Smoke Chamber Parging

What the smoke chamber is

A fireplace opening is much wider than the flue above it. The smoke chamber is the transition between the two: a space that starts at the damper and narrows as it rises to meet the bottom of the flue. Smoke passes through it on every fire.

Builders form those sloping walls in one of two ways. Some lay the brick in steps, each course set a little further in than the one below, which is called corbelling. Others finish the walls with a smooth coat of mortar. Our guide What is a smoke chamber? explains its shape and role in more detail.

Because it sits above the damper and out of sight, the smoke chamber is one of the least-inspected parts of a fireplace. It's also one of the places where heat, smoke and creosote are concentrated, which is why it gets attention during a thorough sweep and inspection.

Why stepped walls are a problem

Building codes and fireplace construction guidance generally call for corbelled smoke chamber walls to be parged smooth, and they limit how steeply the walls can slope. Your town's construction office can confirm what applies to your home. CSIA also notes that stepped brick in a smoke chamber is a sign of potential hazards. Many older fireplaces were built without that smooth finish, or their original parging has cracked and fallen away.

Signs of a smoke chamber problem

• Heavy creosote above the damper, even after a sweep
• Bits of mortar or brick falling onto the damper or smoke shelf
• An inspection report mentioning corbelled, unparged or gapped smoke chamber walls
• Smoke spilling into the room at the start of a fire (one of many possible causes; see why smoke comes into the room)
• Strong creosote odors

You usually can't see the smoke chamber from the hearth. Most homeowners find out about it from an inspection.

How we assess it

The smoke chamber is checked from below, through the damper opening, and from above with a chimney camera where the flue allows. We look at the shape of the walls, whether they're stepped or smooth, any existing parging and its condition, open joints and gaps, and the type and depth of creosote. We sweep before the camera goes in, so the walls can be seen clearly.

We also look at the damper and the flue just above, because repairs in one area often affect the others. You get photos and our findings before any repair is scheduled.

We'll also tell you whether the chamber's shape, apart from its surface, looks likely to affect how the fireplace drafts. That helps set realistic expectations for what parging can change.

How parging is done

1. CleanCreosote and loose material are removed so the coating bonds to sound masonry.
2. RepairOpen joints and loose brick are addressed first.
3. ApplyA refractory mortar or heat-resistant smoke chamber coating is troweled or sprayed onto the walls to fill the steps and gaps and create a smooth surface.
4. CureThe material cures according to the manufacturer's instructions before the fireplace is used.
5. DocumentPhotos show the finished surface.

Products differ, and some are made specifically for smoke chambers with their own application and curing requirements. The choice depends on the chamber's shape and condition.

The smoke shelf and damper area

At the bottom of the smoke chamber, behind the damper, many masonry fireplaces have a smoke shelf: a flat ledge that catches debris and rain falling down the flue. It's also where creosote flakes, nesting material and broken tile tend to collect.

Parging work usually includes looking at and cleaning the smoke shelf, since debris there can fall back into the firebox or block the damper. If the shelf has a lot of water staining or broken tile pieces, it points to something higher up: a missing cap, a crown leak or a damaged flue. Those are worth tracing before the smoke chamber is resurfaced, so the new surface isn't sitting under a continuing leak.

The damper frame sits right where the smoke chamber begins. If the damper is rusted, warped or loose in its frame, it's often sensible to deal with it at the same time. See damper repair and replacement.

What parging won't fix

• A badly shaped smoke chamber that's too steep or uneven may affect draft even after parging
• A cracked or undersized flue above (see flue repair)
• A damper that doesn't seal (see dampers)
• A smoky fireplace caused by house pressure, an oversized opening or a short chimney

What affects the scope

• The size and shape of the smoke chamber
• Depth of the steps and any gaps
• Amount of creosote to remove first
• Access through the damper; a throat damper may limit access from below
• Whether masonry repairs are needed first

Next step

If an inspection mentioned your smoke chamber, or you see heavy creosote above the damper, request an assessment. A team member will contact you to confirm a time.

Request a smoke chamber assessment

Questions homeowners ask

Sources

1. CSIA: Frequently asked questions