Firebox Repair

What the firebox is made of

In a masonry fireplace, the firebox walls and floor are lined with firebrick, a dense brick made to withstand high heat. The bricks are laid with refractory mortar, which is a heat-resistant mortar, in thin joints. Behind the firebrick is ordinary masonry. Building codes and fireplace construction guidance generally call for firebrick laid in heat-resistant mortar with thin joints, because the lining's job is to protect everything behind it.

Factory-built fireplaces use refractory panels instead of brick. Those panels are part of a listed system. Our guide Inside the firebox covers both types.

Common firebox damage

The table shows common possibilities. The cause and how serious it is are confirmed on site.

Why it matters

Gaps in the firebox lining can let heat and embers reach the masonry behind it and, in some fireplaces, combustible framing nearby. Damaged firebrick also gets worse with every fire, since heat cycling widens existing cracks and open joints. Water entering from above can speed this up: if the firebox shows rust stains or crumbling mortar, the cause may be higher in the chimney.

The firebox is also the part of the chimney system you see every day, so it's often where people first notice something is wrong higher up. White staining on the firebrick, rust on the damper above it, or mortar crumbling only at the back wall can all point to water coming down the chimney. Fixing the firebox without fixing the source means repeating the repair.

How we assess it

We examine each wall and the floor of the firebox, tap firebrick to check for loose units, and measure crack widths and joint depths. We check the hearth and the area where the firebox meets the damper and smoke chamber. If water damage is part of the picture, we look higher: the damper, smoke chamber and flue with a chimney camera where it allows, and the crown and cap from the roof when it's safe.

For a factory-built fireplace, we identify the unit and its manufacturer so replacement panels can be matched. You get photos and our findings before any repair is scheduled.

Repair options

Repointing joints

Failing mortar joints are cleaned out and refilled with refractory mortar. This is the most common firebox repair.

Replacing firebrick

Cracked-through, loose or spalled bricks are cut out and replaced with new firebrick laid in refractory mortar.

Rebuilding the firebox

When damage is widespread, the back wall is bulging, or repairs keep failing, rebuilding the firebox lining is often more lasting than patching brick by brick.

Factory-built panels

Damaged refractory panels are replaced with parts specified by the fireplace manufacturer, when they're still available. If they aren't, we'll explain what that means for the unit. See fireplace repair.

Choosing between them

The deciding question is whether the firebrick lining is still a sound, continuous layer. If most bricks are tight and the problems are joints and a handful of units, repair is usually enough. If bricks are loose across a whole wall, or the wall itself has moved, patching leaves you with a lining that's only as good as its weakest section. We'll show you which bricks and joints are affected so you can see the extent for yourself.

Helping the firebox last

Firebrick is made for heat, but how you use the fireplace still affects how fast it wears.

• Avoid very large, hot fires. Overfiring a fireplace stresses the firebrick and the mortar joints more than a moderate fire.
• Don't pour water on a fire. Sudden cooling of hot firebrick can crack it. Let a fire burn down or use the methods the manufacturer recommends for a factory-built unit.
• Use a grate. Raising the fire off the floor improves air flow and keeps logs from sitting directly on the hearth brick.
• Burn dry wood. Wet wood smolders and adds moisture and creosote. See our firewood guide.
• Keep water out from above. A cap and a sound crown protect the firebox as much as the flue.
• Look after a fire season. A quick look with a flashlight each year catches open joints while they're still a small repair.

Limits and what affects the scope

• Number of damaged bricks and joints
• Whether the masonry behind the firebrick is sound
• Firebox size and shape; older or unusual fireplaces may need cut bricks
• Availability of panels for factory-built units
• Water damage, which needs its source fixed or it will return
• Refractory mortar needs to cure as the manufacturer specifies before a fire is lit

Next step

If you see cracked or loose firebrick, crumbling mortar or a damaged panel, request an assessment. A photo of the inside of the firebox helps. A team member will contact you to confirm a time.

Request a firebox assessment

Questions homeowners ask