Chimney Anatomy Infographic: Cap to Ash Pit
A labelled diagram of a masonry chimney and fireplace, with a short description of what each part does and how it fails.
The twelve parts, from top to bottom
A masonry chimney has twelve main parts. The table lists each one in order from the top, with what it does and how it fails.
| # | Part | What it does | How it fails |
|---|---|---|---|
| 1 | Cap | Metal cover with a screen over the flue opening. Keeps out rain, animals and leaves and stops sparks. | Rusts through, blows off, or was never fitted. |
| 2 | Crown | Sloped concrete or mortar slab sealing the top of the stack around the liner. Sheds water away from the flue. | Cracks from freeze and thaw and lets water into the masonry. |
| 3 | Flue liner | The inner tube, usually clay tile or stainless steel, that carries smoke out and protects the brick and the house from heat. | Tiles crack or joints open; metal liners corrode. |
| 4 | Chimney stack | The brick or stone structure around the liner. | Mortar joints erode and brick faces flake off (spalling). |
| 5 | Flashing | Sheet metal sealing the joint between the chimney and the roof. | Lifts or its sealant fails, causing roof leaks. |
| 6 | Smoke chamber | The funnel-shaped space above the firebox that narrows smoke into the flue. | Rough or stepped brickwork collects creosote; mortar coating cracks. |
| 7 | Smoke shelf | A flat ledge behind the damper that catches rain and debris and deflects downdrafts. | Fills with debris and soot. |
| 8 | Damper | A metal plate that opens to let smoke out and closes to keep heated air in when there is no fire. | Rusts, warps or seizes so it will not fully open or seal. |
| 9 | Lintel | The steel or masonry beam across the top of the fireplace opening that carries the weight above. | Rusts and expands, cracking the brick around it. |
| 10 | Firebox | The chamber where the fire burns, lined with firebrick. | Mortar joints between firebricks open up; bricks crack. |
| 11 | Hearth | The non-combustible floor of the firebox and the area extending into the room. | Cracks, or is too small for the opening. |
| 12 | Ash pit | A cavity below the firebox, reached through an ash dump door in the hearth, where ashes collect for removal through a cleanout door. | Overfills; cleanout door rusts or is left open. |
Which parts keep water out, and which control smoke
It helps to think of a chimney as two systems. The weather system is the cap, crown, flashing and the outer masonry. When it fails, you get leaks, stains and crumbling brick. The venting system is the liner, smoke chamber, smoke shelf and damper. When it fails, you get smoke in the room, poor draft, or heat reaching parts of the house it should not.
Not every chimney has every part. Factory-built fireplaces use a metal chimney instead of a masonry stack, and many homes have no ash pit.
If an inspection report mentions one of these parts, this diagram shows where it is. For what the inspection itself covers, see our chimney inspection page, and for fixes, chimney repair.
Common questions
What is the difference between a chimney and a flue?
The chimney is the whole structure. The flue is the passage inside it that carries smoke and gases out. One chimney can contain more than one flue, for example one for a fireplace and one for a furnace. Each appliance or fireplace should have its own flue.
What is the difference between a chimney cap and a chimney crown?
The cap is a metal cover over the flue opening. The crown is the concrete or mortar slab that seals the top of the masonry around the flue. A chimney needs both. A failed cap lets water down the flue; a failed crown lets it into the brickwork.
Where is the damper in a fireplace?
In most masonry fireplaces it is just above the firebox, at the base of the smoke chamber, worked by a handle or lever inside the fireplace opening. Some chimneys have a top-sealing damper at the cap instead, operated by a cable.
What does a smoke chamber do?
The smoke chamber sits just above the firebox and funnels smoke from the wide fireplace opening into the narrower flue. Its walls should be smooth. Rough or stepped brickwork slows the smoke and collects creosote, which is why the smoke chamber is cleaned during a sweep.
What is a smoke shelf?
A smoke shelf is the flat ledge behind the damper at the base of the smoke chamber. It catches rain and debris that fall down the flue and helps turn back downdrafts. Because it collects soot and debris, it is cleaned out during a chimney sweep.