Can I patch crumbling mortar joints inside my firebox myself or should I hire a mason in Ottawa and what's the cost?

Patching crumbling mortar joints inside the firebox is one of the few masonry repairs where a capable DIYer can do a reasonable job — but only under specific conditions. If the damage is limited to surface crumbling of mortar joints with the surrounding firebrick still solid and intact, a DIY repoint using proper refractory mortar is a legitimate option. If the firebrick themselves are cracked, spalled, or loose, or if the damage extends beyond a few joints, you need a professional mason.

The single most important thing to understand is that you cannot use regular mortar, Portland cement, or general-purpose masonry products inside a firebox. The firebox reaches temperatures of 800 to 1,000 degrees Celsius during a hot fire, and standard mortar will crack, crumble, and fail within a season. You need refractory mortar — also called high-temperature mortar or furnace cement — rated for continuous exposure above 1,200 degrees Celsius. Products like Rutland or Sairset refractory mortar are available at Ottawa hearth shops and some building supply stores. Expect to pay $25 to $50 for a small tub sufficient for minor joint repairs.

For a DIY repair, the process is straightforward. Let the firebox cool completely — at least 24 hours after the last fire. Rake out the loose, crumbling mortar to a depth of about 12 to 20 millimetres using a cold chisel and wire brush, removing all soft or powdery material until you reach sound substrate. Vacuum out the dust thoroughly. Dampen the joints lightly with water before applying the refractory mortar — dry brick will pull moisture out of the mortar too quickly and weaken the bond. Pack the refractory mortar firmly into the joints using a pointing trowel or even a gloved finger for tight spots, finishing flush or slightly recessed with the brick face. Allow it to cure for 24 to 48 hours before lighting a fire, and then cure the repair gradually with a series of small fires over the first week rather than going straight to a full blaze.

Where this becomes a professional job is when the damage is more extensive. If you find loose or cracked firebrick, mortar joints that are missing entirely, cracks running through the brick rather than just the joints, or any sign that the firebox back wall is bowing or shifting, stop and call a mason. A compromised firebox structure is a fire and carbon monoxide hazard — hot gases and flames can penetrate through damaged joints into the surrounding framing. In Ottawa, where fireplaces run hard for six months of the year, a weakened firebox deteriorates faster than in milder climates.

A professional mason in Ottawa will charge $500 to $1,500 for a firebox repoint depending on the extent of deterioration. Full firebox rebuilds — replacing damaged firebrick and relining the firebox — run $2,000 to $4,500 depending on the fireplace size and access. If the mason discovers that the smoke chamber above the firebox is also deteriorating (common in older Ottawa homes), add another $800 to $2,000 for smoke chamber parging with refractory mortar.

One important note: if you have a wood-burning fireplace or stove, schedule a WETT inspection before or alongside any firebox repair work. A WETT-certified chimney professional can assess whether the damage is cosmetic or structural, and their report will satisfy your insurance company's requirements. You can browse fireplace and chimney professionals through the Ottawa Construction Network directory at justynrookcontracting.com/directory if you want a qualified set of eyes on the firebox before deciding whether to DIY or hire out.