Can a chimney with deteriorated mortar joints still be used safely or should I stop using the fireplace?

If the mortar joints on your chimney are visibly crumbling, cracked, or receded, you should stop using the fireplace until a qualified mason and a WETT-certified chimney inspector have assessed the chimney's condition. Using a fireplace connected to a chimney with deteriorated mortar joints carries real risks — carbon monoxide leakage, chimney fire, and water damage — that range from inconvenient to life-threatening depending on the severity of the deterioration.

The concern is not primarily about the exterior mortar joints you can see from the ground, though those are an indicator of the chimney's overall condition. The critical issue is the interior mortar joints inside the flue — the liner that contains combustion gases and heat. If the exterior mortar is crumbling after years of Toronto's freeze-thaw cycling, the interior joints are likely in similar or worse condition due to the additional stress of heat cycling, acidic flue gases, and creosote exposure. Deteriorated interior mortar joints can allow combustion gases, including carbon monoxide (CO), to leak through the chimney walls into your home's living spaces. Carbon monoxide is odourless and colourless — you will not know it is leaking until a CO detector alarms or, worse, someone becomes ill.

The second risk is chimney fire. When interior mortar joints deteriorate, gaps and voids form in the flue liner. Creosote — the combustible tar-like residue that builds up inside wood-burning chimneys — can accumulate in these voids. If a chimney fire ignites (which happens more frequently than most homeowners realize), flames and extreme heat can reach the chimney's structural masonry and adjacent wood framing through the deteriorated joints. A chimney fire in a sound chimney is contained by the intact flue liner; in a chimney with failed mortar joints, the fire can extend to the home's structure.

Before using the fireplace again, get two assessments. First, have a WETT-certified chimney sweep (Wood Energy Technology Transfer — the Canadian standard for chimney and wood-burning appliance inspection) perform a Level 2 inspection, which includes a visual examination of the flue interior, typically using a camera inserted from the top. This inspection identifies deteriorated mortar joints, cracked tiles, creosote buildup, and structural deficiencies inside the flue. A Level 2 WETT inspection in the GTA typically costs $200–$500 and is well worth the investment for peace of mind. Second, have a qualified mason assess the exterior mortar joints to determine whether repointing, partial rebuild, or other structural work is needed.

If the interior flue is sound and only the exterior mortar joints need repointing, you may be able to continue using the fireplace while scheduling the exterior repair — but confirm this with the WETT inspector. If the interior flue liner has deteriorated mortar joints, the chimney needs interior repair before use. Options include parging the interior flue (applying a new morite coat inside the flue, typically $1,000–$3,000), installing a stainless steel chimney liner ($1,500–$4,000 installed, which provides a new, sealed flue inside the existing chimney), or in severe cases, a partial chimney rebuild.

Exterior chimney repointing in the GTA runs $1,000–$5,000 depending on size and accessibility, with scaffolding adding $500–$1,500. A chimney crown repair or replacement adds $500–$2,000. Installing a chimney cap — a stainless steel or copper cover that keeps rain, snow, and animals out of the flue — costs $200–$800 and is one of the most cost-effective ways to slow future mortar deterioration by keeping the bulk of water out of the chimney.

Under the Ontario Building Code and Ontario Fire Code, chimneys serving solid-fuel appliances must maintain a continuous, intact flue liner. WSIB coverage should be verified for any contractor working on your chimney. Do not attempt chimney flue repair yourself — it requires specialized tools, materials, and working at height. Toronto Parging can connect you with qualified chimney masons for a professional assessment.