Although a seemingly simple topic, rooftop dunnage can be challenging to the uninitiated since most textbooks and technical resources are silent on the topic.
Structural engineer’s perspective
Cuono: From a structural point of view, rooftop dunnage involves designing a raised series of beams (usually steel) bearing on posts or bearing walls to support mechanical equipment (usually on a roof top). Some structural issues to consider include wind and seismic load, gravity loading, vibrations, painting, galvanizing, roofing maintenance, etc. One difficulty that may arise in new construction is last minute changes in the mechanical system that would require dunnage in an area where it was not originally planned. This is particularly taxing on lighter buildings, especially open web joist framing. One way of handling future changes is to determine some general parameters with the architect and mechanical engineer and plan for a design that accommodates a range of loading conditions anywhere in that particular area.
The challenges are greater on existing buildings. First the gravity loading has to be resolved; although some minor mechanical units may only weigh a few hundred pounds, large chillers and other units have significant weight. If existing drawings are available, a routine analysis can be made of the existing framing to determine if it is adequate to carry the additional loading. Where no documentation is available, especially on older buildings, an investigation is needed to determine the existing framing and its condition. Usually the best option is to locate new dunnage posts over existing columns. The new load on any individual column may only be a fraction of the total existing axial load. Where close to existing load bearing parapet walls, the dunnage can also be supported on these walls in beam pockets. A slip connection (possibly a Teflon sheet) may be needed at one end.
Where the dunnage is elevated above the roof, it is usually easiest to add knee bracing to provide lateral stability. Sometimes, moment connection will be needed.
Dunnage should be galvanized or at least painted with an exterior paint and always maintained. Some codes require a minimum thickness for painted steel. Vibration isolators are also needed to prevent disturbance of the building below.
Bates: Most rooftops in metropolitan areas exhibit some form of dunnage supporting air conditioner compressors, fan units, bulkheads, and even satellite dishes. Beams are typically found to be embedded in perimeter parapet walls but occasionally bear on top of the parapet or on structural steel posts within the roof construction.
It is critical that steel, embedded at parapet walls, is properly treated to prevent rusting. If beams become obsolete, they should be removed in their entirety and the masonry restored. Any forgotten components that are allowed to rust will lead to jacking and displacement of the parapet.
When conditions permit, beam pockets should be employed at parapet walls to allow continued access to beam ends for painting and inspection. It is important to remember that beam pockets will inevitably extend past the concealed edge of integral counter flashings and may indeed pass through the wall entirely. Therefore proper waterproofing of the pocket is essential, especially when the parapet is constructed of cored masonry units that would otherwise conduct rainwater behind the flashings. The floor of the pocket should be flashed and positively pitched back to the roof area.
Dunnage should not interrupt perimeter flashings and should be placed high enough to ensure adequate clearance beneath for roofing maintenance. If posts are used to support beams, clearance must be maintained to access flashings or pitch pockets at penetrations.
Care should be taken to ensure adequate bearing area with special consideration given to party walls and chimneys, where the available masonry thickness may be limited. Beams should not be permitted to pass through active chimney flues.
When planning dunnage for equipment, the designer must consider egress, safe equipment access, and fall protection near building edges. Historically, dunnage beams were installed at parapet intersections where the catty-corner beam placement minimized spans. This arrangement, however, limited safe access to only one side of the equipment, leaving the maintenance staff to clamber precipitously behind the equipment to access the remaining sides. Platforms, railings, and safety lines must be considered to permit safe access. Beams spanning from parapets to other rooftop structures should not block or impede required access or egress.
Ciro Cuono, P.E., LEED AP, is an associate at Hage Engineering. He can be reached at firstname.lastname@example.org. Robert C. Bates, RA, AIA, is a principal at Walter B. Melvin Architects, LLC, in New York. He can be reached at email@example.com.