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Book Review: NYC Building Code, 5, Neighboring Walls

In yesterday’s discussion, I skipped over a piece of section 3309, “Protection of adjoining property” in the 2014 code and the 2022 code.  The reason was simply that the missing piece, how to deal with your neighbor’s walls adjacent to your construction site, is big enough to be discussed on its own.

The context for this discussion is the difference between party walls and independent walls. Both need to be protected during adjacent construction, but the issues are a bt different. A party wall is not simply a wall adjacent to a side lot line, which is an incorrect usage I’ve seen quite often. It’s a wall shared by two buildings and, typically, straddling the lot line. By far, the most common party walls in New York are those built by a developer of a row of houses, a row of tenements, or sometimes even a row of loft buildings. To save expense and to free up a foot or so of lot width for interior use, the side walls of the buildings in the row – the walls at the lot lines perpendicular to the street – are party walls shared by the buildings on each side. The incredibly beautiful drawing above shows what I mean. It’s a piece of a survey of two rowhouses, with everything extraneous to this discussion removed. The street is off to the left, out of frame, and the heavy line is the property line. The left two-thirds is the main block of the houses, and there’s party wall there, with the thickness of the wall indicated by the lines on either side of the property line. The right third shows a rear extension in the house on the top and a fence (the line that looks like this –X–) for the rear yard on the bottom. The rear extension has an independent wall, which is entirely on its lot.

It’s relatively easy to create a party wall when you’re building both structures at the same time. You create a legal mechanism to share the wall – for example, property A giving an easement to property B for use of the half the wall on property A, and vice versa – and then build the wall centered on the property line. It’s much more difficult when the buildings have different owners and are not being constricted in tandem, so rear-yard extensions typically have independent walls and are slightly narrower than the original main portion of the buildings.

Section 3309.4.2 of the code, which I would have discussed yesterday if I was going purely in sequential oder, concerns support specifically for party walls during excavation and foundation construction in addition to the general protections discussed yesterday. It boils down to: just because you’re not using the party wall anymore as you’re digging foundations for your new building, that doesn’t mean your neighbor doesn’t still need it in good condition. And you can reuse it for your own purposes if you want.

Section 3309.8 is called, simply enough, Adjoining walls. It describes various ways in which the people performing construction next to a wall have to protect the structural integrity of side walls, including making sure they’re anchored to the interior structure on the other side, and making sure openings are “bricked up” – a great old phrase that’s a bit out of date but I don’t mind. One of the five items to be addressed is purely for party walls: making sure the stubs of pocketed beams are removed and the pockets closed.

Finally, section 3309.9, Weatherproof Integrity of Adjoining Buildings, addresses non-structural issues for exposed walls. It’s all pretty self-explanatory except for “Properly sealing all cornices, where cut”: most cornices on small buildings are hollow, consisting of sheet metal or wood sheeting over a wood frame. If you have a row of houses with cornices, the void spaces inside often are continuous from one building to the next, so if you demolish one cornice, the void spaces in the adjacent cornices will be open to the weather and will quickly become home to birds or squirrels.

Part 1: Underpinning
Part 2: Peer Review
Part 3: Support of Excavation
Part 4: Protecting Neighboring Property