The smoke chamber and smoke shelf, explained

Where it sits in a masonry fireplace

Built of masonry, a fireplace is a stack of parts with separate jobs, and the smoke chamber is the one hidden between the part you can see and the part you cannot. Reading from the floor up:

• Hearth and hearth extension: the noncombustible floor under and in front of the fire.
• Firebox: the lined box where the fire burns.
• Throat: the narrowed opening at the top of the firebox.
• Damper: the plate or lid that closes the throat when the fireplace is idle.
• Smoke shelf: the flat ledge just behind the damper.
• Smoke chamber: the tapered space rising from the damper to the flue.
• Flue: the lined passage that carries smoke out through the chimney.

What the chamber does

A fireplace opening is wide and shallow. A flue is a narrow tube. Something has to join the two, and that something is the smoke chamber, its front, back and side walls leaning inward until they meet the first flue tile. A chamber that tapers evenly lets the rising smoke speed up and stay together as it enters the flue.

The smoke shelf works at the bottom of that space. Cold air sometimes drops down a chimney, and the shelf helps turn it back up instead of letting it blow straight into the firebox. It is also where anything falling down the flue lands, from soot and loose mortar to nest material and rainwater.

The chamber also has to match the flue above it. The code sizes a fireplace flue in relation to the fireplace opening (section R1003.15), and the chamber is the transition between those two sizes. A chamber that is too tall, too short or badly shaped for the opening and flue it connects can hurt the draft even when every brick in it is sound.

What the Florida Building Code asks of it

Sections R1001.8 and R1001.8.1 of the Florida Building Code, Residential, cover smoke chambers. The requirements are short and specific:

• Walls of solid masonry, hollow units grouted solid, stone or concrete, 8 inches thick in total.
• With a lining of firebrick 2 inches or more thick, or vitrified clay 5/8 inch or more, the walls may total 6 inches, lining included.
• A smooth parge coat inside, of refractory mortar that meets ASTM C199.
• A height, throat to flue, that does not exceed the width of the fireplace opening.
• Where walls are rolled or sloped, or lined with prefabricated panels, a limit of 45 degrees from vertical.
• For corbeled brick, a limit of 30 degrees from vertical.

Corbeled, rolled and lined chambers

Chambers are built in three basic ways. The traditional one is corbeling: each course of brick is set a little further in than the one below, so the walls step inward like an upside-down staircase. It is simple to lay, but it leaves a ledge at every course, and the code's smooth parge coat is what covers those steps.

Rolled or sloped chambers are laid so the inner face forms a continuous slope, and they need less finishing. Lined chambers use prefabricated panels of refractory material set against the masonry. Whatever the method, the finished inside surface is meant to be smooth and continuous from the damper to the flue.

You cannot tell which method was used from the room, and older fireplaces were not all finished to today's code. That is one reason a chamber is judged after cleaning, when the steps, the slope and any parging can actually be seen.

What goes wrong up there

The chamber is out of sight, so its problems go unnoticed until a sweep or an inspection finds them. The findings fall into a few groups, each with a different consequence.

• Corbeled steps that were left without parging, giving creosote a shelf at every course.
• Voids and open joints between bricks, letting heat and smoke into the chimney wall.
• Cracks from repeated heating and cooling.
• An open seam at the base of the first flue tile.
• A smoke shelf buried in debris, holding moisture against the damper.
• Glazed creosote coating the chamber walls.

Why a rough chamber matters

Three things follow from a rough or damaged chamber. Deposits build up on the steps and in the voids, and those deposits burn, so a chamber can be the place a chimney fire starts. Smoke tumbling over ledges moves less cleanly into the flue, which can contribute to a fireplace that smokes into the room. And where walls are cracked or thin, the heat of the fire gets closer to the framing around the chimney than the design intended.

None of that is visible from the hearth. A fireplace can look clean and draw well enough while the chamber above the damper holds a thick layer of creosote on unparged brick.

Signs in the room that point to the chamber

Because the chamber cannot be seen, its problems show up indirectly. None of these proves the chamber is at fault, but each one justifies a look at the chamber when the chimney is next checked.

• Smoke rolling out along the top of the fireplace opening when a fire is going.
• Black staining on the fireplace face just above the opening.
• Crumbs of mortar, brick or soot collecting on the damper or the firebox floor between fires.
• A strong smoky odor from a cold fireplace.
• The wall above the fireplace getting noticeably warm during a fire.

How a smoke chamber gets checked and repaired

Every chimney is due a yearly inspection under NFPA 211, and the smoke chamber is part of it. With the damper open, the lower part of the chamber is visible from the firebox under a strong light. The upper part and the joint at the flue need a closer look after the chamber has been cleaned.

Repair means cleaning, resetting any loose brick, and parging the walls smooth with refractory mortar or a chamber parging system used by its maker's instructions. Our smoke chamber repair page describes that work in detail.