Party Walls and Shared Structure

Party walls, the shared masonry at the edge of nearly every assembled site in old New York, sit at the bottom of what Michael Stern calls assemblage, where the law on the ground is far older than zoning. Structure magazine’s survey of the type begins with Vitruvius, who recorded around 30 BC that Roman houses already used bearing walls set along property lines to carry the floors of both neighbors, and that Roman statutes of the period fixed the rights and duties of the owners who shared them.² Everything since has been an attempt to say what either neighbor may do to a wall that belongs to both.

1666, London

The wall became a fire barrier after the Great Fire of London. Structure records that the laws passed in its aftermath required masonry in perimeter walls, party walls included, so that the wall’s structural job and its fire job were joined from then on.² The Department of Buildings’ course on construction safety reaches a century further for the customary law, quoting Diderot’s encyclopedia of 1765 on Paris practice: a builder who left no gap at his boundary could not stop his wall becoming a party wall, the neighbor could build against it on paying half its cost and half the land it stood on, and neither could alter it without the other’s agreement.⁷ Structure adds that the colonies developed their party wall rules separately, which is why practice still differs from state to state, and that George Washington issued a party wall regulation for the District of Columbia in 1791.²

1813 to 1849, the fire limits

New York wrote its first rules after its own fires. The Department’s course cites an 1813 state law requiring buildings within the fire limits to be of stone or brick with party and fire walls rising at least six inches above the roof, and an 1849 extension north of 32nd Street that fixed brick or stone party walls at 12 inches for a building over 35 feet and 8 inches below that.⁷ Structure dates the first fire ordinances mandating brick or stone perimeter walls to 1830, and describes the consequence by 1900: block after block of row houses sharing walls, and even six and seven story buildings keeping to masonry party walls as long as the lots stayed narrow.²

Figure

How thick a bearing wall had to be

1. 01
   Four stories, 50 feetWalls at least 12 inches thick throughout.
2. 02
   Five stories, 60 feet16 inches at the first story, 12 inches above.
3. 03
   Six stories, 75 feet20 inches at the first story, 16 at the second, 12 above that.
4. 04
   FinishesFurring, lath and plaster added up to four inches more.
5. 05
   The resultA 16 foot wide six story house kept only 12 feet of clear width on its first floor, which is why neighbors shared one wall rather than building two.

The nineteenth century lot

The wall was an answer to the width of the lot. The Gotham Center’s notes on lot sizes find that most Manhattan buildings at the century’s end were about 20 or 25 feet in width and no more than six stories, built of brick bearing walls and timber joists, and that a bearing wall for a six story house had to be 20 inches thick at the first floor before plaster, so that an independent wall on each side of a narrow lot would have eaten a third of the interior.⁶ Sharing one wall on the line, braced by the floors of both houses, saved that width, and the Gotham Center observes that lot widths were as much a product of building widths as the reverse, with developers subdividing wide parcels to match their house plans.⁶ Structure makes the structural point: two attached unreinforced masonry buildings are a more stable unit than two separate ones, since the shared wall is restrained by floors on both sides and any load pushing it out of plane is carried into the diaphragm opposite.²

1901 to 1916, the codes take over

The first New York codes treated the wall prescriptively. Structure counts 20 entries on party walls in the 1901 code, mostly construction prescriptions, and notes that up to the 1916 code the required thickness came from empirical rules about applied weight, after which codes started to specify firewalls by fire test instead.² Steel changed the engineering. The 1912 Kidder Parker handbook, Structure reports, observed that a skeleton frame put up without a party wall agreement usually could not get a symmetrical footing under the columns beside the shared wall, and recommended cantilevered foundations; the 1915 National Building Code required that any vertical extension of an old party wall inside a new skeleton building be carried entirely on columns and girders.² The same source records the nogging wall, wood studs with random brick between them, allowed as a separation until 1916 and still common, prone to rot where rain gets behind vinyl siding.²

Figure

Party walls by the numbers

brick masonry party walls still standing in the older parts of the city

15,000

the decade by which New York fire ordinances required brick or stone perimeter walls

1830

entries on party walls in the 1901 building code

20

the code year from which a demolisher had to anchor the neighbor's beams into the exposed wall

1968

1968 to 2014, protection during demolition

The modern code is mostly about what happens when one of the pair comes down. Since 1968, Structure explains, a demolisher who finds the neighbor’s floor beams bearing on the party wall must confirm they are anchored into it and add anchorage or bracing if they are not, a requirement confirmed by later collapse investigations that traced failures to loss of stability rather than overstress.² The 2014 code, Structure notes, adopted the model code’s definition, under which any wall on a lot line used jointly by two buildings must be built as a fire wall, and the Department’s training course then sets the standard such a wall must meet: enough structural stability under fire that construction on either side can collapse without bringing the wall down.⁷ The course also cites the administrative provision making repair and maintenance of a party wall the joint responsibility of both owners, and a 2015 building bulletin under which a recorded restrictive declaration on both buildings states that they are subject to the party wall provisions of the code.⁷

2023, Section 3309 read for the neighbor

Old Structures Engineering reads the current Section 3309 for the builder next door, and begins by correcting a common misuse: a party wall is not any wall beside a lot line but one that serves both buildings and usually straddles the line, most often built by the developer of a row of houses, tenements or lofts to save expense and free a foot of width.³ The rear extensions of such houses, built at different times by different owners, typically have independent walls and are slightly narrower.³ The section’s requirements, summarized in the figure that follows, come down to one idea: digging a new foundation does not relieve the digger of keeping the neighbor’s half of the wall sound, and the wall may be reused if wanted.³ The broader protections for the site edge are covered under excavation support as well as sidewalk sheds.

Figure

Protecting the wall next door under Section 3309

1. 01
   SupportKeep a party wall supported during excavation and foundation work even if the new building will not use it.
2. 02
   AnchorMake sure a wall left exposed is fastened to the floors behind it.
3. 03
   CloseBrick up openings, and where the demolished side had beams pocketed into a party wall, cut their stubs and fill the pockets.
4. 04
   WeatherKeep the exposed face watertight.
5. 05
   CorniceSeal cut cornices, whose hollow interiors otherwise run open from one house to the next.

Today, finding the wall before building

The present difficulty is knowing the wall is there. In 2022 the Center for Architecture convened a session on existing party walls that counted about 201,000 attached buildings in the city, observed that the common law governing them has not kept pace with engineered design, and noted that the seismic gap the code now requires between buildings makes new party walls almost impossible and complicates alterations to old ones.⁴ The same session, listed by AIA New York under its building codes committee, was led by the Department of Buildings’ former chief structural engineer, whose forensic findings on collapses in dense old neighborhoods became the basis for changes to the structural and construction chapters of the code.⁵

Structure describes the fieldwork. A topographic survey may not show whether a wall is shared or how thick it is, original drawings rarely survive, and older walls may be built around the remnants of still older ones, so a probe or an exploratory pit at the footing is essential; the rubble foundation under such a wall is typically six inches wider than the wall, and sawing off that toe demands underpinning first, or the old wall begins to lean.² Once a shared wall is confirmed, Structure continues, the proper course is the neighbor’s written acceptance in a party wall agreement, for which the Department has issued a standard form, and where no agreement was ever signed the owners’ rights are implied by the wall’s common use.² The Warshaw Burstein alert shows where the question lands when bulk is sold: the agreement between merged lots should settle who surveys the neighbor before work, who may enter to inspect, what may be fastened to the older wall, and who pays for damage.⁸

The chronology ends on an ordinary block. A Brooklyn owner wrote to Brownstoner’s forum in 2019 that the wood frame house next door, which shares a party wall, was going to auction, and asked what to expect if the buyer built.¹ Two thousand years of rules, from Vitruvius to Section 3309, exist to answer that letter.