What Is a Smoke Chamber?

Where it is and what it does

A fireplace opening is wide and the flue above it is narrow. Something has to connect the two. Going up from the firebox, the order is:

1. The firebox, where the fire burns.
2. The throat, a narrow slot at the top of the firebox where the damper usually sits.
3. The smoke shelf, a ledge behind the damper in many fireplaces.
4. The smoke chamber, which slopes inward from the front and sides.
5. The flue, which begins where the first liner tile sits on top of the smoke chamber.

Brick Industry Association guidance describes the smoke chamber as the part that supports the flue and has sloped side walls converging toward the flue. It does two jobs at once: it carries the weight of the liner and it shapes the flow of smoke so the fireplace drafts smoothly.

Corbeled walls and why they matter

Corbeling means stepping each course of brick a little farther inward than the one below it. Masons have built smoke chambers this way for generations, because it is a simple way to narrow a brick opening without special materials.

The result looks like an upside-down staircase. BIA's construction details show the front and sides corbeled in while the back wall rises straight, with the last courses laid to support the flue liner around its full perimeter.

The steps cause three problems if they are left bare:

• Turbulence. Smoke tumbles over each ledge instead of flowing up a smooth funnel. That can slow draft, especially when the fire is starting.
• Deposits. Every ledge is a shelf where soot and creosote collect, and they are hard to brush clean.
• Thin spots and gaps. The mortar joints between steps are a weak point for heat and corrosive exhaust, and gaps can open toward the surrounding structure over time.

Building codes and masonry construction guidance generally address smoke chamber construction, including wall thickness, how steeply the walls may slope and whether corbelled walls must be parged smooth. Your town's construction office can confirm what applies.

What parging is

Parging is a coat of mortar troweled over the inside walls of the smoke chamber to make them smooth and continuous. BIA guidance says the entire smoke chamber should be parged, and its design note calls for a refractory (heat-resistant) mortar for that coat.

In older fireplaces the parging may be thin, cracked, missing in patches or never applied at all. When a smoke chamber is re-parged today, technicians usually use a refractory product made for the purpose, applied by hand or sprayed and then smoothed. Some systems are also used to fill gaps and rebuild a smoother slope.

Parging is not a cosmetic finish. Its value is in sealing joints and giving the smoke a smoother path.

Common smoke chamber findings

• Bare corbeled brick with no parging.
• Cracked or flaking parging, sometimes with pieces falling onto the smoke shelf.
• Open mortar joints or voids in the walls.
• Heavy soot or creosote on the ledges.
• Poor support for the first liner tile, which CSIA's Level 1 fireplace checklist specifically looks at.
• Heat or fire damage after a chimney fire.

CSIA's checklist for masonry fireplaces covers the condition and smoothness of the smoke chamber walls, whether the flue liner is supported on all four sides, and whether the transition into the flue is smooth and sealed.

Smoke chamber terms you may see on a report

If a report uses a term that is not on this list, ask the person who wrote it. A good report should make sense to the person paying for the work.

Why a hidden part gets so much attention

The smoke chamber sits at the hottest point of the chimney after the firebox. Smoke enters it still hot and full of moisture and combustion byproducts. When it meets a rough, cool, stepped surface, more of that material can condense and stick. That is one reason the smoke chamber is a common place to find heavier deposits than in the straight flue above.

It is also where two different parts of the system meet: the fireplace below and the flue above. Transitions are where gaps tend to form as materials expand and contract at different rates. A smooth, sealed chamber helps keep heat and exhaust inside the masonry where they belong.

Finally, the smoke chamber is the part most likely to be skipped by a quick look. A flashlight from the hearth shows the damper and maybe the shelf, not the walls above. That gap between what is easy to see and what matters is why a full inspection looks past the damper.

Symptoms are clues, not answers

Homeowners sometimes notice a fireplace that smokes when first lit, crumbs of mortar on the damper or smoke shelf, or an odor that seems to come from deep inside the fireplace. A smoke chamber problem can cause any of those. So can a closed or partly closed damper, a cold flue, a house that is pulling air down the chimney, a blocked cap, or an undersized flue.

That is why we do not name a cause from a symptom. If a fireplace smokes, see Why does smoke come into the room? for the range of causes. If you see smoke entering the room and cannot stop it, put the fire out safely and stop using the fireplace until it is checked.

How a smoke chamber is inspected

The smoke chamber is hard to see from the room. With a flashlight through an open throat damper you may see the bottom few courses. A good look usually takes a chimney camera, a mirror and light, or a view from above with the cap removed.

On our visits, the assessment includes the smoke chamber when the fireplace allows access, and the findings we send include photos so you can see what we saw. If a chimney fire is suspected, the smoke chamber is one of the areas checked closely. See chimney fire damage inspection.

Repair in general terms

The usual repair for a sound but rough or deteriorated smoke chamber is cleaning followed by parging with a refractory product. Where there are larger voids or loose brick, those are repaired first. If the damage is part of a wider problem, such as a failing liner or a firebox that needs rebuilding, the smoke chamber work is planned together with the rest so the transitions line up.

Whether parging is needed, and how much, depends on what the inspection finds. See smoke chamber parging for how we approach it.

Questions homeowners ask

Sources

1. Brick Industry Association, Technical Note 19: Residential Fireplace Design (PDF)
2. Brick Industry Association, Technical Note 19A: Residential Fireplaces, Details and Construction (PDF)
3. CSIA: Level 1 inspection checklist, masonry fireplace (PDF)

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