How big and how thick a hearth extension has to be

Hearth and hearth extension: two parts, one purpose

The hearth proper is the firebox floor, the surface the fire sits on. The hearth extension carries that noncombustible floor out into the room, in front of the opening and past each side of it. Its job is to catch what leaves the fire, such as a rolling log, a spray of embers or ash from the shovel, on something that will not burn.

The hearth extension code in Florida is the Florida Building Code, Residential, 8th edition (2023), which covers masonry fireplace hearths in section R1001.9. It calls for concrete or masonry construction for both parts, resting on noncombustible supports and reinforced enough to carry the slab itself and anything placed on it. It also says nothing combustible may be left touching the underside once construction is finished, which rules out leaving wood forms or plywood in place under a poured extension.

The size rule in section R1001.10

Hearth size requirements depend on the area of the fireplace opening. For an opening smaller than 6 square feet, the extension reaches at least 16 inches out in front and at least 8 inches past each side. Once the opening reaches 6 square feet, the minimums grow to 20 inches in front and 12 inches beyond each side.

Measuring the opening and doing the math

Measure the width of the opening and its height, from the hearth up to the underside of the lintel, in inches. Multiply them, then divide by 144 to get square feet. The threshold is 6 square feet, or 864 square inches, and an opening that lands exactly on 6 square feet takes the larger rule.

Three examples show how it works. An opening 36 inches wide and 24 inches high is 864 square inches, exactly 6 square feet, so it needs 20 inches in front and 12 on each side: an extension at least 60 inches wide (36 plus 12 plus 12) and 20 inches deep. An opening 32 inches wide and 26 inches high is 832 square inches, about 5.8 square feet, so the smaller rule applies: 48 inches wide (32 plus 8 plus 8) and 16 inches deep. A 42 by 30 inch opening is 1,260 square inches, 8.75 square feet, and needs 66 by 20 inches.

Checking your own hearth with a tape

The front measurement runs from the face of the fireplace opening straight out into the room to the front edge of the noncombustible extension. The side measurements run from each side edge of the opening outward along the face. Measure at floor level, and measure the noncombustible material only, not a wood border or a threshold strip that frames it.

Write down the opening width, the opening height and the three extension measurements, then work out the opening area as described above. If the front measurement or either side comes up short of the rule for your opening size, the extension is undersized. A raised hearth is measured the same way, from the opening to the edge of the noncombustible surface in front of it.

Thickness: the 2-inch rule and the raised-firebox exception

Under section R1001.9.1 the hearth itself is at least 4 inches thick. Under R1001.9.2 the hearth extension is at least 2 inches thick.

There is one exception. Once the bottom of the opening stands 8 inches or higher over the extension's top surface, the code lets the extension shrink to 3/8 inch, in brick, concrete, stone, tile or any other approved noncombustible. That is the raised-hearth design, where the firebox sits up at knee height and the apron in front is a thin noncombustible layer. The size rule in R1001.10 still applies in full.

What counts as noncombustible on the extension

Concrete, brick, stone, and ceramic or porcelain tile set on a masonry or concrete base all qualify. Wood flooring, laminate, vinyl plank and carpet are not noncombustible, and they cannot form any part of the required extension, however good they look. A wood border or threshold that runs onto the required area has to be cut back to its edge.

The support underneath counts too, because the code asks for noncombustible support as well as a noncombustible top. A tile finish laid over a wood subfloor in front of a masonry fireplace does not meet R1001.9, even though the part you can see is tile.

Prefab hearths: the listing sets the size

The numbers above are for masonry fireplaces. A factory-built fireplace, tested to UL 127, follows section R1004.2 instead: its hearth extension is installed according to the fireplace's listing, it has to be readily distinguishable from the surrounding floor, and listed hearth extension products comply with UL 1618.

The listing can set a different size, and some listings also specify the insulating value of the extension material rather than only its thickness. The installation manual for the model is the document to read. Wood-burning inserts set into a fireplace carry floor protection rules in their manuals, and those can call for more protection in front of the door than the fireplace's extension provides.

Mantels and trim close to the opening

The same part of the code limits combustible trim around the opening. Under section R1001.11, exposed combustible mantels and trim on the front of a masonry fireplace sit at least 6 inches from the opening. Within 12 inches of the opening, combustible material may stand out from the face by no more than 1/8 inch per inch of distance from the opening.

So a piece of trim 8 inches from the edge of the opening may stand out no more than 1 inch from the face, and one 12 inches away may stand out 1.5 inches. Beyond 12 inches, that projection limit no longer applies.

Older hearths that fall short

Fireplaces built before the current code, or altered since, can have extensions that are shorter or narrower than R1001.10 calls for. When work is done on an existing fireplace, the building department rules on how the current code applies. Separately from any legal requirement, an extension that is too small leaves embers landing on flooring, and that deserves fixing for safety alone. Enlarging an extension means cutting back the surrounding floor and building out the concrete or masonry to the required size on noncombustible support.