What We Found When We Opened the Wall
The soft spots near the bay window turned out to be something I hadn't seen before. The original plaster was intact and hard, but someone in the 1980s (based on the electrical box style) had drywalled over it in most of the room. The drywall was hung with screws directly into the plaster, and in several places the screws had pulled through. The drywall was floating against the plaster, attached only in spots, which explained the soft flex when you pressed on it.
Diane's downstairs neighbor Raymond had told her the bay windows in these buildings leaked around the frames, and he was right. There was old water staining on the plaster underneath the drywall in the corner sections, the kind of brown shadow that tells you water got in at some point, dried, and got in again. The drywall in those areas was soft along the bottom edge, and some of it was pulling away from the baseboards.
I took out my phone and took about forty photos before I touched anything. When you open a wall in a building this old and find something unexpected, documentation first.
The Decision About What to Fix vs. What to Leave
We couldn't fix everything. Fully removing the 1980s drywall layer and replastering was not a weekend project, and Diane didn't have budget for that. What we could do was resecure the drywall where it was pulling away, repair the damaged sections at the base of the bay window, address the ceiling-line crack, and get the room to a state where paint would hold.
For the sections where the drywall was soft from water, we cut out the damaged material. About four square feet total across two areas near the bay window corners. The plaster underneath was solid. We used that as our backing.
The ceiling crack was different. Running along the ceiling line in a building this age, it was classic settlement: the building had moved enough over a century that the joint between the top of the wall and the ceiling had opened. Not a structural problem, but not something a simple compound fill would solve permanently either.
What We Used and Why
For the patches over the plaster substrate: Durabond 45, a setting-type compound that bonds to plaster chemically as it hardens rather than just drying on the surface. It's not sandable after it fully sets, which meant we had to get the surface reasonably flat before it kicked. I mixed small batches, about two cups at a time, and worked fast. Two coats to build up the thickness, then lightweight all-purpose over the top for the finish coat.
Before any of that, we primed the plaster sections with Zinsser Gardz, a hardening primer that penetrates soft and chalky surfaces. This was the step I'd skipped on a previous plaster repair in Somerville and regretted. The primer cost $28 for a quart. The regret cost me an extra half day. Worth it.
For the ceiling crack: I opened it into a V-groove, ran a bead of paintable siliconized acrylic caulk, let it cure overnight, and skim-coated over it. Same approach I now use for any crack that's caused by building movement. In a 110-year-old triple-decker in Boston, pretty much every crack at a structural joint is caused by building movement.
The Humidity Problem
It was October and Boston was doing that thing where October feels like March. The apartment was 63 degrees inside with the windows shut, and the humidity reading on Diane's thermostat was 68 percent. Compound dries slowly in those conditions. I had a small box fan blowing across the patched areas and still waited nearly three hours for the first Durabond coat to get hard enough to apply the second. The lightweight compound took overnight, which I hadn't fully planned for. We pushed the painting to the morning of the third day, which was tighter than I'd wanted.
What Raymond from Downstairs Told Us
Raymond knocked on the door on day two to see how it was going. He'd owned his unit in the same building for eleven years and knew the building better than anyone. He told us the bay window leak wasn't new, that it happened every year when the weather started getting below freezing and the expansion joint around the frame opened slightly. He'd caulked his own units from the outside with window and door caulk and had mostly solved it.
He also mentioned that the third floor always ran cold in winter, which we could confirm just from being in the apartment, and that the cold wall surface near the windows was part of what caused condensation and moisture issues. His suggestion: weather-strip the bay window interior thoroughly and add a thin rigid foam backer between the exterior wall and any new drywall in those corners.
I hadn't done that on this project because we'd already patched and the wall was closed, but I passed the advice along to Diane. If she ever redoes the bay window section properly, that foam backing is the right approach for Boston's climate in a building that old.
How It Looked When We Were Done
Honestly? Pretty good. Not perfect. There were two spots where I could see the patch texture was slightly different from the surrounding wall under raking light, and I told Diane that before I left. Under normal room lighting with furniture in front of it, you'd never notice. Under paint with the right sheen (she went with eggshell), the repairs were invisible from the doorway.
The ceiling crack I was most worried about. Those repairs in buildings with ongoing settlement can reopen within a year if the building moves again. I explained the caulk-plus-skim approach to Diane and told her if it comes back, it'll come back in the same place and the fix will be the same fix. That's not failure. That's just maintenance in a 110-year-old building in Boston. Raymond told her the same thing, which made me feel better about saying it.