The principle
NPS Preservation Brief 2 says repointing mortar should be softer, measured in compressive strength, and more vapor permeable than the brick it holds. Then the mortar, which is cheap to replace, takes the movement and lets moisture out. When the mortar is harder than the brick, the brick takes the stress and can crack or spall at the edges.
The Rowhouse Manual puts it the same way for Philadelphia rowhouses: mortar “needs to have similar properties to the brick so that it will expand and contract at the same rate.”
ASTM mortar types
Modern mortars are graded by type. NPS gives these approximate compressive strengths:
| Type | Approx. strength (psi) | Note |
|---|---|---|
| M | 2,500 | Hardest; rarely right for old brick |
| S | 1,800 | |
| N | 750 | |
| O | 350 | Softer, lime-rich |
| K | 75 | Very soft, high lime |
The brief describes Type O (1:2:9 cement-lime-sand), Type K (1:3:11) and straight lime-and-sand mortar as high-lime mortars, with more lime than Portland cement. Which mix is right depends on the brick and how exposed the wall is, which is why a mortar analysis or test panel helps.
What to ask your mason
- What mix will you use, and why is it right for this brick?
- Will you rake out joints by hand or with a grinder? (NPS warns power saws can damage soft brick.)
- Will you do a sample area first so I can see the color and joint profile?
- How will you keep the new mortar from drying too fast in hot weather or freezing in cold?
“Masonry cement” and patching
NPS says masonry cement is not recommended for repointing historic buildings. Smearing new mortar over old joints without raking them out, sometimes called a “slurry” or “skim” job, doesn’t bond and won’t last.