Repointing historic brick with modern high-portland mortar is the single most damaging thing a contractor can do to an old chimney. The new mortar is harder than the brick, so the brick becomes the sacrificial element and spalls within a few winters.
Mortar is supposed to be the weak point
In a correctly built masonry chimney the mortar is deliberately softer than the brick. That is not a compromise, it is the design. Masonry moves, and something has to absorb that movement. Better the mortar, which can be ground out and replaced, than the brick, which cannot.
Nineteenth-century construction, which Plano’s historic core has in quantity, used soft lime-rich mortars that did this job well. They erode over decades, and repointing is routine expected maintenance.
What goes wrong with modern mortar
Modern portland-based mortars are considerably harder and much less permeable than historic lime mortar. Put them into a joint between soft old bricks and two things happen.
First, the hardness relationship inverts. The brick is now the softer element, so when the masonry moves, the brick takes the stress and its face fractures. Second, the reduced permeability traps moisture that would previously have evaporated through the joints, forcing it out through the brick face instead, where freeze-thaw pops it off.
Both produce the same result: spalling brick faces. It is irreversible, and matching replacement brick on nineteenth-century masonry is rarely perfect.
What proper repointing looks like
Deteriorated mortar is ground out to a proper depth, generally two to two and a half times the joint width, not simply scratched at the surface. Fresh mortar matched in strength and appearance is packed in layers, then tooled to match the original joint profile so it sheds water as designed.
Smearing new mortar over old joints is the shortcut that looks acceptable for one season and then falls out. We assess the existing masonry and match to it rather than defaulting to whatever mix is on the truck. Details on our tuckpointing page.
One timing note: mortar needs sustained temperatures above roughly 40 degrees to cure. Repointing in this area realistically runs spring through early autumn.
Talk to a local chimney pro
Plano’s downtown is well enough preserved that it stood in for a 1930s streetscape on film. If you own masonry of that vintage, it deserves matched materials. See our Plano page.
Written by Josh Amber, co-owner of Ember Chimney Sweepers, Oswego IL. Published September 30, 2026.