Somebody had repointed the bottom six courses of a rowhouse front with a grey cement mortar, struck it flat, and run the joint about a quarter inch wide onto the face of each brick. From across the street the wall looked like it had been drawn with a marker. Two winters later the brick faces above that band were spalling, because the mortar was now the hardest thing in the wall and the moisture had nowhere else to go. Repointing is the most common masonry job on an old rowhouse and the easiest one to get catastrophically wrong, and almost all of the risk sits in two decisions: the mix and the joint profile.

The brick decides the mortar, not the other way round

Handmade brick from the nineteenth century is soft. Compressive strengths well under 2,000 psi are common, and the outer skin — the fire skin from the clamp or kiln — is the only weather-resistant part of it. A mortar joint is supposed to be the sacrificial element in that assembly: soft enough to crush slightly as the wall moves, permeable enough to let water leave through the joint rather than through the brick. Modern Portland-rich mortars routinely exceed the strength of the brick around them, and when that happens the wall relieves stress through the brick faces instead. That is the mechanism behind spalling above a hard repoint, and no amount of sealer fixes it.

Getting the mix right means testing the old one

A mortar analysis is inexpensive relative to the job — a sample of original mortar taken from a protected location, dissolved in acid, and the surviving aggregate sieved and examined tells you the sand colour, the grading and roughly the binder ratio. That analysis is what lets a replacement mortar match visually as well as mechanically, because most of the colour in a joint comes from the sand, not from pigment. For soft historic brick the target is generally a lime-rich mix; the common specification language sits around ASTM C270 Type O or a straight lime-and-sand mortar for the softest work, rather than the Type N or S used in modern construction. Sand matching matters more than people expect, and experienced masonry contractors in Philadelphia will usually have a short list of pits whose sand reads correctly against local brick.

Cutting out without wrecking the arris

Joints should be raked to roughly two to two and a half times the joint width — on a 3/8 in joint that is about 3/4 to 1 in — and squared off at the back, not feathered. The tool matters. An angle grinder run along a hand-struck joint widens it, wanders into the brick and leaves saw kerfs at every corner, and that joint can never be made to look right afterwards because the edges of the brick are gone. Hand chisels and small hammers are slower and are the correct tool on irregular historic joints. Pace the work: a skilled hand cuts and points somewhere around 30 to 60 square feet of wall a day on fine joints, and a price that assumes far more than that is a price for a grinder.

Profile and finish: where the face of the building lives

The joint profile is the building’s expression. A flush or slightly recessed joint struck back to the brick edge leaves each brick reading as an individual unit. A struck or weathered joint throws a shadow line. A raised or “buttered” joint spread onto the brick face shrinks every brick visually and flattens the whole elevation — that is what makes a bad repoint look like a smear rather than a wall. Match the existing profile by finding an original sheltered area, usually under a sill or behind a downspout, and copy what is there. Finish the joint at thumbprint hardness rather than immediately, and brush it back so the sand grains show. New lime-rich work also needs protection: damp burlap or a misting regime for several days, because lime mortar that dries too fast never gains strength.

Scope, sequencing and cost

Whole-wall repointing is rarely necessary and frequently damaging. The sensible scope is defined by survey: joints that are eroded more than about 1/4 in, joints that are cracked or hollow-sounding, and areas around known water paths such as failed flashing or a leaking gutter. Fix the water source first — repointing under a broken downspout is repainting a leak. Expect a rowhouse front elevation to be priced per square foot of wall, commonly $15 to $35 depending on access, joint condition and whether hand tools are specified, with scaffold hire as a separate line. A correctly specified lime repoint should last 50 years or more; a cement repoint on soft brick often visibly fails in ten, and takes brick with it.

Frequently asked questions

How can a homeowner tell whether an earlier repoint used the wrong mortar? Scratch a joint with a key. If the mortar resists a key while the brick edges are crumbling, the mortar is harder than the brick, and the wall is being asked to relieve stress in the wrong place.

Does repointing stop water getting into a wall? It stops water entering through eroded joints, which is one path among several. Failed flashing, a cracked sill, and roof or gutter overflow are frequently the real source, and repointing alone will not address them.

Is it ever correct to use a cement-based mortar on an old rowhouse? On hard, later machine-made brick with existing cement joints, sometimes. On soft handmade brick, a cement-rich mix transfers the wear from the joint to the brick face, which is the outcome the work is supposed to prevent.