On a Georgetown rowhouse I inspected last spring, someone had repointed the front elevation with a hard gray mortar that looked crisp and new. The problem was everywhere around it: the faces of the old bricks were spalling off, popping and flaking in a way the untouched side elevation was not doing at all. The repair had not saved the wall. It was destroying it, one brick face at a time, and the owner had paid good money for the damage.
Why old brick needs soft mortar
Brick made before roughly the early twentieth century is softer and more porous than modern brick, and the lime mortar it was laid in was softer still. That softness is not a defect. It is the design. A masonry wall moves and takes on moisture, and in a lime-mortar wall the mortar is the sacrificial part: it flexes, it lets water pass through and evaporate out of the joints, and when it wears it can be replaced without touching the brick. The joint is meant to be weaker than the brick.
Modern Portland cement mortar reverses that relationship. It is harder and less permeable than the old brick around it. Now the brick is the weak, sacrificial part. Moisture that used to leave through the joints has to leave through the brick faces instead, and when it freezes or carries salts, it blows the face off the brick. That is the spalling I saw in Georgetown. The mortar wins and the wall loses.
How the wrong repair does its damage
The mechanism is worth understanding because it explains why the damage shows up in the brick, not the mortar. Water always finds the easiest way out of a wall. Repoint a soft old wall with hard cement and you have sealed the joints, so the next path out is through the brick body. In a climate with real freeze-thaw cycles, and DC gets enough of them, that trapped water expands as it freezes just under the brick face and pops it off. Add the salts from de-icing or groundwater and you get the same result faster. Within a few winters, a wall that stood for a century starts shedding its faces.
The National Park Service Preservation Briefs, particularly the one on repointing mortar joints in historic masonry, lay this out in detail and are the standard reference preservation contractors actually use. If a contractor has not heard of them, that tells you something.
What a correct repointing looks like
Matching historic mortar is a real skill. The repair mortar should be softer than the brick, usually a lime-based mix with little or no Portland cement, matched to the original in strength, color, and joint profile. The old joints are raked out by hand or with careful power tools to a consistent depth, not ground out aggressively in a way that widens the joint and damages brick edges. Then the new mortar is packed in tight and tooled to match the original profile, because the joint shape affects how water sheds off the wall.
This is slow, and it is not cheap, but it is a fraction of the cost of replacing spalled brick later with salvaged period brick that matches. In historic districts, it is also frequently required, and the review board will care what mix you used.
What it costs, and what the wrong version costs later
Correct historic repointing typically runs $15 to $35 per square foot depending on access, joint condition, and how much of the wall needs it. Full-height scaffolding on a rowhouse pushes the number up. That sounds like a lot until you price the alternative: once brick faces have spalled, you are into brick replacement with matched salvage at $30 to $60 per square foot and up, plus the repointing you should have done in the first place. The cheap hard-mortar repair is the single most expensive thing you can do to an old wall. If you are trying to sort out which cracks and losses are genuinely urgent versus cosmetic on an older building, it helps to read how to tell shared party-wall risk apart from surface wear before you scope a job.
Matching historic mortar is a real skill, so for old brick it is worth finding Staten Island masonry who does period repointing rather than whoever is cheapest that week.
Frequently asked questions
How do I know if my mortar is lime or cement? Age is the first clue: pre-1920s masonry is usually lime-based. A simple field test is hardness, since lime mortar can be scratched with a key and old brick is soft, while cement mortar resists it. A mason who does historic work can confirm it in minutes.
Is it ever okay to use Portland cement on old brick? As a rule, no, not on soft historic brick. The whole problem is the mismatch in hardness and permeability. A little cement is sometimes added to a lime mix for workability, but a hard modern mortar on soft old brick is the mistake this whole article is about.
Can spalled brick be repaired, or does it have to be replaced? Once the face has popped off, that brick cannot be un-spalled. Minor cases can sometimes be turned or patched, but significant spalling means replacement with matched brick, which is why stopping the cause, the wrong mortar, matters more than cosmetics.