Mismatch

That picture shows a new wall being constructed in a historic building, replacing one that was badly damaged. We’re inside the building looking out; the half-height pier in the middle will eventually separate two windows.

If you look closely at the pier (click on the pic to enlarge) you’ll see that the joints in the veneer (the wythe of brick that projects on top and on the right) do not line up with the joints in the back-up (the two wythes to the left/behind). There’s a simple reason: the veneer has narrow buttered joints to match the historic appearance, while the back-up has standard 3/8-inch joints. The difference in bed-joint height allows nine courses of veneer brick in the distance of eight courses of back-up.

In modern construction, there is a simple solution to this kind of mismatch: we use flexible metal ties to connect the veneer to the back up. This is ordinary construction these days, since we use those ties to attach veneer to concrete block back-up in new buildings.

What did people do in the past with mismatches? Uh…they cheated. A large percentage of the nineteenth-century buildings we work on have veneer that’s tied to the back up at the window lintels and sills and nowhere else. In small well-built buildings that detail often works for 150 years, since the narrow joints in the veneer and the solidly-filled collar joint between the veneer and the back-up combine to keep water penetration to a minimum. Eventually, the collar-joint mortar and the veneer bed-joint mortar start to deteriorate and the veneer peels off the building. The movement of the veneer can look like severe structural damage but it’s really just an original construction flaw, caused by a geometric mismatch, coming to light.