The 601 Congress Street Roof Garden is housed on a 14-story building that is stepped at the 12th floor, leaving an open roof area with direct access from adjacent offices and visibility from the floors above. One of the building design goals was to achieve LEED Certification and Sasaki took this opportunity to create a terrace and garden that would be an amenity for the occupants as well as another sustainable design component of the project. The green roof portion of the 12th floor terrace is approximately 11,000 square feet. 

Soil depth was limited due to structural loading limitations. Allowable depth varies from 6" at the perimeter to about 12" where low points occur in the sloped roof drainage system. The lightweight soil mix for roof planting was specified as a custom pre-mixed blend composed of 55% rotary kiln expanded lightweight aggregate (expanded shale), graded sand and treated compost derived from cranberry waste. 

It was decided for safety reasons that access to the planted area would be restricted to maintenance personnel. However, the terrace provides seating space with views of the surrounding urban landscape and harbor. A glass railing separates the paved terrace from the planted area without obscuring the visual impact of plant forms, colors and textures, including seasonal changes in appearance. The planting scheme is composed primarily of natural grass masses that vary in height from one to two feet, punctuated by drifts of taller ornamental Miscanthus grass. Beds of low-growing flowering Sedums provide additional interest, color and contrast in the green roof’s foreground.

Drought tolerant plant materials were selected that will cover and shade the ground plane. After the plants are established maintenance requirements are minimal. Annual pruning of the ornamental grasses to a 6" height in the spring, and soil nutrient replenishment once or twice a year via the drip irrigation fertilizer injector system are the only requirements. 

A drainage/water storage/aeration system, manufactured by American Hydrotech, was installed over a layer of closed cell extruded polystyrene insulation. The drainage system consisted of lightweight panels of 100% recycled polyethylene, molded to form water retention cups and drainage channels and engineered to promote irrigation through capillary action and evaporation into the soil/vegetation layer. 

The structural concrete roof slab was waterproofed with a hot, fluid applied, rubberized asphalt monolithic membrane, produced with a minimum 25% recycled content, manufactured by American Hydrotech (MM 6125 EV). The membrane was applied in two coats, with a layer of fabric reinforcement between layers, to a thickness of 215 mils. 

American Hydrotech's Rainwater Retention and Run-Off Calculator, which is based on testing they have performed, indicates that approximately 70% of rainfall water is retained by the planting soil and drainage mat assembly, assuming an average 8" soil depth and roof slope of less than 10%. The retained water is taken up by plant root systems, reducing the need for irrigation.

The building is located across the harbor from Boston's Logan Airport. Although there is no quantitative data available, it is clear that drainage, soil and planting layers provide additional buffering of noise from planes passing overhead. Other benefits provided by the green roof include; reduction in the urban heat island effect, reduction in glare from the roof, decrease in energy costs, creating habitat for birds, butterflies and insect species, and improved air quality by reducing CO2 levels, increasing oxygen output and filtering and binding airborne dust and other particles.

The 601 Congress green roof is an excellent example of how green roofs can achieve multiple benefits such as energy savings, noise reduction and biodiversity restoration while at the same time providing an enhanced amenity space for employees.