Building an Outdoor Fireplace With Cinder Blocks

Block as the Shell, Not the Firebox Face

Concrete block is a fine structural material and a poor fire surface. Unilock, which makes concrete wall units, warns that excessive heat may cause concrete units to break down over time if they are not designed for high temperatures, and recommends a metal liner or fire-rated brick wherever fire meets block. A cinder block fireplace works when the block carries the weight and something else takes the heat.

The 2023 Florida Building Code, Residential, sets the details in chapter 10 for masonry fireplaces. Outdoors, those sections are the standard a building official will hold a block fireplace to.

Grouted Solid, Not Hollow

Open block cores have no place around a fire. Section R1001.5 allows a firebox of solid masonry units, hollow masonry units grouted solid, stone or concrete, and section R1003.10 builds the chimney walls the same way, at least 4 inches nominal. Grout fills the cells so the wall acts as solid masonry, and the lintel over the opening has to be non-combustible, with at least 4 inches of bearing on each side under section R1001.7.

The Fire Brick Lining

The firebox walls take the fire, so their thickness is set in the code. With a fire brick lining at least 2 inches thick, the back and side walls total at least 8 inches of solid masonry including the lining. Without a lining, they need at least 10 inches of solid masonry.

• Fire brick meeting ASTM C27 or C1261.
• Laid with medium duty refractory mortar meeting ASTM C199.
• Joints between fire bricks no wider than 1/4 inch.
• A firebox at least 20 inches deep, under section R1001.6.

Smoke Chamber and Flue Inside the Block

Above the firebox, the block keeps working as structure. Section R1001.8 builds the smoke chamber of solid masonry or grouted block, parged smooth inside with refractory mortar, and section R1003.11 requires the chimney to be lined. A clay flue liner inside the block chimney is one way to meet it.

The liner has to be big enough. Under section R1003.15, a square flue needs at least a tenth of the fireplace opening's area, a round one a twelfth, and a long rectangular one an eighth. Our guide to outdoor fireplace dimensions works through the numbers.

The Footing Under the Block

Block is heavy, and a block fireplace and chimney stack it high on a small footprint. Section R1001.2 calls for a footing of concrete or solid masonry at least 12 inches thick, 6 inches wider than the fireplace on every side and at least 12 inches below grade. Hernando County's permit exemption list says any slab poured to support vertical construction needs a permit, which a fireplace footing does. More on that in our guide to outdoor fireplace footings.

Why Block Fireplaces Crack Apart

When a cinder block outdoor fireplace fails, one of the code items above is usually missing.

• No fire brick lining, so the block faces the fire directly and breaks down.
• Open, ungrouted cores that crack around the firebox.
• No flue liner, so heat and smoke work straight on the block chimney.
• No footing, so the fireplace settles on a patio slab or pavers and cracks from the base.
• No cap, so rain fills the cells and the firebox from above.

Finishing It Properly

Stucco, stone veneer or a painted finish goes on the outside of the block once the fireplace is built right inside. Building or rebuilding one is our outdoor fireplace work, from the footing to the cap, starting with a free inspection and a written estimate.

Our experienced masons work for a local, family-owned company, and we are Proudly American. Plain spoken in the Proudly American way, we will tell you if an existing block fireplace can be lined or should come down.