The Green Roof of The Chicago City Hall Completed in 2001, the green roof of the City Hall of Chicago is one of the most noteworthy roofs in the already very green city. The roof intends to be a living laboratory and has been monitored since 2002 to identify the most effective species of plants and substrate depths for future green roofs in the Chicagoland area. There are three sections of the roof with varying substrate depths of 4, 6 and 18 inches. There are over 100 different species of plants on the roof; native prairie and woodland grasses and forbs, hardy ornamental perennials and grasses, several species of native and ornamental shrubs, and two varieties of trees. The substrate used is a lightweight German-engineered soil mixture. The roof’s exiting drainage system was left in tact, as we parts of its original waterproof membrane, which was also enhanced. Results from monitoring the cooling effects during the garden's first summer showed a roof surface temperature reduction of 70 degrees and an air temperature reduction of 15 degrees.

History of Green Roofs
While roof gardens have existed since the ancient civilizations of the Tigris and Euphrates River valleys, only recently due to modern building techniques and materials have roof gardens been able to fully integrate themselves into the structural and sheltering qualities of roofs. Scandinavian design has proved to be very influential in the construction of modern green roofs. As turf grass was readily available, designers sandwiched birch bark and twigs (providing a waterproof barrier and a drainage layer) between the turf and wooden boards (providing structural support). Europe has historically been a leader in modern green roof installation, with Germany leading the movement. In 2002, one in every ten flat-roofed buildings in Germany featured a green roof.

Benefits of Green Roof Technology
*Aesthetic enhancement of site
*Potential for food production
*Insulating and cooling properties / Energy efficiency
*Enrichment of biodiversity
*Water management and retention
*Enhancement of air quality
*Assisting in countering “urban heat island effect”
*Absorb sound and help reduce noise pollution
*Fire Prevention
*Job creation (estimates at 1350 new jobs / year)
*Potential for recreational space

Construction Materials
For Green Roofs
· Waterproof Membrane
The first component of a green roof to touch the physical structure is a waterproof membrane or laminate. Three types of waterproof membranes exit: the built-up membrane, the single-ply membrane and the fluid applied laminate. Built up membranes are the most common and are usually made of a bitumen/asphalt combination, but they have durability problems and have a lifespan of only 15-20 years. Single-ply membranes are generally sheets of a synthetic plastic or rubber material which is then sealed down around drainage points. These membranes are generally very effective but poor seals can be problematic in that they are susceptible to damage from overgrown roots and UV rays. Fluid applied membranes are liquids applied to the roof that then create a seal when set. They are generally foolproof in that they are easy to apply and eliminate the problem on joints, but are the most expensive of the three membranes.
· Root Protection Barrier
Generally most buildings use rolls of PVC to lay out over a newly waterproofed roof surface. These barriers are necessary because were a membrane to contain a substance with any organic material (such as asphalt) and no barrier existed, roots would grow down and penetrate the waterproof layer, therefore ending its effectiveness. Their circular or triangular shape also marginally adds to a roof’s weight bearing ability.
· Drainage Layer
As shown by the above two layers, a fundamental principle behind green roof design is to drain water that is not either absorbed by the roots or stored in the substrate at the highest level possible, and avoid it ever coming in contact with the waterproof membrane. The two main types of drainage layer materials include granular materials (gravel, stone chips, scoria, pumice, etc.), porous mats, and polystyrene modules. Granular materials are the least expensive and have the advantage of creating additional space for root growth. Porous mats act like sponges to effectively hold and slowly drain moisture. These mats are generally considered less effective in that the slow draining process has potential to negatively effect plant growth through rot due to excess water storage, one of the key reasons to maintain a drainage layer. Additionally, these layers are important because wetter green roofs negatively affect their insulating abilities.
· Growing Substrate
One may be surprised to learn that simple garden soil is often too heavy and too fertile for most roofs and is only appropriate for a green roof with a substantial substrate depth of 20 inches or more. There are a variety of materials that can be used as growing substrates effectively described in chart 1-A.

Planting Guidelines
As illustrated in chart 1-B, the depth of a roof’s substrate plays a large role in determining what species of plants that can be grown on certain green roofs. While the chart gives a more specific idea of what types of plants can be grown on varying depth levels, it is important to take climate and place into strong consideration when planting for a green roof. Qualities like a plant’s ability to withstand summer droughts and intense exposure to sun and heat in desert climates are important to note, as is selecting plants that are capable of thriving with saturated soils in wet or tropical climates.

Design Options for Green Roofs
There is also reason to put in much thought into the design or composition of the plantings on a roof, as each layout offers specific benefits and drawbacks. The three broad categories of plant combinations for green roofs are: monocultures, individual species mixtures and plant communities. Monocultured roofs are most commonly established by being sowed from seed. They have the advantage in their “lack of worry factor” as every plant has the exact same needs and can save time by eliminating the individual plant care that more diverse designs can require., But monocultures can be somewhat dull aesthetically, do little to enhance a region’s biodiversity and have increased vulnerability to being entirely wiped out by disease or stress. Individual species mixtures is a quite broad heading, as it represents the wide area between monocultures and intentional plant groupings. Species mixtures can be applied through methods of vegetation establishment. This type of green roof is generally the best for the experienced gardener as it is the option where the designer has the most control. Intentional plant communities are commonly established by being sown from seeds that are all sold together in a package. They have the advantages of being relatively self-sustaining, have a low maintenance requirement and have a naturalistic appearance.

Methods of Vegetation Establishment
There are three primary ways to go about establishing a plant community. The first is the old fashioned way of growing plants, by direct application of seeds and cuttings to the soil. Once a roof’s surface area has become greater than 200 square feet, growing green roof plants from seed becomes the most cost-effective option. The primary disadvantage in growing plants from seed on green roof is that time must be allowed for the plants to fully grow and develop in order for the roof to achieve its maximum potential. Simply put, a green roof of only substrate with seeds in the ground doesn’t provide the same benefits that a fully grown and mature green roof would. The second option of establishing vegetation on a green roof is by planting plugs or pot-grown plants. This is a likely the best option for roofs with small surface areas, although the technique can be applied to large roofs as well. It allows the designer much freedom and thus most roofs that are primarily planted from plugs fall into the individual species mixture category. A plug planted roof can have a very naturalistic appearance; plants can be planted quite close to one another. The final method of vegetation establishment is through mats of vegetation. Usually consisting of a combination of sedums, vegetation mats also have the advantage of instant effect and that they usually cover 90-100% of the roof with greenery. Their major disadvantage is that they are largely inaccessible and often require a system or plan for irrigation for them to be fully healthy and thus be fully functional.