Repairing a firebox - Fine Homebuilding Question & Answer
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Repairing a firebox

Q: The fireplace in my house was rebuilt sometime in the late 1960s. The walls of the firebox are still in beautiful condition, but the floor seems to be deteriorating. The mortar between the bricks is thinning, and some of the firebricks are spalling. I’d like to repair the floor. Do I need a special type of mortar? Could a thin coat of cement be applied over the existing floor to even it out? Should the spalling bricks be replaced?

A: John Carroll, a mason in Durham, North Carolina, replies: As you suspect, there is a special mortar for laying up fireboxes; it is called refractory mortar. But don’t assume that your fireplace was built with this mortar. When I started laying bricks in the 1970s, refractory mortar was not required by the building code, was difficult to find, and, consequently, was rarely used. Well into the 1990s, bricklayers used portland-cement-based masonry mortars, or they concocted a fireclay mortar by mixing together portland cement, crushed fireclay, and sand. These code-approved mortars haven’t fared well in households that use their fireplaces regularly. The problem is simple: Portland cement does not hold up well when subjected to the cycle of heating and cooling that occurs in a firebox. The deteriorating mortar on the floor of your firebox is almost certainly a portland-cement mortar.

Recently, most major building codes have added clauses requiring refractory mortar. The most widely used code, the International Residential Code (IRC), for example, states that  firebrick “shall be laid with medium-duty refractory mortar conforming to ASTM C 199.” Refractory mortar doesn’t use portland cement as a binder; instead, either calcium aluminate or sodium silicate is the binder. Over the long haul, refractory mortar performs far better than portland-cement mortar. Refractory mortar is available at masonry-supply yards or fireplace stores.

For your problem, you can take one of three approaches. You can simply rake out and repoint the joints between the bricks with refractory mortar (see the list of suppliers below). Another approach would be to apply a thin coat of refractory cement over the floor. For this, use a refractory cement formulated for repairs, such as Rutland FSC (Rutland Products; 800-544-1307), Rescobond 3000G (Resco Products; 888-283-5505), or HeatStop Flue Glue (HeatStop Refractory Mortars; 716-667-2321). A final approach is to remove the damaged bricks and replace them with a castable refractory cement. For this, use Rutland Castable Refractory Cement or HeatCast-40. Whichever approach you take, make sure you scrub the surface clean and vacuum any dust before you make the repair.

From Fine Homebuilding 164, pp. 104 July 1, 2004

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