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Parts of a chimney explained
A chimney is a system, not just a brick stack. This guide walks through each part from the top down and explains what it does and what can go wrong with it.
The short answer: a masonry chimney has a cap and crown at the top, a flue and liner running up the inside of the brick stack, and flashing where the stack meets the roof. At the fireplace there is a smoke chamber, smoke shelf, damper, firebox and hearth. Each part has a job, and a problem with one can affect the others.
From the yard or the living room you only see a small part of a chimney. The diagram below is a simplified side view of a typical masonry chimney with a fireplace. Select a part, or a number, to see what it does.
A simplified side view of a typical masonry chimney with a fireplace. Real chimneys vary.
Covers the top of the flue to keep rain, debris and animals out. Some caps also include a mesh screen. A cap that is rusted, damaged or missing is usually replaced.
The sloped slab at the top of the masonry, usually concrete or mortar. It sheds water away from the flue and the brick below it, so cracks in the crown can let water into the chimney.
The brick, stone or block and mortar that form the chimney stack and the chimney breast. Water, freezing and thawing wear down mortar joints and brick faces over time.
Metal that seals the joint between the chimney and the roof. It is usually made of base flashing on the roof and counterflashing set into the mortar joints. Failed flashing is a common path for water.
The flue is the passage that carries smoke and gases out of the house. The liner, often clay tile or metal, lines it and protects the masonry and the house. A cracked, missing or deteriorated liner is a reason to have the chimney assessed.
The space directly above the smoke shelf that narrows from the fireplace to the base of the flue. Smooth walls help smoke flow upward, and rough or gapped masonry here can affect how well a fireplace draws.
The ledge at the bottom of the smoke chamber, behind the damper. It helps deflect downdrafts and catches debris and rainwater that come down the flue.
A plate at the throat of the fireplace, just in front of the smoke shelf. It opens when you burn a fire and closes when you do not. A top-sealing damper is an alternative that sits at the top of the chimney.
Where the fire burns, lined with firebrick. Cracked or missing firebrick and mortar joints in the firebox are worth having looked at.
The noncombustible floor of the fireplace and the extension in front of the opening, which protects the floor from sparks and embers.
The top: cap and crown
The cap sits over the flue opening. It keeps rain, debris and animals out of the flue, and some caps include a mesh screen. The crown is the sloped slab, usually concrete or mortar, that covers the top of the masonry around the flue. It is built to shed water away from the flue and from the brick below it.
A cap protects the flue opening and a crown protects the masonry. They are different parts with different jobs. Because the crown takes the weather directly, cracks in it are one of the most common ways water gets into a masonry chimney.
The stack: masonry and flashing
The masonry is the brick, stone or block and mortar that form the outside of the chimney, both the stack above the roof and the chimney breast inside the house. Rain, freezing and thawing wear down mortar joints and brick faces over time.
Flashing is the metal that seals the joint where the chimney meets the roof. It is usually made of base flashing on the roof plus counterflashing set into the mortar joints of the chimney. When flashing fails, water can run in beside the chimney, so it is a common place to look when a stain shows up on a ceiling or wall near the chimney.
Inside the stack: the flue and liner
The flue is the passage that carries smoke and combustion gases out of the house. The liner lines the flue. It may be clay tile, metal or cast in place. Its job is to contain the products of combustion and to protect the masonry and the house around it.
The liner is hidden from view, so damage there often goes unnoticed. That is why a Level 2 inspection includes a video scan of the flue interior. Our guide to inspection levels explains when that applies.
At the fireplace
Starting at the bottom, the hearth is the noncombustible floor of the fireplace and the extension in front of the opening. The firebox is where the fire burns, lined with firebrick.
Above the firebox is the throat, where the damper sits. The damper is a plate that opens when you burn a fire and closes when you do not. Directly behind the damper is the smoke shelf, a ledge that helps deflect downdrafts and catches debris and rainwater that come down the flue.
Above the shelf is the smoke chamber. It narrows from the width of the fireplace to the base of the flue and guides smoke upward. Smooth walls help; rough or gapped masonry can affect draft. See our Morristown smoke chamber project for an example.
A top-sealing damper is an alternative that sits at the top of the chimney and seals the flue from above.
Factory-built chimneys and chase covers
Not every chimney is masonry. A factory-built fireplace uses a metal chimney, often inside a framed enclosure called a chase. The top of the chase is covered by a chase cover, which is a different part from a crown. Factory-built systems usually do not have a smoke shelf or a masonry smoke chamber.
What you can safely look at
- From the ground, with binoculars if needed: is there a cap, is the top of the chimney visibly cracked, is mortar missing or brick flaking?
- Inside: does the damper open and close freely, and is there debris or pieces of mortar or tile in the firebox?
Please do not climb onto the roof or the chimney, and do not try to repair a liner or flashing yourself.
When to have it assessed
- Water stains or dampness near the chimney, or in the attic
- Smoke that does not rise the way it should
- Pieces of mortar or flue tile in the firebox
- A damaged or missing cap, or a cracked crown
- After a chimney fire, storm damage, or before buying or selling a home
If a fire alarm or carbon monoxide alarm sounds, leave the building and call emergency services. Do not wait to have the chimney looked at.
Have a question about your chimney?
Call or send a request and we will help you work out what it needs.