Imagine a city where every roof is green with plant life. From atop a tall building all you see are plants, flowers, and trees in bloom. That time may be sooner than we think, and we have to prepare now.

Structural engineer’s perspective
Cuono: A green roof presents the challenge of a potentially very heavy loading condition. From a structural engineering point of view, the two basic types of green roofs, extensive and intensive, differ only in the amount and unit weight of the growing media. An extensive roof may have 6 inches of soil whereas an intensive roof may have upwards of 24 inches or 200 or more pounds per square foot (psf) of loading. Any change to the occupancy or live load must be considered, too.

The challenge is not great on new buildings since the loading can be planned for in advance. Additionally, it is likely that the majority of green roofs in the coming years will rest on existing buildings, so we will focus on those challenges. The first steps in verifying if an existing roof can be retrofitted for a new and much heavier loading is to research the existing building (such as building departments records and plans); document the existing conditions through on-site surveys and probes; and conduct material testing (such as concrete cores, radar, etc.). A thorough engineering analysis will then determine the extent of available additional loading capacity and the necessary reinforcing (if any), which could be a deal maker or breaker for an owner. Where a very heavy loading is proposed, one compromise that an engineer could present is to have a mix of shallow media/soil over the majority of the area, up to a depth equal to the calculated additional remaining capacity, and a deeper/heavier media (perhaps for a tree) at areas concentrated over the building columns where the capacity is likely much greater. This may be a way of developing a scheme with little or no additional reinforcing while still allowing for some mix of plant life.

Other useful tools available to the engineer may be to remove existing loose roof fill, pavers, or any non-structural layer that could help offset the new weight. This may be an efficient way of balancing the loads from a shallower extensive roof. Also, if allowed by the local building official, one could research the original design snow load and compare this to the current design load. The change in codes over the years has tended towered a lighter snow load (such as from 30 or 40 psf, down to 22 psf in some municipalities), except in areas of drift and build up.

Architect’s perspective
Bates: Once structural concerns have been satisfied, the designer should consider the following to ensure the performance and longevity of the waterproofing and drainage systems beneath an extensive or intensive green roof.

The waterproofing membrane must be chosen carefully, as some manufacturers will not warranty roofs that are exposed to continuous moisture. There are many, of course, that do including liquid-applied and modified bitumen systems. If there is considerable pitch on the roof (more than 20 percent), the soil contact surface may require terracing with insulation or other means to avoid soil slump toward the drain.

While certain manufacturers permit the application of membranes on a level surface, it is still critical that the path to the roof drain remain unobstructed. The use of thick drainage mats and root barrier systems serve to maximize subsurface flow to roof drains.

Roof drains for intensive green roofs must also meet special design criteria. Often manufacturers require a gravel skirt around drains to permit visual inspection and maintenance. If the drain bowl is buried, an inspection port or access hatch will be required. Drains can also be outfitted with a traditional perforated stand pipe from the bowl to the soil surface to provide multiple points of entry for absorbed rain water. The installation of an emergency overflow scupper is also recommended since a drain clog may go unnoticed for days and permit an unacceptable accumulation of rainwater. Such an excess of water could lead to root deterioration or, more seriously, structural failure.

It is also important to remember that traditional masonry roof parapets and bulkhead or penthouse walls are not designed to be buried or act as retaining walls against lateral thrust. Counter-flashings at the base of the parapet are susceptible to infiltration through capillary action if buried. Masonry parapets will also need to dry from both sides.

Raising the level of soils within parapet walls also effectively reduces the height of the parapet, creating a potentially hazardous condition. The parapet height would need to be extended in such a case.

Ciro Cuono, P.E., LEED AP, (left)is an associate at Hage Engineering. 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. He can be reached at rbates@wbmelvin.com.