Green Roofs on Parking Structures

A green roof is a roof covered with plants and growing medium over a waterproofing membrane, drainage layer and root barrier.

A green roof stacks plants and growing medium over drainage, a root barrier and waterproofing on the structural deck. The saturated weight and the need to reach the membrane for repairs make it far easier to plan into a new garage than to add later.

On a parking structure, a green roof usually means giving up the top deck for parking and adding significant weight. In return it manages stormwater, cools the roof and the area around it, and can create usable open space. It works best when planned in the original design, because the structure must carry the extra load.

Types

Extensive: shallow growing medium, a few inches deep, with low-maintenance plants such as sedums; lighter and cheaper.
Intensive: deeper soil that supports lawns, shrubs and trees; much heavier and more like a park.
Semi-intensive: in between.

Trade-offs

Saturated weight must be carried by the structure; retrofitting an existing garage may not be possible.
Lost parking on the top level, sometimes offset by the value of the open space.
Waterproofing must be excellent and accessible for repair.
Benefits: stormwater retention, lower heat island effect, habitat and amenity. See stormwater detention.
A solar canopy competes for the same roof.

The Parkonomics view

A garage roof can park cars, grow plants or make power. Deciding which one early, before the structure is designed, is what makes the other two options affordable later.