Inside the Firebox

The parts of a firebox

• Back wall: the wall behind the fire, which takes the most heat.
• Side walls (covings): angled walls that help reflect heat into the room.
• Inner hearth: the floor of the firebox, where the fire sits.
• Lintel: the steel or masonry support that spans the top of the opening.
• Throat and damper: the opening at the top that leads to the smoke chamber. See dampers.
• Hearth extension: the noncombustible floor that projects into the room.
• Ash dump (some fireplaces): a small door in the inner hearth leading to an ash pit below.

Firebrick and why ordinary brick is different

Firebrick is a dense, usually light-colored brick made from fireclay and fired to resist high heat and repeated heating and cooling. Brick Industry Association guidance for fireplaces points to firebrick conforming to ASTM C27 or ASTM C64 for the firebox lining.

Some older fireplaces were built with ordinary red brick inside the firebox. BIA notes that a durable solid brick can be used in some cases, but firebrick is the preferred lining. CSIA's Level 1 checklist for masonry fireplaces records whether the firebox is built of firebrick or common brick, because they wear differently.

Firebricks are usually laid in a separate layer in front of the structural masonry. BIA construction details show a gap between the firebox lining and the surrounding brickwork, filled with a compressible noncombustible material, so the hot lining can expand without pushing on the outer walls.

Refractory mortar and thin joints

The mortar in a firebox is not the same mortar used on the outside of the chimney. Standard portland cement mortar is not made for direct fire. Firebox joints are laid with a refractory or fireclay mortar intended for high heat.

The joints are also thin. BIA's construction details describe fireclay mortar joints of roughly 1/16 to 3/16 of an inch, and its design note limits refractory joints to about 1/4 inch. Thin joints matter because mortar is the weaker part of the wall when exposed to flame. Wide joints, or joints patched with the wrong material, tend to erode and fall out sooner.

BIA also advises allowing new masonry fireboxes to cure for a period before the first fire. Repairs follow the instructions of the product used.

The hearth and hearth extension

The inner hearth is the floor of the firebox. BIA guidance calls for a solid, noncombustible inner hearth several inches thick.

The hearth extension is the noncombustible surface in front of and beside the opening. Its job is to catch sparks, rolling logs and embers before they reach wood flooring or carpet. Its minimum size depends on the size of the fireplace opening. BIA's design note gives typical minimums: for openings under about 6 square feet, at least 16 inches in front and 8 inches to each side; for larger openings, at least 20 inches in front and 12 inches to each side. CSIA's Level 1 checklist includes checking hearth extension size against the opening.

These are design guidance figures, not a statement of code. Building codes generally require a hearth extension sized to the opening, and your town's construction office can confirm what applies. A thin tile or stone surface laid over a wood subfloor is not the same thing as a properly built hearth extension, even if it looks similar.

Clearances around the opening

Wood mantels and trim close to the fireplace opening can overheat. CSIA's Level 1 fireplace checklist looks at the distance from the opening to combustible trim and mantels, with larger distances required for parts that project farther out. BIA's construction details describe the same idea: the closer a combustible piece is to the opening, the less it is allowed to stick out.

Homeowners often add or change a mantel during a renovation without thinking about this. If you are planning one, have the clearances checked against the building code that applies in your town and the materials you plan to use.

Firebox shape, depth and draft

The proportions of a firebox affect how well it drafts and how much heat it sends into the room. A taller, wider opening needs a larger flue to carry the smoke. A firebox that is too shallow for its opening can let smoke roll out the front. Brick Industry Association design guidance relates the fireplace opening size to the required flue size for that reason.

Building codes and fireplace design guidance generally set a minimum firebox depth for ordinary fireplaces, while some traditional designs, such as the tall, shallow Rumford fireplace, follow their own proportions. The angled side walls you see in many fireboxes are there to reflect heat forward rather than letting it go straight up the flue.

Homeowners sometimes try to change these proportions, for example by raising the hearth with a new grate platform or narrowing the opening with glass doors. Each change can affect draft. If the fireplace started smoking after a change like that, mention it during the inspection.

Everyday habits that are easier on the firebox

• Use a grate or andirons sized for the firebox, so logs sit off the floor and air reaches the fire.
• Build moderate fires. Overloading the firebox puts extra heat stress on the brick and the damper above.
• Leave a thin bed of ash if you like, but do not let ash pile up against the walls or the damper.
• Let fires burn down on their own rather than dousing hot firebrick with water. Sudden cooling stresses the brick.
• Keep the screen or doors closed while the fire burns.
• Burn only dry, seasoned wood. See Burning the right firewood.

How fireboxes wear

The right-hand column lists possibilities, not conclusions. Cracks that look minor can sometimes reach deeper, and some alarming-looking stains are surface only. An inspection decides which it is.

Repair options in general

Joint repair. Eroded joints are cleaned out and refilled with a refractory mortar made for fireboxes.

Brick replacement. Individual cracked or broken firebricks can be cut out and replaced.

Panel or wall rebuild. When the back or side walls have widespread damage, they are taken down and rebuilt with new firebrick.

Hearth repair. Cracked hearth extensions are repaired or replaced. An undersized hearth extension may need to be enlarged.

If water is reaching the firebox, the source above, such as a missing cap or a failed crown, needs to be addressed too, or the repair will not last. Our firebox repair page explains how we assess and plan this work.

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.