A Roof Over Your Head: Choosing The Right Roofing Materials - Renew

The vast majority of roofs on Australian homes are conventional—Colorbond/galvanised, tiles, slate or shingles. However, there is another type of roof that can perform better thermally than any conventional materials—a green roof.

A green roof is simply a roof designed to grow plants. A layer of growing medium—soil or lightweight aggregate—is laid on a suitably prepared and designed roof structure, which may include waterproofing, root barrier and drainage system. The roof is planted out with appropriate plant species.

The insulating and thermal mass properties of a green roof can help stabilise internal room temperatures and reduce sound ingress while helping the house to blend into the surrounding landscape—ideal for homes in wilderness areas.

Other advantages include the reduction of the urban heat island effect, where large surfaces of concrete and bitumen absorb heat rather than reflecting it, reduced stormwater runoff, improved air quality and creation of habitat for wildlife.

Green roofs fall into two main categories: extensive and intensive.

Extensive roofs are those with growing medium thickness under 200 mm. They have a relatively low roof loading of up to 120 kg/m2 and use low mass plants like sedum or other low growing succulents, ground covers and mosses, grasses and small ferns. Extensive green roofs are normally designed to require minimal care and are usually only accessed for maintenance, so most extensive roofs will only grow simple plants that require little effort.

Extensive roofs can be laid on lightweight roofs using structures not much different from those suited to Colorbond roofs, for instance, and in some cases, subject to structural limitations, can be laid on top of existing lightweight roofs.

Intensive green roofs use a growing medium depth over 200 mm with much higher roof loads of up to 700 kg/m2 or more. They can include high mass plants like large shrubs and small trees, and are designed to be used and maintained regularly, so they can even be used for growing herbs and vegies.

As you might expect, only certain structures are suitable for green roofs due to the high mass on the roof. Buildings made of concrete and stone or other materials with high compressive strengths are required—heavy steel or timber structures can be suitable, but the engineering must be carefully calculated.

Green roofs can be either flat or near-flat (the usual format for an intensive roof) or sloping. A sloping roof increases the water run-off from the roof during rain, so less water is retained and waterproofing is less critical. In an intensive roof, waterproofing must not only last the life of the building, it must also be able to withstand plant root penetration from more aggressive plants such as small trees.

This leads us to the costs of green roofs. A simple extensive roof may cost up to a few hundred dollars per square metre, not including any building permits and fees, roof strengthening or moving of roof infrastructure. An intensive roof can easily cost into the thousands of dollars per square metre due to the high structural strength and levels of waterproofing and root barrier required—and the cost of the much thicker layer of growing medium and more expensive plants than extensive roofs normally have. Actual cost will depend on many factors.

For most domestic situations, an extensive roof is probably the best option, but if your garden space is otherwise limited, then an intensive green roof can double as an outdoor living space, potentially offsetting the higher cost of this sort of roof.

Further reading:

A useful guide on planning a green roof is the Growing Green Guide. Your Home green roof page Green Roofs Australasia Wikipedia green roof page