Cantilevers Over Neighbouring Buildings

Cantilevers over neighbouring buildings, the overhangs by which the floor area that Michael Stern’s assemblage subject tracks on paper becomes visible from the sidewalk, have a plain cause: an owner next door who will sell the air over a roof but not the roof itself. ArchDaily’s 2014 piece on the form quotes a developer saying that when people will not sell their property but will part with the air above it, the cantilever becomes almost a necessity, and an architect calling development in the city a three dimensional chess game in which a site able to use every one of its available development rights is rare.¹ The object this entry examines is the building that results, read from the street upward.

Why the upper floors are wider than the base

Seen from below, such a tower inverts the old setback silhouette. ArchDaily’s example is an 18 story condominium on East 77th Street with two eight foot cantilevers, one floating 17 feet above the airspace of a high school and the other 36 feet above a courtyard, and it quotes the architect’s remark that where the 1920s built straight up with wedding cake setbacks, a cantilever lets the floors grow larger toward the top, where space is worth most.¹ The school sold development rights it had no intention of using; the builder gained longer apartment layouts on every upper floor.¹ The same article lists other overhangs that make use of a neighbor’s unused space, among them a 57th Street tower planned to project over the landmark Art Students League, whose sale of spare bulk for that project 6sqft later reported in its account of the Municipal Art Society’s study of as of right towers.¹⁴

The rights come first and the shape follows. 99% Invisible points to a Manhattan tower whose 24 foot overhang above a low brick neighbor records the purchase of that neighbor’s airspace, and recalls the older case of a Midtown office tower built out over a church whose congregation gave permission on condition that a new church be built on the corner of the block, so that the bulk moved while the congregation stayed where it was.³ What is bought is floor area, through the routes described under air rights and usually by the merger described under zoning lot mergers; what makes the overhang lawful toward the neighbor is a set of easements. The Warshaw Burstein alert on such transactions lists among the usual terms of a development agreement an easement over the seller’s light and air that caps the lower building and its rooftop equipment, together with construction easements that let the builder survey, inspect and attach protection to the neighbor and that allocate damage and insurance.⁶ That easement gets fuller treatment elsewhere, and the wall the two lots may share under party walls.

Figure

The Westly's three reaches

the lot frontage at 251 West 91st Street from which the tower rises

50 feet

the first overhang, on floor eight

25 feet

the second, at the eleventh floor

40 feet

the third, at the fourteenth floor

55 feet

What holds it up and what the code demands

Higher up, the object is a structure problem. Facades+ profiled the Westly, a tower of roughly 140,000 square feet rising from a lot just 50 feet across at 251 West 91st Street that steps out three times over its northern neighbor: 25 feet on floor eight, then 40 on floor eleven and 55 on floor fourteen.² Where comparable towers use diagonal trusses, the architects and their structural engineer picked a Vierendeel frame in concrete, a rectilinear system that keeps the window openings regular, with anchors drilled into rock beneath the footings to steady the lopsided mass.² Each cantilevered floor was poured on a provisional steel deck branching from the superstructure, braced with diagonal steel that was cut away by torch once the concrete had set and the floor was carrying itself, three to four weeks for each of the three.² Setting the first one meant hoisting steel over the neighbor’s roof, for which the Department of Buildings granted after hours permission so the crew could keep going for 61 consecutive hours.² The cladding returns the building to its context: a limestone veneered rainscreen that reaches just over a foot beyond the structure to the property line, with mineral wool and spray foam behind it.² Related structural devices appear under transfer structures.

The city has since written the overhang into its fire code. Section 705.12 of the Building Code, as amended by Local Law 77 of 2023 with effect from June 10 of that year, applies wherever part of a building reaches out by more than a foot above a neighboring structure or lot, and requires the projecting part to be protected according to an engineering study of fire that the buildings commissioner will accept, with its structure, its underside and its walls and openings treated no less strictly than the building type and a lot line condition would require.⁵ The analysis must assume the worst about the building below: a design fire that burns to burnout with every sprinkler failed, no firefighters arriving, all interior compartments and stair enclosures removed, and more fuel than the occupancy would normally hold, with the commissioner free to demand more data where the building below is small or absent.⁵

Figure

What Section 705.12 asks of an overhang

1. 01
   ThresholdA horizontal reach of more than one foot above a neighboring structure or lot brings the section into play.
2. 02
   AnalysisAn engineering study of fire that the buildings commissioner will accept, covering the structure, the underside and the exterior walls and openings of the projecting portion.
3. 03
   ScenarioThe analysis assumes a fire in the building below that burns out with sprinklers failed, no fire department response, interior compartments removed and a fuel load above the normal for its occupancy.
4. 04
   AccessThe Fire Department must approve the overhang for access to buildings and roofs.
5. 05
   ReviewA fire engineer from outside, engaged by the owner and with no hand in the design, reviews the analysis and certifies full conformity.
6. 06
   ResponsibilityThe registered architect or engineer who signed the drawings keeps responsibility for the analysis regardless of the review.

Two signatures close the file. The Fire Department must approve the overhang for access to the buildings and roofs beneath it, and a fire engineer from outside, engaged by the owner and with no hand in the design, must review the analysis and certify that it conforms to the code in full, while the design professional who submitted it keeps full responsibility for its conclusions.⁵ Outside review of this sort has a parallel in special inspections. The object, then, is three transactions stacked: a purchase of floor area, an easement over a roof, and an analysis of what happens if that roof is on fire. The tower leaning over its neighbor is the visible residue of all three.