Look at the firebox walls and at the chimney where it leaves the roof. Firebrick laid in real mortar joints, under a brick or stone chimney with clay flue tiles on the upper end, is a masonry fireplace. Smooth or molded panels, a metal frame around the opening, a round metal pipe above the damper and a framed chimney with a metal lid is a prefab, which the code calls factory-built. The manufacturer's label, if you can find it, settles the question.
The answer changes which repairs are allowed, which parts can be used, and whether an insert or gas log set is an option at all. It is worth ten minutes with a flashlight before anyone quotes for work.
Five checks you can do from the hearth and the yard
None of these needs a ladder or a tool beyond a flashlight. Taken together they give a clear answer on most houses.
- Firebox walls. Separate firebricks with recessed mortar joints point to masonry. Large panels with a pressed or printed brick pattern and very shallow, uniform lines point to factory-built.
- The opening. A metal frame or trim strip around the firebox opening, often with slots or louvers above or below it, is typical of factory-built units.
- Looking up past the damper. A rectangular throat into a brick smoke chamber points to masonry. A round metal pipe, sometimes with a smaller pipe inside it, points to factory-built.
- The chimney outside. Brick or stone with a clay tile or two poking out of a cement crown is masonry. A boxy chimney clad in stucco or siding, closed by a flat metal plate with a round pipe and hood coming through it, is almost always a framed chase over a metal flue.
- The attic. Where the chimney passes through, solid masonry is one thing. A metal pipe inside a framed box with a gap around it is another.
The label is what settles it
The 2025 California Residential Code (R1004.1) says factory-built fireplaces shall be listed and labeled, installed in accordance with the conditions of the listing, and tested in accordance with UL 127. Factory-built chimneys have a matching rule (R1005.1): listed, labeled, and installed and terminated per the manufacturer's installation instructions.
UL 127 is the standard for factory-built fireplaces; its current edition is the 10th, dated November 21, 2024. Its scope covers the fire chamber, chimney, roof assembly and other related parts that are entirely factory-made and intended for assembly in the field. In other words, the fireplace and its chimney are one tested system, not a firebox with any pipe attached.
The label gives you the manufacturer and model. With those, you can find the installation manual, the list of approved replacement parts, and whether an insert or gas log set is permitted. A masonry fireplace has no such label, because it was built on site.
Three setups that confuse the question
A masonry fireplace with a stainless liner in the flue is still a masonry fireplace. The liner was added inside the original chimney, usually during a relining, and the firebox and smoke chamber are still brick.
An insert sitting inside a masonry fireplace is a separate appliance, with its own label, installed into the old opening. The fireplace around it is masonry; the insert is not. Both matter when anything is repaired.
A brick-clad chase is the reverse case. Some framed chimneys carry a thin brick veneer, so the outside looks like masonry while the flue inside is a listed metal pipe. The firebox and the pipe above the damper will tell you which it is.
Tell us what you have noticed and request an estimate. Estimates are free.
What changes if yours is factory-built
Repairs follow the listing. Cracked refractory panels are replaced with the panels specified for that model rather than rebuilt in firebrick. Damaged flue sections, firestops and termination caps are replaced with parts listed for that system. Mixing brands, or adding a cap from a different system, takes the installation outside its listing.
The weather protection is different too. There is no crown. A metal chase cover closes the framed chase, and when it rusts, water runs into the framing where nobody sees it. Rust stains at the chimney's upper end or on the exterior cladding are an early warning on this type.
Adding an insert or converting to gas is possible only where the fireplace manufacturer's instructions and the insert's own listing both allow it. Some factory-built fireplaces are not designed to take an insert at all.
What changes if yours is masonry
A masonry fireplace is repaired with masonry methods. Open joints in the firebox are raked out and refilled with refractory mortar, and cracked firebricks are cut out and replaced. The smoke chamber can be parged. The crown, the flue tiles and the brick above the roof are separate jobs, each covered by a different trade method.
A masonry fireplace is also the more common candidate for an insert, because the old firebox and chimney can house one with a liner run up the flue to connect it. Whether that is sensible depends on the condition of the chimney it goes into.
Adding or replacing a wood-burning fireplace in the basin
The type question matters again if you are thinking about replacing a fireplace. CRC R1004.1.1 requires factory-built wood-burning fireplaces to be qualified at the EPA Voluntary Fireplace Program Phase 2 emissions level. South Coast AQMD Rule 445 separately says no wood-burning device may be permanently installed in new development, meaning construction begun on or after March 9, 2009, and limits what types of permanently installed device can be sold or installed.
Those rules have exemptions, and how they apply to a given house is for the local building department and the district to confirm. In the City of Los Angeles that department is LADBS. The rules for burning on no-burn days are a separate question, covered in the article on whether you can burn wood in Los Angeles.
When you do not need anyone to tell you
If the label is readable and the manual is on file, you already know what you have. Paying for a visit just to identify the type is not a good use of money.
The same applies to fine crazing on refractory panels that has not changed over a season. That is normal thermal wear on a factory-built firebox, and watching it is a legitimate decision. The visit becomes worthwhile when a panel is cracked through, a piece is missing, the chase cover is rusting, or you plan to add an insert or gas logs and need to know whether the fireplace allows it.
Next steps for each type
Once you know which type it is, the right page is easier to pick.
Cracked firebrick, open joints and worn refractory panels are covered on firebox repair. A framed chimney with a rusted or flat metal lid needs a chase cover repair or replacement. If you are weighing up an insert, the fireplace insert installation page explains what has to be true of the fireplace first.
Sources & Further Reading
Service areas
Los Angeles and cities across the county, with the local conditions that affect chimneys there.
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