The addition detail that generates the most callbacks on my projects is not the roof and it is not the flashing. It is the vertical line where new brick meets old, on a Garden City colonial where the original wall was laid in 1948 and the new one went up last spring. Six months later there is a crack you can slide a nickel into.
Two walls, two rates of movement
An old wall has finished most of its settling and most of its moisture-related shrinkage. A new wall has not. New brick expands slightly and irreversibly as it takes up moisture after firing — on the order of a few hundredths of a percent, which over a 30-foot run is a visible amount. New foundations settle. Old ones already have.
Tie the two together rigidly and the difference has to go somewhere. It goes into a crack, and the crack usually appears at the junction because that is the weakest plane.
There are two legitimate ways to handle this and they are opposites.
Option one: toothing in
You cut back the old wall in a stepped pattern and interlock the new brick into it, course by course, so the two act as one wall. This is right when the two foundations are genuinely continuous — the new footing is at the same depth, on the same bearing, tied with dowels — and when matching brick is available.
Done properly it is invisible. Done on mismatched footings it guarantees a crack, because you have removed the one plane where movement could have been accommodated.
Toothing runs $45 to $90 per linear foot of junction and it is slow, careful work. It also demands a brick match, which on a 1948 wall usually means salvage.
Option two: a proper vertical movement joint
You butt the new wall against the old with a clean vertical joint, tie the two with slip anchors that transfer lateral load but allow vertical and in-plane movement, and seal the joint with backer rod and an elastomeric sealant.
This is what I specify on most residential additions. It admits that the walls will move differently and gives the movement a designed place to happen. The joint reads as a deliberate line rather than as damage, and it costs $12 to $30 per linear foot instead of $45 to $90.
The mistakes I see: no anchors at all, so the new wall has no lateral restraint; anchors that are rigid, which defeats the joint; and mortar squeezed into the joint during laying, which turns it back into a rigid connection.
Getting the match right
Whichever detail you use, the two walls have to look like relatives. That means matching size first — a 1948 brick and a modern modular brick differ enough in height that three courses will not line up — then texture, then color. Color is the one owners obsess over and the one that matters least, because new brick weathers toward the old within a few years and mortar color drives the visual reading of the wall more than the units do.
Mortar match matters structurally as well. New work laid against an old soft-brick wall should use a mix compatible with the old, not whatever the crew has in the mixer.
On a typical single-story addition the masonry junction detail is maybe two percent of the budget and it is the detail people notice for the next thirty years. It is worth making a Nassau County home additions builder commit to the junction detail and the brick source in writing before the foundation is poured, because both decisions get much more expensive once the footing is in.
Frequently asked questions
Can I just butt the new wall against the old and caulk it? That is essentially the movement joint approach, but only if you also install slip anchors and use a proper sealant joint with backer rod. Caulk alone, with no anchors, leaves the new wall laterally unrestrained.
How do I find matching brick for a mid-century wall? Architectural salvage yards, demolition contractors, and sometimes the back of your own house — taking brick from a rear or garage wall for the visible junction and infilling the donor area with modern brick is a common trick.
Will the crack come back if I just repoint it? Yes, if the underlying detail is rigid. Repointing a movement crack with mortar puts a hard filler into a joint that needs to flex. The lasting repair is to convert it into a sealant joint: cut it out cleanly, install backer rod, and seal it.