Do You Need Fire Brick in a Fire Pit?

What Direct Heat Does to Block and Pavers

The makers of the blocks themselves are the clearest source here. Unilock's fire pit advice says excessive heat may cause concrete units to break down over time if the pit is not designed for high temperatures, and recommends lining an above-ground pit with a metal liner or fire-rated brick. Belgard's fire pit questions add that direct heat from a wood fire can crack or discolor pavers, and advise a metal fire ring or insert to prevent that damage.

That is why many block fire pit kits ship with a steel insert. Unilock describes its kit insert as holding the walls in shape and directing heat toward the center, protecting the wall material from drying out.

River Rock and Porous Stone

Rounded river rock looks natural around a fire and is the wrong choice. Belgard tells homeowners to avoid porous materials such as river rock or sandstone, which can crack or even explode under high heat, and to use dense, heat-resistant stone or engineered block instead. Keep decorative rock outside the steel ring, away from the fire itself.

Fire Brick, Steel Liner or Thick Solid Masonry

No Florida code section is written for a backyard fire pit, but the masonry fireplace sections of the 2023 Florida Building Code, Residential, show what a fire-facing wall is expected to be. Section R1001.5 allows three ways to build a firebox wall:

• A firebrick lining at least 2 inches thick, inside a wall that totals at least 8 inches of solid masonry.
• No lining, but at least 10 inches of solid masonry, which is how ordinary solid brick can face a fire.
• Hollow masonry units only when they are grouted solid; open cores are not a fire wall.

The Mortar That Survives the Heat

The same section is specific about the materials. Firebrick has to meet ASTM C27 or C1261, it is laid with medium duty refractory mortar meeting ASTM C199, and the joints between firebricks may be no wider than 1/4 inch. Ordinary masonry mortar is not what the code asks for against the fire, and thin joints leave less of any mortar exposed to it.

Block pit kits that come with a steel insert keep the fire off the block and its joints altogether. A pit built from loose brick or block with no liner and no refractory mortar is the one that cracks apart.

Refractory Panels in an Outdoor Fireplace

Factory-built outdoor fireplaces line their fireboxes with refractory panels instead of loose fire brick, and their makers set the replacement point. One outdoor wood fireplace maker says minor cracking from expansion and contraction is normal, and that a panel should be replaced when a crack is wider than 1/4 inch, when metal shows behind it or when large pieces fall out. The same rule of thumb is a fair test for fire brick in a pit: hairline cracks are expected, open gaps and missing pieces are not.

Checking a Fire Pit You Already Have

A few minutes with the pit cold tells you whether the lining is doing its job.

• Look for cracked, flaking or crumbling block faces on the inside of the ring.
• Check whether pavers next to the pit are discolored or cracking from heat.
• See whether a steel ring is present, rusted through or sitting loose.
• Look for river rock or sandstone inside the fire area.
• Check fire brick joints for missing mortar wider than about 1/4 inch.

Relining a Block Fire Pit

A heat-damaged block pit can often be saved with a new steel ring or a fire brick lining, as long as the outer wall is still sound. Our guide to fire pit liners and rings covers fitting a ring. Larger rebuilds, and fire pits built into an outdoor kitchen or next to an outdoor fireplace, are outdoor fireplace work for us.

Bring in an experienced mason from a family-run, locally owned company; the free inspection of the pit comes with a written estimate. We are Proudly American, and a Proudly American family crew will tell you if a ring is all it needs.