· by Michael Blair, Nashville's Masonry
Our post on why mortar fails before the brick does is about which mortar goes back into the wall. This one is about the work itself, because a repointing job can use exactly the right mortar and still fail early if the joint was cut too shallow, filled in one thick pass or tooled at the wrong moment. The steps below follow the Brick Industry Association’s Technical Note 46 on maintenance of brick masonry, and they are the things to watch for when a crew is working on your wall.
How deep should old mortar be cut out?
To a uniform depth of at least twice the width of the joint, which BIA says generally comes to about 3/4 inch, and further if the mortar at that depth is still loose. A shallow cut leaves a thin skin of new mortar sitting on crumbling old mortar, and a thin skin has nothing to hold it.
Uniform matters as much as deep. BIA notes that a pointer’s grinder will not clear the short vertical head joints to full depth without cutting into the brick on either side, so those are finished by hand with a toothing chisel. A crew that runs the grinder down every joint and moves on has skipped the slow part of the job.
The back of the cut should be clean and square. The National Park Service’s Preservation Brief 2 asks for the old mortar to come out cleanly, leaving square corners at the back of the joint, and for the joints to be rinsed with a water jet before any new mortar goes in.
Why does new mortar go in thin layers?
So the joint is filled solid from the back to the face. BIA calls for repointing mortar to be placed in layers no more than 1/4 inch thick, so a joint cut to 3/4 inch takes at least three passes. One thick pass pressed in from the front leaves the back of the joint to chance.
Before any of it goes in, the mortar is prehydrated. BIA describes mixing it dry first, to a damp consistency that will hold its shape when squeezed into a ball, then letting it stand for 1 to 1-1/2 hours before the rest of the water is added. The joints should be damp when the mortar goes in, with no water standing on the surface, a point BIA and the Park Service both make.
The finish is timed. BIA says to tool the joints when the mortar is thumbprint hard, not while it is still soft enough to smear across the brick face.
How is new mortar matched to the old?
Against samples that have cured, not against wet mortar in the bucket. BIA advises comparing samples wet to wet and dry to dry, because the same mortar reads differently in each state. Where pigment is needed to get there, the note calls for metallic oxide pigments at no more than 10 percent of the weight of the portland cement, and carbon black at no more than 2 percent.
Matching also covers how much of the wall is done. BIA advises repointing all the joints in the affected area of the wall rather than picking out a joint here and there. A scatter of new joints among old ones shows from the street long after the color has settled, while a whole panel repointed to a corner or an opening reads as one wall.
Who should do the repointing
BIA makes a point worth repeating: a mason who is excellent at laying brick may not be qualified to repoint it. Cutting out old joints without chipping the brick edges, clearing head joints by hand and packing mortar in thin lifts is slower, fussier work than building a new wall.
When we price brick repair and repointing on a house in Brentwood or anywhere else around Nashville, we write down how deep the joints will be cut, how the head joints will be cleared, and how much of the wall is being repointed. If an estimate in front of you does not say those three things, ask. Send us photos of the wall and we will tell you what we see.
Sources
- Brick Industry Association, Technical Note 46, Maintenance of Brick Masonry (December 2017) — Remove mortar to a uniform depth at least twice the joint width, generally 3/4 inch, or to sound mortar; grinder cannot clear head joints without damaging brick, chisel the rest; prehydrate 1 to 1-1/2 hours; layers no more than 1/4 inch; tool thumbprint hard; damp joints, no surface water; repoint all joints in the affected area; metallic oxide pigment up to 10 percent, carbon black 2 percent; wet-to-wet and dry-to-dry comparison; an excellent mason may not be qualified for repointing.
- National Park Service, Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings (Mack and Speweik, 1998) — Old mortar removed cleanly with square corners at the back of the joint; joints rinsed with a water jet; damp joints with no standing water; power grinders can damage masonry in unskilled hands.
FAQ
Repointing Brick: How the Job Should Be Done — questions
What is repointing brick?
Repointing is cutting deteriorated mortar out of the joints to a set depth and replacing it with new mortar. BIA calls for a cut of at least twice the joint width, generally about 3/4 inch, then new mortar packed in layers no thicker than 1/4 inch and tooled when thumbprint hard. The brick itself stays in place.
Can you repoint brick yourself?
A small, low area is within reach of a careful homeowner, but it is slow work. The joints have to be cut to a uniform depth without chipping the brick, head joints cleared by chisel, the mortar prehydrated and packed in thin layers. BIA notes that even an excellent mason may not be qualified to repoint.
Does repointing damage brick?
It can, if the cutting is careless. BIA warns that a grinder cannot clear head joints to full depth without damaging the brick beside them, which is why those are finished by chisel, and the Park Service cautions that power grinders in unskilled hands damage masonry. Done properly, only the mortar is removed.
Why did tuckpointing fail?
Look at how it was done. Joints cut shallower than twice their width, head joints left shallow where the grinder could not reach, mortar packed in one thick pass instead of 1/4-inch layers, or joints tooled before thumbprint hard all depart from BIA’s Technical Note 46. A mortar harder than the original is the other thing to check.