Extensive Green Roofs: The Essential Guide

We give you the low down on extensive green roofs, outlining what’s involved and why you should create them. Extensive green roofs are a form of Sustainable Urban Drainage System (SUDS) that can provide many ecological, economic, biodiversity, and aesthetic advantages over traditional tile, metal, or felt roofs. In this essential guide, we take you through everything you need to know about extensive green roofs.

Intensive or Extensive? An extensive green roof is a vegetated roof that does not require an additional watering system. Typically, this limits the range of plants that can be accommodated to herbaceous perennials and succulents. Intensive green roofs, on the other hand, can accommodate larger species, such as woody perennials, but require greater engineering and are often more expensive to install and maintain.

Benefits of Extensive Green Roofs

In her excellent article Permeable Paving: The Essential Guide, Marta Ratajszcak highlighted the need for storm water to effectively drain through the soil to avoid localized flooding. Another way to prevent flooding through SUDS is to prevent some of the water from reaching the ground or drainage system. According to the Green Roof Centre, extensive green roofs are able to store up to 80 percent of summer storm water and between 10 percent and 35 percent of winter rain that falls on the roof. This water is stored within the plants and substrate on the roof, thus mitigating the effects of storms.

Extensive green roofs turn a potentially dead and barren hardscape into a verdant oasis of vegetation within the city. In addition to the biodiversity benefit of incorporating vegetation into a previously unplanted area, local wildlife is provided with shelter, foraging, and nesting habitat. As Oana Anghelache points out in her article Why Should You Have Grass on a Roof, a green roof can insulate both sound and heat. The additional layers of drainage material, substrate, and planting used in extensive green roofs have an insulating effect on the building to which they are applied. This in turn saves on heating and cooling costs. The aesthetic benefits of extensive green roofs can add to the value of properties. With the principles of biophilic design being increasingly popular in urban design, the need for incorporating more natural stimuli into the urban grain is becoming a priority. Properties that achieve ecological benefits while providing attractive green views can attract a commercial premium. Fire, Health and Safety Arguably, the Germans are the world leaders in specifying extensive green roofs. The German FFL Standards (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau) set out criteria for the specification and installation of green roofs, which include recommendations such as leaving a 0.5 meter vegetation-free zone around walls to act as a fire break. This is vitally important, as the green roof could potentially dry out in summer and become a fire hazard. The fire break stops the fire from easily spreading from one area of a building to another via the green roof. Another concern is the angle of the roof. While it is possible to turn an existing flat roof of less than 2 degrees into a verdant green roof, such a shallow fall can present difficulties for drainage. On the other hand, roofs with an angle greater than 10 degrees present health and safety issues for installation and maintenance. For these reasons, a landscape architect will usually defer to a specialist when dealing with roofs outside of these parameters. When specifying green roofs, it is important to consider weight. A simple extensive green roof can weigh anywhere between 60 to 150 kg/m2 (13.0 to 30.0 lb/sq.ft) when wet. One also needs to consider the occasional load bearing of maintenance workers accessing the roof. It is therefore vital to seek the proper advice of a structural engineer. WATCH: Greenroofs 101 from Greenroofs.com

Typical Extensive Green Roof Construction

A typical extensive green roof is made up of various layers of materials. Usually, these include: • An insulating layer • A waterproof membrane • A drainage layer • A filter fabric • Gravel • Another filter layer • Substrate/growing medium • Plants Not all layers are needed in every roof. For example, if the roof is insulated immediately below the roof deck, then the insulation can be left out. Therefore, it is important to liaise carefully with the architect when specifying green roofs.

Types of Extensive Green Roof

There are three main types of extensive green roof that the architect or landscape architect can employ. They each have their advantages and disadvantages. Sedum Mat/Seed The simplest and cheapest method of installing an existing or proposed roof is the Sedum green roof. Being succulent plants, Sedums require only the minimal build-up of substrate growing medium and are relatively easy to maintain. Sedum mats can even be rolled out much like turf to instantly green an extensive green roof. For their weight, sedums can carry a lot of water, making them an ideal choice to use for storm water mitigation.

Biodiverse Wild-Flower Meadow For a biodiverse extensive green roof, specialist wildflower mixes can be sown. This option offers unparalleled biodiversity, while also providing an important nectar and habitat source for local wildlife. When specifying or using wildflowers, it is important to check which species are indigenous to the local area to get the maximum biodiversity benefit. Pictorial Meadow One of my personal favorites is the pictorial meadow. This utilizes native and non-native wildflowers to create a long-lasting and beautiful floral display that adds value to any development. Possibly the world’s greatest authority on both pictorial meadows and extensive green roofs is Professor Nigel Dunnett. Check out his awesome book Planting Green Roofs and Living Walls, co-written with Noel Kingsbury. WATCH: Silent but powerful; a time lapse of an Extensive Green Roof Installation Whether its a biodiverse green roof for local wildlife and ecological enhancement or a simple sedum mat extensive green roof, it is important to keep in mind the load-baring capacity of the existing or proposed building for which you are specifying the green roof. The benefits of enhanced biodiversity, SUDS, and aesthetics should convince you that it is well worth designing a green roof for your next development proposal. If you are still unsure, why not check out Planting Green Roofs and Living Walls, available here. Article written by Ashley Penn Return to Homepage

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