replacing. When water gets into brick and freezes, the face of the brick can pop off. That is called spalling, and it has its own guide: Why is my chimney brick flaking?

Flashing. Where the chimney passes through the roof there is a gap the roofer cannot shingle over. Flashing closes that gap in two layers. Base (step) flashing is tucked under the shingles and turned up against the brick. Counterflashing is set into a mortar joint and folds down over the base flashing. BIA guidance gives minimum overlaps for both layers so that water running down the brick is directed onto the roof. On the uphill side of wider chimneys, a small peaked roof called a cricket helps split the water flow so it does not pond against the brick.

Most chimney leaks we are asked about start at the crown, the flashing or porous masonry. Our chimney leak guide walks through how those sources are told apart.

The flue and the liner

The flue is the open passage the smoke travels through. The liner is the material that forms the walls of that passage. In most older masonry chimneys the liner is a stack of square or rectangular clay tiles, set in mortar, with the brick wrapped around them. Newer and relined chimneys may have a stainless steel liner or a poured, cast-in-place liner.

The liner keeps heat and corrosive combustion products away from the brick and from nearby wood framing. It also gives the smoke a smooth, correctly sized path, which helps draft. The Chimney Safety Institute of America (CSIA) describes a liner as a clay, ceramic or metal conduit that contains combustion products, directs them outside and protects the chimney walls from heat and corrosion.

A chimney can have more than one flue. A common New Jersey setup is one flue for the fireplace and a second for the furnace or boiler. Each appliance normally needs its own flue unless the system was specifically designed and approved to share one. For the difference in terms, see Flue vs. liner.

Below the flue: smoke chamber, damper and firebox

Smoke chamber. Above the fireplace opening, the walls step inward to funnel smoke from the wide firebox into the narrow flue. That funnel is the smoke chamber. In many older fireplaces the walls were built by stepping each course of brick inward, which leaves a rough, stair-stepped surface. Masonry guidance calls for the smoke chamber to be parged (coated smooth). More in What is a smoke chamber?

Smoke shelf. Many fireplaces have a flat or curved ledge behind the damper at the base of the smoke chamber. It collects debris that falls down the flue.

Throat and damper. The throat is the narrow slot between the top of the firebox and the smoke chamber. A throat damper is a cast-iron or steel plate set in that slot. Some homes instead have a top-sealing damper mounted at the top of the flue and operated by a cable. See Throat dampers vs. top-sealing dampers.

Firebox and hearth. The firebox is the brick-lined chamber where wood burns. It is usually lined with firebrick laid in thin refractory or fireclay mortar joints. The hearth is the noncombustible floor of the firebox, plus the hearth extension that projects into the room to catch sparks and rolling embers. Details in Inside the firebox.

Ash dump and cleanout. Some fireplaces have a small metal door in the firebox floor that drops ash into a chamber below, emptied through a cleanout door in the basement or outside.

Each part, its job, and what tends to go wrong

This table is a general guide. It does not tell you what is wrong with your chimney. Several of these problems look alike from the ground, and some can only be seen from inside the flue.

How an inspection walks through these parts

CSIA's published Level 1 checklists for masonry chimneys and fireplaces follow roughly the same top-to-bottom order as this page: termination and cap, crown, flashing, flue condition and cleanliness, smoke chamber, damper, firebox, hearth extension and clearances to nearby combustible trim.

On our visits, every job starts with an assessment. We check what can safely be reached from the roof and attic, use a chimney camera when the flue allows it, and send you findings with photos before any repair is scheduled. If you are not sure what you are looking at in a report, a part-by-part map like the one above is the best place to start.

Questions homeowners ask

Sources

1. Brick Industry Association, Technical Note 19: Residential Fireplace Design (PDF)
2. Brick Industry Association, Technical Note 19B: Residential Chimneys, Design and Construction (archived PDF copy)
3. CSIA: About chimney liners
4. CSIA: Level 1 inspection checklist, masonry chimney (PDF)
5. CSIA: Level 1 inspection checklist, masonry fireplace (PDF)

This guide is general information. It can't diagnose a specific chimney; an inspection can.