Roofs are designed to shed water quickly and protect the inside of the home from environmental damage. But to support a roof garden, the roof must bear the weight of soil, water, plants, all the layers that protect the roof from water and roots, as well as water reservoirs. Since most roofs are not designed to do so, green roof technology is designed to adapt existing roofs or re-build a roof designed to carry a garden.
No matter the roof type, a layer of waterproof sheeting is installed on top of the roof. This layer must be durable and often has a protective layer placed on top to prevent tears or holes during the installation of the rest of the green roof. Thermal plastic sheeting, for example, is durable, waterproof and provides a small measure of insulation. Liquid applied asphalt is often used for large areas to avoid having to overlap sheeting.
A major concern with planting on top of roofs is the ability of roots to slowly work their way through concrete, brick, tile and other roofing material. Roots can slowly break apart any of these structures, but they can't work through a weed blocking layer. Polyethylene root barrier sheeting is designed specifically to deter roots. On top of this layer, additional padding to prevent tears and holes is often added. Even a small tear will allow roots to penetrate and widen the hole, causing structural damage over time.
The rest of the layers can be as high- or low-tech as you wish. The padding on top of the waterproof and root barrier layers can be cardboard or polyurethane board. On top of this is the drainage layer. Volcanic rock works well, but pre-fabricated water preserving and channeling systems provide more water control. Fine mesh sheeting or just simple blankets keep the soil separate from the drainage layer.
Depending on how intense, deep or complicated the garden roof is, more equipment might be involved. Irrigation systems and water capture systems such as rain barrels are useful for areas with intense heat or wind or for high-maintenance roof gardens. Some gardens include reinforced walkways for pedestrians enjoying the roof garden if the roof can carry the weight.
The key element to a roof garden is the load capacity of the roof itself. A structural architect must determine the load capacity of the roof. In some cases, a new roof must be installed to carry the weight of the soil, water, plants and people who will walk in the planted roof garden. Roofs with extreme angles may only tolerate ground cover vegetation, while flat roofs are the most compatible with intensive roof gardens with trees, shrubs and walkways.