Plazas as vegetated roofs
For plazas over occupied space, an important design option is the vegetated roof—plantings atop the waterproofing system. In retrofit applications, this addition offers many benefits:
- cooler climate—in cities, vegetated roofs reduce the heat island effect, keeping buildings and their surroundings cooler and cutting down on energy consumption;
- stormwater control—plants and growing media filter pollutants and reduce stormwater runoff, decreasing loads on drains and sewers and cutting down on flooding;
- noise reduction—beyond providing a thermal buffer, the insulating properties of plants and growing media tend to reduce ambient noise within the building;
- aesthetics—by limiting pavement and hardscape, vegetation at the plaza level enhances and softens the view, providing a pleasant environment for building users and others in the community;
- snow and ice management—in cold climates, converting portions of a plaza to a vegetated roof reduces the total area requiring snow removal in the winter, saving maintenance costs and reducing liability; and
- corporate or institutional image—a ‘green’ roof visibly affirms the owner’s commitment to sustainable design, which can be deemed just as important as the environmental benefits themselves.
There are two types of vegetated roofs—intensive and extensive—the choice of which is usually dictated by the weight-bearing limitations of the structure. Intensive roofs are thicker, heavier, and generally higher-maintenance; they can include lawn grasses, larger shrubs, and even small trees. Despite the advent of light engineered growing media, the weight of these systems can be substantial. Extensive roofing systems, in contrast, are thin and relatively lightweight; however, these systems usually sustain only small hardy plants, such as sedum and mosses.
Before adding vegetated areas over occupied space, there are many considerations to evaluate, the most immediate and limiting typically being the load-carrying capacity of the plaza deck. In terms of structural considerations, though, vegetated roofs in a plaza setting have significant advantages over their counterparts on the rooftop. These assemblies—even extensive ones—are heavy compared with traditional roofs, and this extra weight can limit their use on existing buildings.
Typically, this limitation is not a problem for plazas. The weight of extensive vegetated roofs is comparable to that of many plaza paving systems, so ounce-for-ounce replacement of pavers with plants and growing media is often possible. Additionally, plazas must be designed for large potential live loads from vehicles and crowds; once converted to a vegetated assembly, that area must sustain only the occasional gardener or two. By restricting access, the design professional may convert the excess live loadbearing capacity into additional support capability for plantings.
Being several stories off the ground, the benefit of a vegetated roof atop a building is it can be designed to be more or less self-sustaining. Requiring only occasional weeding and fertilizing, an extensive roof is low-maintenance, but it can become an unattractive jumble of desiccated plant matter during the drier summer months. To avoid having a plaza that looks more brown than green, irrigation measures must be incorporated into most plaza landscape designs.
The most attractive plaza vegetated roofs employ a combination of both intensive and extensive roofing systems. Taller, decorative intensive plants and grasses can be strategically located within a mosaic carpet of extensive plantings to provide the appearance of a lush and vibrant meadow.
| VEGETATED ROOFS AND CODE REQUIREMENTS |
  With careful planning, owners of existing plazas can use code requirements to their advantage in incorporating sophisticated planting designs impossible on an ordinary roof. Code requirements for live load are different for plazas than for roofs. Regardless of a plaza’s actual use, current code anticipates the potential for large loads due to crowds or events, and requires a high live load capacity for the structure (typically 4.8 kPa [100 psf]). Roofs, conversely, are not expected to receive such loading, so the code stipulates a far smaller live load capacity (typically 0.96 to 1.44 kPa [20 to 30 psf]). If a plaza area is converted to a vegetated roof, the reduction in live load may be used advantageously to permit a more robust planting assembly, provided people are kept off the vegetated areas. Heavier, more substantial plantings and decorative features impossible on a roof may be employed in a plaza installation thanks to the structural capacity inherent to the existing plaza deck. However, such repurposing of plaza space requires the new planting beds be protected from incidental use that could bring the total load above structural capacity. Installation of fences, railings, and/or gates must clearly indicate to pedestrians these areas are off-limits. |
