Portland cement mortar on old brick is the mortar mistake that ruins Chicago brick. When mortar is harder and less permeable than the brick, moisture and stress move into the brick instead of the joint, and the faces spall or delaminate. National Park Service guidance says repointing mortar must be softer and more vapor-permeable than the brick and no harder than the original mortar. Chicago code does not prescribe a mortar type, so this comes down to your contractor's knowledge.
Portland cement mortar on old brick is the single most expensive tuckpointing mistake, because it damages the brick itself, not just the joint. The mortar looks great. The wall around it slowly comes apart. Here is why, how to spot it, and how it gets fixed.
Why is hard mortar bad for old brick?
The job of mortar is to be the sacrificial part of the wall. The National Park Service (NPS) says the joint should relieve stress in the wall, not the brick, and that repointing mortar must be softer (lower compressive strength) and more vapor-permeable than the brick, and no harder than the original mortar (NPS Preservation Brief 2). It calls the assumption that harder or stronger means better a common error. A mortar stronger than the brick causes cracking and spalling of the brick that cannot be easily repaired (NPS).
The Brick Industry Association (BIA) says the same thing in its repointing guidance: the compressive strength of the repointing mortar should be similar to or weaker than the original mortar, to avoid irreparable brick damage. BIA's note is aimed at modern brick buildings. The NPS guidance is aimed at historic masonry (BIA). For old Chicago brick, the historic principle is the one that applies.
How does the mortar damage the brick?
Water moves through a wall in both directions, and it has to be able to leave. Mortar joints are normally the path of least resistance. When the mortar is less permeable than soft brick, moisture and dissolved salts leave through the brick instead. Salt crystallizing inside the brick can make the face spall or delaminate. The NPS shows trapped moisture eventually popping the surface off bricks behind hard portland mortar (NPS). Freeze-thaw of saturated brick is another cause of the same kind of damage (BIA).
So hard mortar does not fail the way people expect. The joint stays sound. The brick beside it is what starts to go.
What does "hard" and "soft" mean for mortar types?
Mortar is sorted into ASTM C270 types by strength. In decreasing order, with typical compressive strengths: Type M about 2,500 psi, Type S 1,800, Type N 750, Type O 350 and Type K 75 (NPS). More lime makes a mortar softer and more permeable (NPS). For historic masonry, the NPS points to lime-rich mixes such as Type O or Type K, or a lime and sand mix (NPS).
The BIA recommends Type N for normal use, including most veneer (BIA). That is guidance for modern brick. It does not make Type N the answer for every old wall. The right mortar for your wall depends on the brick and the original mortar, and a mason who has not looked at either is guessing.
Chicago's exterior maintenance code requires masonry joints to be weather resistant and water tight, and the City requires licensed mason contractors and permits. In the text we read, it does not prescribe a mortar type. The softer-mortar rule is industry and preservation guidance, not a City mandate (City code, 14X-3-303).
What are the warning signs hard mortar is hurting your brick?
- Brick faces flaking, peeling or popping off beside new joints, while the joints themselves look intact.
- Joint lines standing proud of the brick, with a gray, dense look.
- Brick edges and corners crumbling along the joint.
- White salt deposits (efflorescence) returning after the tuckpointing. Salts need water and a path to the surface (BIA).
- A wall that was repointed in patches, where the patched sections are flaking and the untouched sections are not.
On older Chicago buildings, we often see a mix of old and new work on the same wall: original softer joints in one section, hard gray joints in another from a past repair. The damaged brick is nearly always next to the hard joints, not the old ones.
How is it fixed?
The hard mortar has to come out, carefully, without damaging the brick any further. The NPS says hand chisels are the least damaging method for removing old mortar (NPS). Bricks that lost their faces to spalling are replaced where the loss is serious, and the wall is repointed with a softer, more permeable mortar matched to the brick and the original joints. If the wall has been repointed in patches, the scope can grow. See the tuckpointing mistakes list for the other errors that usually come along with this one, and Type N vs Type S mortar for how mortar types compare.
Brick that has lost its face is not repaired by repointing. The longer hard mortar stays in the wall, the more bricks have to be replaced when it finally comes out. Every winter adds to the damage. Spalling brick can also become a safety problem if pieces come loose over a sidewalk.
What to ask your contractor
Ask what mortar they plan to use, in what proportions, and why it suits your brick. Ask whether they will match it to the original mortar, and whether they will do a test panel. A contractor who answers "Type S, it is stronger" is describing the problem. For a second opinion or an estimate on a wall you suspect has already been damaged, talk to our tuckpointing crew, and read the Chicago tuckpointing guide for the rest of the process. For how to read the wall yourself, see signs your brick needs tuckpointing.
Common questions
Straight portland cement mortar is generally too hard for old brick. National Park Service guidance says repointing mortar should be softer and more permeable than the brick and no harder than the original mortar. Lime-rich mixes are the historic standard.
There is no single answer, because it depends on the brick and the original mortar. For historic masonry, the NPS points to lime-rich mixes such as Type O or Type K, or lime and sand. Have a mason check your wall before choosing.
Not in the text we reviewed. The code requires masonry joints to be weather resistant and water tight, and the City requires licensed mason contractors and permits. Mortar type is a matter of industry and preservation guidance.
Look for brick faces flaking or popping off beside new joints that look intact, crumbling edges along the joint, and gray, dense joints that stand proud of the brick. Damage next to new mortar and not next to old mortar is the clue.
Sources
- National Park Service: Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings (PDF)
- Brick Industry Association: Repointing Brick Masonry (PDF)
- Brick Industry Association: Technical Note 8: Mortars for Brickwork (PDF)
- Brick Industry Association: Technical Note 23A: Efflorescence, Causes and Mechanisms (PDF)
Facts last checked: October 7, 2026.
This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.