In cities across the nation, green space and open space are being developed on rooftops both new and old. Designing for a new building is a fairly straightforward process, however, retrofitting an older building for this application requires knowledge of historic systems and careful investigation.
Structural engineer’s perspective
Cuono: Many of the residential structures in our building stock, especially in older cities, are steel-framed buildings with concrete cinder slabs. Used extensively in the 1920s and 1930s, this structural framing system consisted of a cinder concrete reinforced with a draped wire mesh acting as a catenary in tension. These cinder slabs are often light weight and typically spanned 5 to 8 feet. A common practice on roofs was to use cinder fill to pitch the roofing. This was usually at least 6 inches deep. In our own practice we have noted some roofs with more than a foot of loose pitched fill.
When a client comes forward with a project to add a deck or green space to the roof for these types of buildings, the challenge to the structural engineer is to determine if the additional weight is acceptable. In the absence of full structural documentation, a field investigation is required, complete with probes, cores, and testing at the slabs, beams, and connections to determine the structural capacity.
To accommodate the new additional loads, the engineer can research the live load used at the time the building was designed and built. As an example, the live (snow) load in New York City during the 1920s and 1930s was 40 pounds per square foot (psf). This was lowered to 30 psf in later years and may be even lower in the new IBC/ASCE 7 format. The net difference in live loads can help offset some of the new additional load.
Another useful tool relates to the loose cinder used for the roof pitching. If it is found that this fill is indeed loose and not bonded to the structural slab, it can be removed and replaced with lighter weight material. This change in dead weight would also help to carry the additional loads. Another possible solution is to remove the loose fill and replace it with a new reinforced slab to provide additional load capacity at the slabs. In some cases, studs could be welded to the beams to create a composite system with the newly poured slab and increase the framing capacities. In the event where a cinder slab cannot be reinforced or relied upon, a new plinth can be created using column stubs that bear on existing columns (with excess capacities), which in turn support a new layer of framing.
Architect’s perspective
Bates: Installation of decking, paving, or green roof systems on draped-mesh cinder concrete roof slabs requires a thorough understanding of the existing slab construction, roofing, and drainage systems. While most setback terraces in 1920s-era highrises were constructed to receive stone or tile paving, roof areas typically were not. In planning a new finish for roof tops, the architect could confirm whether or not the roof was constructed with a paving surface. While clues such as direct elevator access may suggest historic communal use of roof areas, only probes and field measurements can confirm the adequacy of the structural slab to sustain any new loads.
Special consideration should also be given to roof areas adjacent to reconstructed parapet walls, where the cinder fill may have been partially removed or otherwise compromised. If removal of the cinder fill layer is deemed necessary to reduce the dead load in anticipation of new finishes, the design team should anticipate implementation of long-term weather protection.
Extreme care must be taken during removals to avoid damage to the steel mesh where it emerges from the concrete slab and drapes over the top flange of the structural beams. While the shape of the formed cinder concrete slab may redistribute some load to the bottom flange of the beam, it is the mesh, and its continuity, that actually supports the slab. If the mesh is found to be discontinuous, it may require post-tensioning or attachment to the beam flange. All exposed steel mesh and beam flanges must also be fireproofed.
Drain and utility pipes that have been concealed in the fill will require inspection and, possibly, repair as cinders in fill are known to create an acidic atmosphere, especially when exposed to moisture at failing roof areas. Historically, cinder fill was separated from perimeter parapets and bulkhead walls by a 1 to 2 inch expansion joint often filled with pitch during roofing installation. It is important to maintain such joints when using new cementitious fill materials.
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Ciro Cuono, P.E., LEED AP, is an associate at Hage Engineering, in New York. His experience spans renovations and new construction projects, including historic renovations, cultural, civic, residential, and commercial projects. He can be reached at ccuono@hageengineering.com. Robert C. Bates, RA, AIA, is a principal at Walter B. Melvin Architects, LLC, in New York. His experience includes restoration and reconstruction of exterior envelopes on historic buildings in and around New York. He can be reached at rbates@wbmelvin.com.