Masonry Outdoor Fireplaces: the Footing, the Flue, and Why the Cheap Ones Crack

An outdoor fireplace is the tallest, heaviest, hottest thing in most backyards. It concentrates a lot of weight on a small footprint and then cycles between ambient temperature and several hundred degrees.

Those two facts explain essentially every failure we are called to look at.

Failure one: the footing

This is the most common and the most expensive.

A masonry fireplace needs a footing designed for its weight, thick enough, extending beyond the structure on all sides, on soil that was prepared. What it frequently gets instead is to be built on top of an existing patio slab that was poured four inches thick for people to walk on.

What happens next is predictable. The slab cannot carry the concentrated load, it settles under the fireplace, and the fireplace cracks — usually with a vertical crack running up the face, wider at the top.

A vertical crack that is wider at the top than at the bottom means the base rotated. That is a foundation problem, and repointing the crack does nothing.

On expansive clay, which is most of this valley, a shallow or undersized footing also rides the seasonal soil movement. The preparation that prevents it is the same as for any structure here, and it is in what goes under a concrete pad in Temecula and Murrieta.

Failure two: the wrong mortar in the firebox

The firebox is lined with firebrick, and firebrick is set with refractory mortar. Refractory mortar is formulated to survive the temperature cycling.

Ordinary masonry mortar is not. Used in a firebox, it degrades — it crumbles out of the joints over a season or two, the firebrick starts to loosen, and eventually heat reaches material that was never intended to see it.

This shortcut is invisible at handover and obvious three winters later. If you are commissioning a fireplace, ask specifically what the firebox is lined with and what mortar is being used in it. The answer should be firebrick and refractory mortar, without hesitation.

Failure three: no allowance for movement

Masonry expands when it heats and contracts when it cools. A fireplace that is rigidly attached to an adjoining wall, a seat wall or a veneer with no allowance for that movement will crack at the joint between them.

The tell: a crack exactly at the line where the fireplace meets something else, straight and clean. That is not settlement, that is thermal movement with nowhere to go.

Failure four: the flue is wrong

A chimney works because the flue is proportioned to the firebox opening. Too small and smoke comes into the seating area. Too large and the flue never develops a proper draft, which also produces smoke and, over time, more creosote.

Symptoms of a flue problem: the fireplace smokes when first lit, smokes when the wind is from a particular direction, or never seems to draw properly no matter how the fire is built.

The chimney height requirements matter here too, and they are in outdoor fireplaces in Riverside County. The same principles that govern a house chimney apply, and they are covered in chimney retrofit and reconstruction in Los Angeles.

Failure five: water

An outdoor fireplace has no roof over it. Rain enters the flue and soaks into the masonry, and saturated masonry that then gets heated is a fast route to spalling — the face of the stone or brick popping off.

Two defenses: a chimney cap, which keeps the bulk of the water out of the flue and is inexpensive, and appropriate treatment of the masonry so it is not absorbing water like a sponge.

A chimney cap also serves as the spark arrestor, which matters more if you are in a fire zone. See outdoor fireplaces in a very high fire hazard severity zone.

Reading your own fireplace

• Vertical crack up the face, wider at the top: footing settlement or rotation.

• Crack exactly where the fireplace meets an adjacent wall: thermal movement.

• Crumbling joints inside the firebox: wrong mortar.

• Face of the stone or brick flaking off: water plus heat.

• Smoke entering the seating area: flue sizing, chimney height, or a nearby structure interfering with draft.

• Stepped cracking through the veneer: movement below, following the mortar joints.

What a real repair involves

For a firebox with failed mortar: the firebrick comes out and goes back in with refractory mortar. That is a real repair and it lasts.

For a settlement crack: the cause has to be addressed first. Repointing a crack caused by a moving footing gives you a tidy crack that reopens. The honest options are underpinning the footing or, if the movement is severe, rebuilding from the base.

For thermal cracking at a joint: the joint gets detailed to allow movement rather than resist it.

The pattern is the same as everywhere else in construction. Fix the cause, then fix the symptom. Doing it the other way around is how people pay twice, which is exactly the argument in which concrete cracks actually matter.

A cracking fireplace is telling you something specific about what is under it or inside it. Book a free 15-minute call or call (323) 707-4979 and describe where the cracks are. Location usually identifies the cause.

Prefer to watch before you call? X Plan Nation is The Ultimate Pro's YouTube channel, where we take construction apart step by step so you can tell good work from work that will come back.

The Ultimate Pro, Inc. · California contractor license 831700 · Class B general building · 18481 Grand Avenue, Lake Elsinore, CA 92530 · Serving Los Angeles County, Orange County, Riverside County, San Bernardino County and the Inland Empire since 1996.