Green rooftop gardens are growing in popularity throughout the United States. They not only provide an aesthetically appealing addition to a house, but also insulate the house. The leaves of the garden reflect sunlight and drastically decrease the need for air conditioning. In cities, the gardens also filter and moderate the temperature of runoff water, eliminating chemicals that may end up in the sewers or drinking water. To install the garden you cannot simply throw dirt on a roof; you must go through a process.
Consider what type of rooftop garden you would like and your roof can support. Intensive roof gardens are traditional accessible gardens with large shrubs and trees requiring much attention. Generally, these roofs are flat and must support 4 to 6 feet of soil and 150 pounds per square foot (not your typical roof). Extensive gardens require little maintenance and are mainly for environmental purposes. For this, your roof must be able to support 15 to 35 pounds per square foot.
Remove any existing roofing you have. It may be best to start this process when your roof needs repair anyway. Apply a waterproofing membrane to protect your house. Look for one that will not decay from the acid some plants release. The best waterproof membrane is called "monolithic membrane," which is a rubberized asphalt applied as a hot liquid. This layer is essential to protect your house.
Install other layers of protection over the waterproof membrane. Typically an insulation layer comes next and then a chemical root barrier sheet goes on top of that. You don't want anything piercing the waterproof membrane. Place a drainage layer on top of that. This is typically made with lightweight gravel or a recycled polyethylene material resembling egg crates. A geosynthetic filter mat lies above and below the drainage layer to filter small particles.
Obtain a lightweight soil medium for the roof. This typically consists of 75 percent mineral or recycled materials and 25 percent organic material. In most cases, the soil is specially formulated to be fertile for plants, resistant to sparks and rot while staying lightweight even while wet.
Spread the soil over the membranes at a depth that both your roof and the plants you choose can support. For extensive roofs, this means anywhere from 1 to 7 inches. For extensive roofs, this means up to 6 feet. Place a wind blanket over the soil until you plant the plants and they take root. This will prevent your installed soil from blowing away.