Repairing a Chimney on a Historic or Landmarked Home

Why old chimneys need different treatment

Chimneys on nineteenth and early twentieth century brownstones, rowhouses and Victorian houses were built with softer, more porous brick and lime-based mortar. That combination flexes slightly with temperature and moisture and lets water that soaks in dry back out, mostly through the mortar joints. The joints are meant to be the weaker, sacrificial part: they wear and get repointed every few decades while the brick lasts.

Modern repairs often break that balance. A hard, cement-rich mortar packed into old joints is stronger and less permeable than the brick around it. Water and movement then go through the brick instead of the joint, and the brick faces crack and pop off. The result is spalled brick on a chimney that was only due for repointing. Many damaged historic chimneys owe more to an earlier repair than to age.

What gets assessed first

A historic repair starts with understanding what is there. The technician looks at the brick (hardness, size, face texture, any glazed or molded units), the mortar (color, aggregate, joint width and tooling), any earlier repairs and what they were made with, and the details at the top: corbels, decorative courses, pots and the crown. Inside, the flue is checked for lining, cracks and whether it is still in use.

The findings decide the method. A chimney with sound brick and washed-out joints needs careful repointing. One with spalled faces from hard mortar needs selective brick replacement and repointing. One that is loose, leaning or cracked through needs partial rebuilding, ideally reusing as much of the original brick as possible.

Matching mortar and brick

Mortar is the decision that matters most. The mix is chosen to be softer and more vapor-permeable than the brick, which on older buildings usually means a lime-rich mortar rather than a standard modern cement mix. Color comes from the sand and any pigment, and is matched to the weathered original rather than to a freshly cut joint. Joint width and tooling are matched too, since a wide, smeared joint on a building with fine joints is obvious from the street.

Replacement brick comes from salvage, from the chimney itself when part of it is rebuilt, or from a matched new unit that is close in size, color and hardness. A brick that is harder than its neighbors causes the same problem as hard mortar.

• Mortar: a lime-rich mix softer than the brick, never a hard cement-rich mortar
• Joint removal: careful raking that protects the brick edges, not aggressive grinding that widens joints
• Damaged brick: salvaged or matched units rather than mismatched modern brick or patching compound
• Cleaning: the gentlest method that works, not strong acid or high-pressure blasting
• Water protection: a sound crown, a cap and breathable details instead of film-forming coatings

Landmarks approvals in New York City

The city's business portal says that if you own a landmark or a building in a designated historic district, you need a permit from the Landmarks Preservation Commission before doing any work, and you also need an LPC permit for any work that needs a Department of Buildings permit. LPC issues three types of permits, and the type depends on the work.

For chimneys, the distinctions are practical. The portal lists cleaning or repairing masonry as an example of work that needs a Permit for Minor Work, even though it does not need a DOB permit. Installing heating equipment that needs a DOB permit but does not change protected features is an example of a Certificate of No Effect. Larger changes, such as rooftop additions, demolitions or removing old features, call for a Certificate of Appropriateness, which involves a public hearing. Taking down or significantly changing a visible chimney falls on that side. If you are not sure whether your building is designated, ask the Landmarks Preservation Commission through 311.

• Permit for Minor Work: changes protected features but needs no DOB permit, such as cleaning or repairing masonry
• Certificate of No Effect: needs a DOB permit but does not change protected features
• Certificate of Appropriateness: changes significant features beyond staff-level guidelines; public hearing

Philadelphia and New Jersey

In Philadelphia, L&I's building permit page says properties on the city's historic register need Historical Commission approval for work, and that L&I will refuse a permit application that lacks it. L&I's permit standard for chimney and vent extensions repeats that historically designated buildings need prior approval from the commission. Rowhouse chimneys are part of the streetscape, so expect interest in brick, mortar color and the profile of the top.

Many New Jersey towns have their own historic preservation commissions and districts, with rules that vary from town to town. Before exterior work on an older house in a historic area, check with the local construction office or preservation commission. This is general information; confirm the rules for your address before work starts.

Keeping the top intact

The top of a historic chimney takes the worst weather and is usually where the damage is. Original corbelling, decorative courses and clay or terra cotta pots are worth keeping, and they can often be reset rather than replaced. A crown that sheds water and overhangs the brick protects everything below. A cap can be chosen to sit low and dark so it reads as part of the chimney rather than a modern addition, and on chimneys with pots, caps made to fit pots are available.

Avoid sealing the masonry with a film-forming coating. Old brick has to dry out, and coatings that trap moisture lead to spalling behind the surface.

Making an old chimney safe to use

Restoring the outside is half the job if you plan to light a fire or run a boiler through the chimney. Many old flues were built without any liner, or with clay tiles that have since cracked or lost their joints. A liner sized for the fireplace or appliance can usually be installed inside the old chimney without changing anything visible from the street, and in a landmarked building, LPC's Certificate of No Effect category covers work that needs a DOB permit but leaves protected features unchanged; ask LPC which permit fits your job. The fireplace itself, often built for coal, may need its own evaluation before wood is burned.

What to expect from the project

Historic work takes longer than standard repair. Mortar has to be matched and sometimes test-patched, brick may need sourcing, approvals may take time, and lime mortars cure more slowly and need protection from rain and frost while they set, which affects scheduling in late fall. Our crews start with a full look inside and out, then provide a written plan covering both the masonry and the flue, so you have everything a commission or a building department may ask about before any work starts.