by alan burchell & rubyana lyon green roof
mar 3 2017

Types of Green Roofs

Each green roof is unique, as their designs and performance vary by region, climate,  and building type and use.  Simpler, shallower, cheaper green roofs with limited variety of plants are called ‘extensive green roofs.’ These systems are easy to install, require little to no maintenance or irrigation, and weigh the least compared to other deeper systems.  They typically manage the minimum amount of  stormwater to qualify for financial incentive programs offered by city governments.

Extensive Green Roof

Extensive Green Roof

At the other end of the spectrum, the deepest (and therefore heaviest), most complex and expensive green roofs, with the widest variety of plant species of all types of green roofs, are called ‘intensive green roofs.’  They will require regular maintenance and a professional irrigation system.  To compare them with extensive green roofs, think sod lawn versus English garden (not really, but you get the idea), or quick-and-dirty versus high-performance.

Intensive Green Roof

Intensive Green Roof

To limit costs and weight, yet still achieve desired aesthetics and performance in some areas of the roof, green roofs are commonly designed with a mix of extensive and intensive areas, or an even average of the two, and these are referred to as ‘semi-intensive’ green roofs.

Semi-Intensive Green Roof

Semi-Intensive Green Roof

The Layers of a Green Roof

Regardless of the depth of growing media (soil) and plant types used above, the underlying architectural elements of most vegetated roof systems are typically the same, with each layer playing a crucial role.

Layers of a Typical Green Roof. Image: Earth Pledge

Layers of a Typical Green Roof. Image: Earth Pledge
Starting from the existing roof deck and moving up, the typical layers of a green roof system are:

Waterproof membrane
In many retrofit cases, this layer already exists and depending on it’s condition (water-tightness), it may or may not need replacing.  Polyvinyl chloride (PVC) and polymer modified bituminous membranes are common materials.

Root barrier
Flexible, tear- and puncture-resistant, this layer keeps roots from penetrating the roofing membrane and causing leaks.

Protection Mat
Protects the root barrier and waterproofing from mechanical damages while also functioning as water and nutrient reservoirs, and reducing subsonic noise.

Drainage layer
This egg carton shaped layer manages water by retaining only the amount needed for plant growth.  The excess is drained away through a channel system molded into the bottom of this layer, carrying it to the roof drains, to prevent a waterlogged system or significant increase in system weight.

Filter cloth
Keeps the growing medium (soil) above from being washed away by the drainage layer below or clogging it up.

Growing medium
Creates a suitable growing environment for a rooftop setting and is designed to be lightweight and drain easily yet still retain some storm water. The engineered mix is often native soil supplemented with organic or mineral additives.

Green Roof Plants
Plant species options vary greatly, depending on local climate, load capacity, budget, and personal preference.  Drought, wind, frost, and heat tolerant plants, like sedums, are best, but some green roofs can support vegetable gardens, flower beds, even trees, to serve as an ecological, rooftop haven!

Possible Plants for a New York Green Roof

Sample of Possible Plants for a New York Green Roof (Click to zoom in and read species names)