Repointing mortar and why hardness matters
Applies to: Assumes brick or stone masonry originally bedded in a lime-based mortar, with joints requiring localised or general repointing.
Last worked on 2015-06-02
In any masonry wall, one of the two materials is going to take the damage. The whole logic of repointing is choosing which.
The sacrificial material
Mortar is meant to be the weaker part. It is easy to cut out and replace; the units around it are not. Where the mortar is softer than the brick or stone, movement, salt and frost damage concentrate in the joint, and the wall is maintained by repointing it every few decades.
Reverse that relationship and the damage moves into the units. A hard mortar around a softer brick concentrates stress at the arris and traps moisture in the unit, and the face spalls off. That damage is not repairable by repointing, because the material that failed is the one you cannot replace.
Why it also affects drying
A traditional wall handles water by absorbing it and releasing it, and the joints are a large part of the surface that does the releasing. A dense impermeable pointing reduces that path, so water that gets in - and it always gets in - has to leave through the units instead.
This is the mechanism behind walls that get damper after being repointed. The repointing did not admit the water; it closed the route by which the wall used to dry.
Preparation is most of the job
Joints must be raked out to a depth greater than their width, or the new material has no key and sits as a thin skin that falls out. Raking must not damage the arris of the units, which is why angle grinders are the usual cause of visible long-term harm to a wall.
The surface must be dampened before the new mortar goes in, or the masonry pulls the water out of the mix before it has cured, and the pointing fails from the back.
Matching the finish
The joint profile changes both appearance and performance. A recessed joint holds water at the top of the recess; a weathered joint sheds it. Matching the original profile is usually right on both counts, because the original was chosen for that wall and that exposure.
Colour and texture come from the aggregate more than from the binder, which is why a matched sand matters more than anything else in getting a repair to disappear.
Sequence for a repointing repair
- Establish what the wall was originally bedded in before deciding what to point it with.
- Rake out to a depth greater than the joint width, by hand where the arris is fragile.
- Brush out all dust and loose material from the raked joint.
- Dampen the joint so the masonry does not draw water from the fresh mortar.
- Fill in layers rather than in one pass on deep joints, allowing each to firm up.
- Finish to the original profile once the mortar has stiffened, not while it is still wet.
- Protect from sun, wind and frost while it cures, which is when most avoidable failures happen.
If there is one thing to carry away: the replaceable material must be the weaker one. Every other decision in repointing follows from that.