Libraries are traditionally stark repositories of knowledge. A key design objective of installing a green roof was the opportunity to generate community interest in green design by making this facility a dynamic teaching tool for sustainable design and environmental awareness. The gently curving roof is visible from the periscope and observation deck and invites visitors to engage in the green roof’s ecology above the street. The Ballard Library project in Seattle, Washington illustrates green building is feasible within a modest budget, presenting the community with an ideal example of benefits realized when sustainable design combines with extraordinary architecture. The green roof occupies approximately 20,500 square feet of the roof at a per square foot cost of $20. 

The green roof consists of a waterproofing membrane, insulation, and drainage/moisture retention elements, which are part of a Garden Roof® Assembly, all supplied by American Hydrotech, Inc. The seamless waterproofing membrane used for the green roof project, Monolithic Membrane 6125®EV-FR (fabric reinforced, environmental grade, 25% recycled content), is a hot fluid-applied, rubberized asphalt that forms a long-lasting, tenacious bond to the substrate. MM6125EV-FR’s unique formulation, which includes inert clay fillers, provides excellent resistance to acids and fertilizers.

One of the most significant components of the garden roof assembly is the water retention/drainage/aeration element. It is comprised of lightweight panels of 100% recycled polyethylene, molded into retention cups and drainage channels. The design allows for the free drainage of excess water, achieving flow rates significantly higher than that of conventional drainage methods while simultaneously promoting irrigation through capillary action and evaporation into the growing media/vegetation level. Faceted planes of the curved roof create six microclimate conditions, each a separate exposure with differing water retention properties, based on slope and orientation. The upper edges slope more steeply, retaining less water but offering the highest opportunity for evapotranspiration. The lower slopes retain more water and are more protected from prevailing breezes. Monitoring of the species planted will tell which plants battle for survival successfully under these very difficult conditions. 

Strom water runoff is filtered through the planted roof or absorbed by the site landscape. Local groups are collaborating to install water-monitoring devices on the green roof. Data will be valuable in assessing the performance of the green roof over the life of the structure. Water usage on the roof and throughout the building is conserved through devices such as a computer controlled irrigation system, low flow fixtures, sensor and timed faucets, and waterless urinals.

The roof top planting is a mix of self-sustaining, drought tolerant, indigenous grasses and sedums planted in a pattern that mimics a windborne casting of seeds. While regular roof maintenance is required, the green roof was designed with mainly fescue and sedum type plants, which minimize any additional maintenance requirements. 

4-to-6 inches of growing medium is used and the mix was a custom blend based on German FLL standards. It was blown onto the roof and a biodegradable coconut fiber mat helps the plants during the establishment period by reducing wind and water erosion.

The green roof was incorporated as part of an overall strategy to reduce and conserve energy costs where possible. Solar (photovoltaic) panels provided by the Seattle City Light Green Power Panel installed on the northern edge of the roof will monitor the amount of electricity captured and collected onsite. Energy generated from these panels is fed back in to the city's power grid, reducing the Library's energy bills. Additionally, various rooftop sensors measure wind speed, direction, sunlight, etc. 

Ballard Library provides an excellent setting to model integrated building design to the community by engaging them with highly visible features such as a green roof.