Repointing an old brick or stone house: why the new mortar must be softer
Updated · KeystoneProGuide editorial team
Why does the mortar on an old house matter so much?
Because in an old wall the mortar is meant to fail first. The National Park Service (NPS) explains that a masonry wall works as a system in which mortar is the softer part. It lets water and the salts it carries pass through the joints rather than the bricks, and because it can be raked out and replaced, it "sacrifices itself for the good of the system". For that to work, the NPS says, the mortar has to be softer than the masonry it holds together.
Put a mortar harder than the brick into those joints and the system runs backwards. The joints stop wearing and the bricks start.
How did old mortar differ from modern mortar?
The NPS gives the history in brief. The basic recipe for historic mortar was water, lime and sand, the lime made by burning limestone or seashells. In the early 1900s some portland cement was added to mixes to harden them. After the Second World War, portland cement all but replaced the much softer lime.
Bricks changed too. The NPS says historic bricks were fired differently from modern ones, which are fired at much higher temperatures and are much harder. Early soft brick and soft lime mortar "breathed together" and moved at about the same rate. Modern hard mortar and old soft brick do not.
That history matters in Pennsylvania. The Census Bureau estimates that a quarter of the state's homes, 25.0 percent, were first built in 1939 or earlier, and in Philadelphia the share is 39.9 percent. Not every one is brick or stone, but those that are were built before the switch to cement-rich mortar.
What goes wrong with hard mortar?
The NPS is direct: "Soft historic masonry and hard modern mortar is a catastrophic mix." Its explanation is that hard mortars high in portland cement are less permeable than the brick around them. The brick, not the joint, then becomes the path for moisture, and with the two materials also moving differently as the wall heats and cools, the hard mortar "will often cause the surrounding masonry to deteriorate or deteriorate faster".
Its example is a sandstone house in Iowa that was repointed and partly parged with portland-rich material. The stone has been wearing away around the hard joints, which now stand out like the walls of a honeycomb. The NPS adds that the people who did it were trying to do responsible maintenance, and that the mistake is a common one.
Are hardware-store mortar mixes safe for old brick?
Not according to the NPS. It says the pre-packaged mortar repair kits sold in hardware stores "contain a high degree of portland cement, which produces a very hard surface", fine for modern materials but disastrous for older masonry. The same caution applies to a contractor's habit of using a standard bagged mix on every job.
How is the right mortar chosen?
By finding out what is there. The NPS advises choosing a pointing mortar that duplicates the old in strength, composition, color and texture, and consulting a preservation professional for the recipe. On historic federal buildings, the General Services Administration (GSA) goes further: the old mortar is analyzed by a laboratory, the new mortar must be lime-based, and its strength may not exceed that of the original mortar or the surrounding stone or brick. Its specification also requires the contractor to show familiarity with historic mortar formulations and curing conditions.
For a sense of the cost, Angi lists a mortar composition analysis at $200 to $500 and a color analysis at $100 to $300, against an average of $1,450 to repoint 100 square feet. HomeGuide prices repointing at $3 to $20 per square foot, and a stone foundation at $5 to $28.
What does matching mean in practice?
Hardness is one part. The NPS lists the others: the new joints should match the old in width and tooling, and the mortar must not be smeared onto the brick faces, which from a distance makes a wall look mottled. It counts mismatched color, width and tooling, along with portland cement and poor preparation, among the marks of poor pointing.
The GSA's method shows how a match is proved before the whole wall is done. It has small test patches made, then a sample area at least 10 feet by 10 feet repointed and cured for 14 days, and the approved sample becomes the standard for the rest of the work. That is a federal standard for public buildings, but asking a mason to do a small test section in an inconspicuous place first is a reasonable request at any scale.
Can old mortar ever have cement in it?
Some does. The NPS dates the first portland additions to the early 1900s, so a house from that period may already have a harder mix, and the GSA's rule is to match whatever the analysis finds while staying no stronger than the original or the masonry. The principle is the same either way: the new mortar should never be the strongest thing in the wall.
Does repointing need approval?
In Philadelphia, if the house is on the city's historic register, the Historical Commission must review masonry pointing, along with cleaning and repair, before the work. The review is free.
For the full sequence of a repointing job, see what to expect on the day, and for the questions that test a contractor's knowledge of old mortar, see questions to ask. Masons are listed on the masonry contractors page.
Sources
- Mortar, Unsung Hero of History, U.S. National Park Service
- Common Problems with Brick Masonry, U.S. National Park Service
- This Masonry is for the Bees!, U.S. National Park Service
- Repointing historic masonry, U.S. General Services Administration
- 2020-2024 ACS 5-Year Narrative Profile: Pennsylvania, U.S. Census Bureau
- 2020-2024 ACS 5-Year Narrative Profile: Philadelphia County, Pennsylvania, U.S. Census Bureau
- How Much Does Repointing Brick Cost? [2026 Data], Angi
- How Much Does Tuckpointing or Repointing Cost? (2026), HomeGuide
- Get approval for work to a historic property, City of Philadelphia