When planning for a green roof, a number of decisions, processes and initial costs must be considered.
Before the green roof installation can begin, a licensed structural engineer or architect should be hired to determine what, if any, upgrades need to be made to the existing roof. Due to the weight of green roofs, some buildings cannot initially support them. Consultants will determine the weight capacity of the building, type of roof membrane, current condition of the roof, height above ground, roof slope and orientation, water supply, and accessibility for installation and maintenance. Additionally, the consultants will determine what irrigation and drainage systems are needed.
Overall, the initial cost of installing a green roof usually includes consultant fees; irrigation and drainage systems; garden materials; plants; and maintenance, transportation and city permits. The average cost varies because of the different green roof options. The weight load of the building and the plantings desired will determine which type of green roof is necessary. The options include either an extensive or intensive green roof. The main differences between these two types of green roofs are the growing media depth and organic contents—which in turn affects the cost and planting options.
With a growing media depth of three to six inches, extensive green roofs are less costly and are only ideal for growing drought-tolerant plants such as sedums and grasses. Little maintenance is required on an extensive green roof. An intensive green roof contains a media depth of a foot or more; thus, an intensive green roof can support plants, ground covers, ornamental grasses, shrubs and trees. Like any garden, maintenance such as watering and weeding is required. For both types of green roofs, plant selection must be adjusted to local environments and conditions.
A common misconception of green roofs is the thought that you can use any type of soil for the growing media. Soil alone does not provide the proper characteristics for a green roof to survive. The growing media must be designed and manufactured to provide the nutrients, water-holding capacity and drainage characteristics to promote a sustainable green roof environment. An example of a type of specifically designed growing media for green roofs is rooflite. Although the growing media is often the biggest expense of installing a green roof, it is the most important aspect because the green roof will not survive without it.
While the initial cost of installing a green roof might be high, the cost savings realized in the long run are tremendous. The roofing membrane will have a longer life span because a green roof protects the roof from ultraviolet rays, severe temperature changes, and physical and wind damage. A green roof saves energy costs because of the added insulation, meaning less heat in the winter and air conditioning in the summer. Additionally, while more maintenance may be necessary on a green roof, fewer repairs are required overall because of the protection green roofs provide. Lastly, depending on the city, often there are reductions in city fees.