What is a green roof?

Introduction  A green roof is not a surface painted green, it is a living surface of plants growing in a soil layer on top of the roof. A thinner or thicker soil layer (or another substrate than soil, as you can read more about in the section Materials) is spread over the roof membrane itself, sometimes with a protective root barrier, often with a drainage layer underneath. On the substrate, draught-tolerant species of plants are grown, usually Sedums on the extensive roof vegetations. This forms an attractive mat that flowers, and changes its appearance with the seasons. Roof vegetation like this one does not at all damage the waterprofing membrane of the roof, in fact it protects it from the uv-light which does damage the membrane over time.

When speaking of green roofs, there is a distinction between extensive and intensive green roofs. An extensive roof has very thin soil layers, draught tolerant plants and requires little or almost no maintenance. The limitation here is in the choice of plants. An intensive roof garden has much thicker soil layers, and can look like an ordinary garden, with trees and shrubs. This of course needs as much maintenance as a garden does, and can only be constructed on a roof that can bear heavy loads. More on this in the section on construction. In between the two extremes, there are all the medium heavy roofs, we can call them semi-intensive.

Extensive. This is a typical low-weight green roof. It is drought tolerant and requires almost no maintenance. Weight 50 kg/m2.	Intensive. A roof can become like any garden or park! The weights are usually over 500 kg/m2, and they require a fair amount of maintenance.
Semi-intensive. This ia a living roof designed for biological diversity. Maintenance is low, because the chosen plant species are so humble. Weight over 200 kg/m2.	Semi-intensive. Believe it or not, but this lush planting is semi-intensive. The soil layer is only 10-15 cm thick, and weighs about 250 kg/m2. Requires weeding and occasional watering.

Using vegetation on the roofs is not a modern invention. Turf roofs has been a traditional building technique in many places. However, in the last decades, modern versions have been developed for the needs we have in the cities today. The differences between a modern green roof and a turf roof are substantial, and this is mainly due to the different purposes, and the materials available nowadays. The main objectives in the early days was to use the turf as insulation, and to stop the waterproofing layer, which was often birch bark, from blowing away.
Today our objectives are mainly environmental, economical, and to improve storm water management, health and aesthetics in our communities. How green roofs can improve our cities and towns in these respects will be described in the following sections. 

 

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