Roof Safety Engineering in NYC: Dunnage, Skylight Guards & Local Law 196

Rooftop Dunnage: Supporting Mechanical Equipment

Rooftop dunnage is structural steel framing — typically steel wide-flange beams, channels, or tube sections welded or bolted into frames — that elevates and distributes the loads of rooftop mechanical, electrical, and plumbing equipment. The term "dunnage" comes from the packing material used in ship holds, but in NYC construction it universally refers to this rooftop steel support structure.

Dunnage serves multiple purposes:

• Elevation above the roof membrane for drainage and maintenance access underneath the units
• Load distribution — spreading the concentrated weight of heavy equipment (HVAC chillers, cooling towers, packaged AHUs) from point loads to the distributed roof structural members below
• Vibration isolation (when combined with spring or pad isolators under the equipment feet)
• Flashing integration — dunnage frames can provide the curb for weathertight flashing at roof penetrations

Rooftop Load Analysis: What Can a NYC Roof Hold?

NYC Building Code Table 1607.1 sets the minimum design roof live load at 20 psf (for accessible flat roofs); ASCE 7-22 Table 4.3-1 sets it at 20 psf for rooftop. However, mechanical equipment generates point loads far exceeding 20 psf over small areas.

The structural engineer's process:

1. Equipment weight compilation: Mechanical engineer provides operating weight, shipping weight, and center of gravity of each rooftop unit
2. Available structural capacity: Review of the existing structural drawings (or field investigation if drawings are unavailable) to identify the roof structure — whether steel beams, concrete flat slab, or steel deck on open web steel joists — and its available capacity after deducting existing framing loads
3. Dunnage design: Steel framing that spans between structural supports to distribute each equipment load. Dunnage members are designed for the equipment weight plus ASCE 7 wind uplift, seismic forces, and NYC snow load (20 psf ground snow, pg)
4. Connection design: The dunnage must connect to the roof structure with adequate anchorage for uplift (equipment can act as a wind sail) and lateral loads

Skylight Fall Protection

Skylights are among the most hazardous rooftop features for construction and maintenance workers. An unprotected skylight over a light well or interior atrium can be a fatal fall hazard — the glass or plastic glazing is typically not rated to support a person's weight.

NYC Building Code and OSHA requirements for skylight protection:

• OSHA 29 CFR 1926.502(i): Skylights on construction projects must be covered with covers capable of supporting at least 200 lbs without failure, or guarded with a standard guardrail on all sides
• NYC Building Code §1015: Openings in roofs, including skylights, that are accessible require guards (42 inches minimum height) where the fall height exceeds 30 inches
• Local Law 196 (BCNYC Chapter 33/Subchapter 19): Construction sites must identify and guard all fall hazards; PE-stamped fall protection plans are required for projects above 15 feet
• Skylight guard specifications: Standard skylight guards are steel or aluminum welded grate frames that bolt to the curb framing around the skylight. The structural engineer confirms the curb framing capacity for the guard connection forces

Local Law 196: Fall Protection for Workers

NYC Local Law 196 of 2017 created stricter worker fall protection requirements for NYC construction sites, effective September 2020. For rooftop work, key engineer obligations:

• Fall protection anchorage: Workers using personal fall arrest systems (PFAS) need engineered anchor points. OSHA requires each anchor support 5,000 lbs per attached worker. The structural engineer specifies or designs anchors cast into concrete, bolted to steel, or installed into masonry parapets — and verifies the base structure can carry that load
• Parapet compliance: Where parapets are less than 42 inches high (the minimum guardrail height under BCNYC §1015), temporary or permanent guardrail systems are required. The engineer designs the guardrail post anchorage
• Fall protection plans: For projects above 15 feet, a PE-stamped Site Safety Plan must include a fall protection section identifying all roof-level hazards and measures

Parapet Engineering and Guard Requirements

NYC Building Code §1015.7 requires parapets on roofs accessible to building occupants or maintenance workers to be at least 42 inches high measured from the finished roof surface. Many older NYC buildings have low parapets (18–30 inches) that do not meet current code.

When a change-of-use, roof alteration, or code compliance project triggers current code compliance, the structural engineer:

• Analyzes existing parapet construction (unreinforced masonry, concrete, steel) for lateral wind load capacity at 42 inches height
• Designs parapet enhancements (masonry reinforcing, parapet extension, or supplemental steel guardrail system) to achieve adequate wind resistance
• Coordinates with the building envelope specialist if parapet waterproofing or flashing must be disturbed

Permanent Building Maintenance Systems (PBMS/BMU)

Buildings with curtain wall or glass facades require periodic exterior cleaning and facade inspection — most access facades via building maintenance units (BMU) or suspended scaffolding. The structural support for PBMS (roof davits, trolley tracks, parapet-mounted swing stage anchors) must be designed by a structural engineer to ANSI/ASSE A120.1 and NYC DOB requirements. PBMS anchors are typically designed for 2,500 lbs suspended load per anchor point (a suspended scaffold), with the structural system carrying the dead weight plus a 4:1 safety factor.

Solar Panel Structural Engineering

Solar panel arrays on NYC rooftops are an increasingly common structural engineering assignment. The engineer evaluates:

• Ballasted vs. penetrating mount systems — ballasted systems add dead load and require roof structure capacity confirmation; penetrating systems require waterproofing coordination and anchor design
• Wind uplift on panels — NYC wind speeds (115 mph basic wind speed per ASCE 7, Exposure Category B/C) create significant uplift on tilted panels
• Combined loading — snow + wind + panel dead load combinations
• DOB permit requirement: an Alt-2 permit is required for solar panel installations that involve structural alterations or significant load additions

NYC DOB Permits for Rooftop Work