More About Green Roofs

Green Roof Types – Extensive, Intensive and Hybrid

Green roofs are living roofs. There a number of different types of green roofs. They vary based on the depth of soil, the type of plants, its weight, the way it is used and the number of improvements on the roof. Generally speaking, green roofs are described as extensive, intensive, or hybrid (a combination of extensive and intensive elements.)

An extensive green roof at FIT, Manhattan

Extensive Green Roofs

An extensive green roof is a light weight, low maintenance choice good for large areas with maximum benefits and little human access. An extensive green roof can placed on top of either a sloped or flat roof.

The planting medium in extensive green roofs ranges from 1.6 to 6 inches deep, and while deeper systems have been installed shallower systems are often favored because of they are light weight. Drought-tolerant sedums and grasses are the typical plants used on extensive roofs, since they are shallow-rooted and use little water. Plant diversity on these roofs is minimized standardize moisture needs and provide ease of maintenance. During dry times, these rooftops may require occasional hose watering in areas where there are months between rains.

Installation methods vary, but often extensive roofs can be installed in a very short time providing an instant green effect. Extensive green roofs are the simplest to install and are very often added to existing roofs. Depending on the source you look at these roofs may add 10 to 35 pounds per square foot to a roof’s load.


Community of the Holy Spirit

An intensive roof on St. Hilda’s House, Manhattan

Intensive Green Roofs

An intensive green roof is characterized by deeper planting mediums, more plant variety than in an extensive roof, and increased opportunity for human access and usability.

Intensive green roofs commonly include grasses, ground covers, flowers, shrubs and even trees, which provides biodiversity on the roof top. They will often include paths and walkways that travel between different architectural features to provide space where people can interact with the natural surroundings. Benches, tables, planter boxes, greenhouses, ponds and fountains offer people places to relax, dine or work in park-like settings.

Once the plants are installed and the soil is moist these rooftop green spaces can weigh as much as 150 pounds per square foot. The irrigation and drainage systems have to operate at peak efficiency to reduce the chance of overloading the roof’s structure.


A built-up hybrid green roof on a pool house in Baldwin Hills, Los Angeles

A hybrid green roof on a Baldwin Hills pool house

Hybrid Green Roofs

As the name implies, hybrid roofs are a combination of extensive and intensive roofs. They utilize the best of both green roof types: versatility for different roof styles, wide plant variety, all with less maintenance and cost of an intensive roof.





Methods for Creating Green Roofs – Modular, Mats, and Built-Up

There are a variety of ways to install plants on a green roof. No matter the method, the initial steps are the same. The roof is carefully waterproofed, root barriers, irrigation, drainage and rooftop infrastructure are installed and then the plants are either set in place or planted. Modular and mat systems offer instant green because they’ve been grown offsite. The built up method allows for greater flexibility in design and plant choice, but take a little longer to look like a green roof.


Modular – A modular system usually combines several of the green roof’s layers into pre-manufactured and often pre-planted systems that are placed directly on a roof that has been prepared with a proper water proofing membrane. There are a number of manufacturers of green-roof modules allowing for choices in size, weight and plant variety. A modular green roof allows for fairly easy alterations and additions, instant green and easy access to the roof surface below. Modular systems are frequently used on extensive portions of a green roof.

matsMats – Like the modular systems described above, the mat method uses green roof plantings grown off site and later installed on the green roof after proper waterproofing has occured. Mats are laid much like a carpet or grass sod. They are placed on the roof to provide complete coverage, and often instant green. Over time, mats will grow together, seams will disappear, and with proper care, a solid looking green roof will appear. Mats are frequently used on extensive portions of a green roof.

planted_in_placeBuilt-Up – This type of green roof allows for greater flexibility in design and plant choice. After installing proper water proofing and root barriers and building the designed rooftop infrastructure, such as planter boxes, irrigation, pathways, cisterns, and trellises, the planting medium is spread on the roof, and then plants are added. This is often called “planting in place.” It may take a while for a green roof planted in place to look lush and green. The built-up method is used most often in planting intensive and hybrid green roofs.