What is a green roof system?

Green roof systems are vegetated roof covers, with growing media and plants taking the place of bare membrane, gravel ballast, shingles or tiles. The number of layers and the layer placement vary from system to system and green roof type, however all green roofs include a single to multi-ply waterproofing layer, and drainage with growing media and plants, covering the  entire roof deck surface.  There are two main types of green roofs – extensive or intensive -  although a green roof is often
designed with features of both, and then are referred to as either semi-extensive or semi-intensive.

Commonly, the roof function or objective of the roof space determines the design. Is it just an ecological cover or is it intended for human recreation, vegetable gardening, etc.?  The limiting factors for greenroofs include: the roof loading capacity or maximum dead and live weight loads, determined by a structural engineer; the slope of the roof and perhaps the client’s budget.

Extensive – Also referred to as eco-roofs, and low-profile, have thinner and less numbers of layers,  therefore they are lighter, less expensive and very low maintenance.  Extensive green roofs are built when the primary desire is for an  ecological roof cover with limited human access.  The minimum growing media or soil substrate starts at about 2 1/2” to 6” at most (although vegetative mats can actually have even less than 1" of growth media); the engineered soil media contains 70 – 80% inorganic or mineral material (or higher) to 20 – 30% organic (or less).  Low growing, horizontally spreading root ground  covers with general maximum plant heights of 16 – 24” are ideal.  Alpine-type plants are successful because they are high  drought, wind, frost, and heat tolerant, all necessary attributes for green roofs.  Plants include sedums and other succulents,  flowering herbs, and certain grasses and mosses.  Fully saturated weights range from a low of about 10 – 50 lbs/sq. ft.  In comparison to common river rock ballast which weighs about 12 lbs/sq. ft.  Extensive green roofs can be constructed on slopes  up to 30°, and steeper ones can be installed with raised grids or laths to hold plants and soil media in place.

Intensive – Also referred to as high-profile, look like traditional roof gardens because a much wider variety of plant  material can be included since growing media depths are increased.  The growing media starts from about 8 - 12” and can range  up to 15 feet or more, depending on the loading capacity of the roof and the architectural and plant features that the building owner desires.  The engineered soil media usually contains about 45 - 50% organic material to 50 - 55% mineral, and fully saturated weights range from about 80 -120 lbs/sq. ft. and up.  Architectural accents such as waterfalls, ponds, gazebos, etc. are possible and these greenroofs provide recreation spaces and encourage interaction between people and nature.
Maintenance requirements are also more intensive, and of course, these roofs are relatively flat.