By James Rodriguez, Central Chimney Service.
Repointing a chimney — raking out failed mortar and replacing it — looks like the most straightforward masonry job there is. It is also the one where using the wrong material does more damage than leaving the joints alone would have. The mortar that goes back in has to be softer than the brick around it, and a great deal of the crumbling brick we are asked to look at was caused by repointing done with mortar that was stronger.
Mortar is supposed to be the sacrificial part
In a masonry wall, the mortar joint is designed to be weaker than the units it bonds. Brick expands, contracts, and shifts with temperature and moisture, and the joint absorbs that movement by giving slightly.
Mortar is also the drying path. Moisture that gets into a wall migrates out through the joints, which is why they weather visibly while the brick faces stay intact. A joint deteriorating over decades is the system working as intended. It is maintenance, not failure.
What happens when the mortar is too hard
Modern high-portland mortars are considerably stronger than the lime-rich mortars used in older masonry, and stronger than many of the softer brick they get applied to. When the joint is harder than the brick, the brick becomes the sacrificial part instead.
The movement has to go somewhere, so it goes into the brick face. Moisture that used to leave through the joints now leaves through the brick, carrying salts with it and freezing inside the face during our winters. The result is spalling — brick faces popping off — concentrated exactly where the repointing was done, usually within a handful of seasons. At that point the repair is replacing brick, not replacing mortar.
This matters most on older housing stock. In the range of home ages we work on across Lisle chimney sweep calls and the surrounding towns, the correct mortar for a 1920s chimney and the correct mortar for a 1998 chimney are genuinely different materials, and matching them is a judgment made at the chimney rather than at the counter.
Reading your own joints
You can assess this yourself from the ground with binoculars and, on the accessible lower portion, a key. Run the key along a joint: if it gouges out sand and crumbs easily, the mortar has lost its binder.
Look for joints recessed well behind the brick face, gaps you can see daylight or shadow in, vegetation or moss rooted in a joint, and any joint where you can insert something and meet no resistance. Missing mortar at the top courses and on the weather-facing side is normal and expected — those take the most exposure.
The findings that mean stop rather than schedule: brick that moves when pushed, a visible lean or bulge in the stack, a horizontal crack running through several courses, or a chimney with a stair-step crack down one face. Those are structural questions, not pointing questions.
What waiting costs
Open joints are a direct water path into the core of the chimney. Once water is in the wall, it saturates the brick from behind, freezes, and drives spalling from the inside — and it reaches the flue liner, where it accelerates deterioration you cannot see.
The progression is predictable: joints that need repointing this year become joints plus a dozen replacement brick in three years, and a partial rebuild above the roofline in a few more. The cost curve is steep because each stage adds work that the previous stage would have prevented, which is why the chimney repair conversation is easier the earlier it happens.
Timing and what proper work looks like
Mortar needs temperatures reliably above its minimum for a sustained cure, which in northern Illinois closes the practical window well before hard frost. Late summer through early fall is when this work gets done properly.
Done right, the joints are raked to a depth roughly two to two and a half times the joint width — not skimmed over the surface — the mortar is matched in strength and appearance, it is packed in layers rather than in one pass, and it is tooled to a profile matching the original so the joint sheds water. Smearing new mortar over old is a cosmetic job that traps water behind it and fails within a couple of winters.
A chimney inspection is what establishes which of these you are dealing with, and you get the price before any work starts — it is published on our pricing page. Insured and bonded, 29 years, 7 days a week from 7 AM to 7 PM at (815) 305-5510.
Frequently asked questions about chimney repointing
What is the difference between repointing and tuckpointing?
In everyday use the terms are used interchangeably for replacing deteriorated mortar. Strictly, tuckpointing is a decorative technique using two mortar colors to imitate fine joints. What most chimneys need is repointing: raking out failed mortar and replacing it with a properly matched mix.
Why does the new mortar have to be softer than the brick?
Because the joint is meant to be the sacrificial element that absorbs movement and lets moisture escape. Mortar harder than the brick forces the brick to take that stress instead, which causes the face to spall. Matching the strength of the original mortar is the single most important part of the job.
Can I repoint a chimney myself?
The accessible lower portion is within reach for someone comfortable with masonry, provided the mortar is properly matched. The parts that fail first — the top courses and the crown — are above the roofline, and roof work on a chimney is where the risk is. Getting the mix wrong up there also costs more to undo than to do.
How do I know if my joints need repointing?
Test them. Run a key along the joint: if mortar gouges out as loose sand, the binder is gone. Joints recessed noticeably behind the brick face, visible gaps, and plants growing in a joint all confirm it. Weather-facing sides and top courses go first.
How long should repointing last?
Correctly matched and properly tooled mortar lasts for decades, essentially becoming the new maintenance baseline. Surface-smeared or over-hard mortar can fail within a few winters and often takes brick faces with it, which is why the material choice matters more than the labor.
Can this be done in winter?
Not reliably. Mortar needs sustained temperatures above its minimum to cure, and mortar that freezes before curing never reaches full strength. In our climate this is late-spring-to-early-fall work, which is why waiting until you notice a problem in December means waiting until spring.
Should I seal the chimney after repointing?
It can help, with two conditions: the new mortar must be fully cured first, and the product must be a vapor-permeable penetrating repellent rather than a film-forming sealer. A film traps moisture inside the masonry and produces exactly the spalling it was meant to prevent.
Is a cracked chimney the same problem as open joints?
No, and the distinction matters. Deteriorated joints are maintenance. A crack running through brick and mortar across multiple courses, a lean, or a bulge indicates structural movement or a failing foundation, and that needs assessing before anyone points anything.
Published September 8, 2026 · Last updated August 27, 2026