Fire Brick Mortar: What to Use and How to Lay Firebrick

Fire brick mortar is a refractory mortar: a thin-joint mortar made to bond firebrick and survive repeated heating. Use it to lay or re-set firebrick in a fireplace firebox, a fire pit liner or an oven. Regular masonry mortar breaks down when it sees direct flame, so it does not belong between firebricks.

For a wood-burning fireplace, the best mortar for the firebox is a medium-duty refractory mortar meeting ASTM C199, which is what the residential code calls for, with firebrick joints no wider than 1/4 inch (IRC Chapter 10, section R1001.5). The same type works for a fire pit liner.

Gerrior Masonry, a family-owned mason in Woburn, MA, sets out below how firebrick mortar differs from fire brick glue and other products, which format to buy, and how to lay firebrick so the joints last.

Refractory mortar vs regular mortar

• Refractory mortar: made for heat, used in thin joints, comes premixed (air-set or heat-set) or as a dry mix. Belongs anywhere flame or hot gases touch: firebox walls and floor, fire pit liner, oven.
• Regular masonry mortar (Type N or S): made for weather and load, used in standard joints. Belongs on brick or stone that never sees flame: the face of a fireplace, the outside of a chimney, a fire pit's outer wall.

The common mistake is using one where the other belongs. Portland-based mortar inside a firebox cracks and crumbles; refractory mortar on the exterior of a chimney is often not made to stand up to rain and freezing.

Fire brick glue vs fire brick mortar

Products sold as fire brick glue, firebrick adhesive or high-heat brick adhesive are usually premixed refractory mortar or refractory sealant in a caulk cartridge. They work well for re-setting a loose firebrick or filling thin joints. They are not a substitute for laying a whole firebox, because a cartridge cannot butter full joints well.

• Refractory mortar in a cartridge: re-setting a few firebricks, thin joint repairs.
• High-temperature silicone: sealing around fireplace doors and metal frames where some movement is expected. Not for joints in the firebox.
• Furnace cement: sealing metal-to-masonry gaps such as stove pipe at a thimble.
• Construction adhesive: fine for capping stones on the outside of a fire pit. Never where flame reaches.

Premixed, dry-mix or castable

• Premixed tub or cartridge: ready to use, best for repairs and small jobs. Often air-set.
• Dry-mix refractory mortar: mixed with water on site, better value for laying a full firebox or fire pit liner, and easier to control consistency.
• Castable refractory: not a mortar. It is poured or packed into forms for liners, oven domes and custom shapes. Our guide to heat resistant cement explains when to choose a castable.

Which mortar goes where in a fireplace

• Firebox (walls and floor): firebrick in medium-duty refractory mortar.
• Brick or stone surround and outer hearth: standard masonry mortar such as Type N or S, since these surfaces do not see flame.
• Tile on the surround or hearth: a thinset and grout rated for the heat exposure, over cement backer board.
• Where a hearth meets a wood floor: leave a small flexible joint rather than a rigid grout line, because the floor and the masonry move differently.

How firebrick is laid

1. Clean and dry the surface. Remove loose mortar, soot and any old firebrick that is cracked through.
2. Dry-fit the brick. Firebrick should fit closely so joints stay thin. Cut, do not fill, to close a gap.
3. Butter thin joints. Follow the product's joint range; the code limit for a firebox is 1/4 inch. Some installers dip the brick edge in mortar thinned to a slurry for very tight joints.
4. Tool and clean as you go. Refractory mortar is hard to remove once it sets.
5. Cure on schedule. Let it dry as the label directs, then run small curing fires before a full fire. Heat-set mortars do not reach strength until fired.

Fire pit specifics

For a fire pit, the outer block or stone wall is regular exterior masonry; only the liner needs refractory mortar. Keep joints thin, give the pit a way to drain, and remember that in Massachusetts the liner will be wet and frozen for months. Many refractory mortars are not rated for wet outdoor use, so check the label, and expect to re-point a few liner joints over time. A steel ring insert is a lower-maintenance alternative.

What about a wood-burning stove?

Many stoves hold their firebricks in place with the stove's own frame or clips and need no mortar; check the manual before buying anything. The stove pipe connection may call for furnace cement or the gasket material the stove maker specifies.

When to call a mason

Re-setting one or two firebricks is a reasonable homeowner job. Call for help when many bricks are cracked or loose, when the back wall is bulging, or when cracks continue into the chimney above. Gerrior repairs and rebuilds masonry chimneys through our chimney repair service and builds fire pits as part of our hardscaping services. For dampers, liners and inspections of the flue, a CSIA-certified sweep is the right first call.

Common questions

What is the best mortar for a fireplace?

Inside the firebox, a medium-duty refractory mortar meeting ASTM C199. On the brick or stone face, a standard masonry mortar matched to the brick.

What is the best mortar for a fire pit?

Refractory mortar for a firebrick liner; standard exterior mortar or masonry adhesive for the outer wall and cap.

What is the best glue to use for an outdoor fireplace?

For firebrick, a refractory mortar in a cartridge labeled for outdoor use. For cap stones and veneer that stay cool, an exterior masonry adhesive. Ordinary construction adhesive should never be used where flame reaches.

Can I use regular mortar for firebrick?

No. It cracks and crumbles under direct heat. Use refractory mortar.

Need firebrick, fire pit or chimney work looked at? Gerrior Masonry has served the Woburn area for over 30 years, is fully insured and guarantees its work. Call (781) 933-4157 or request a free estimate.