Green Roofs — Wholebuild UK

Green Roofs - Wholebuild UK

A green roof or ‘living roof’ is a roof that is wholly or partially covered in plants, a growing medium and waterproof drainage membrane. The use of the term “green” refers also to a roof’s environmental credentials rather than to the actual colour of the plants and incorporates other roofing materials such as roofing shingles. A green roof is a suitable alternative to roof tiles.

Green roofs can be seen increasingly seen on commercial projects and social housing developments where in the latter case, the use of Green roofs attract Code for Sustainable Homes Credits for:

Mandatory Element: Yes

Aim: To design surface water drainage for housing developments which avoid, reduce and delay the discharge of rainfall run-off to watercourses and public sewers using SuDS techniques. This will protect receiving waters from pollution and minimise the risk of flooding and other environmental damage in watercourses.

ECO2 1 Code credit is available worth 1.33% points each

Mandatory Element: No

Aim: To enhance the ecological value of a site.

Benefits of Green Roofs

A green roof especially one like a sedum roof serves a number of functions and benefits including:

1. Green roof systems help to combat climate change and the Urban Heat Island Effect (UHIE) in which urban areas are hotter than surrounding rural areas because there are greater numbers of materials (ie: buildings) that retain the heat.

2. Green roofs help people save money; they reduce the need for air conditioning in summer and have some insulating effect in the winter.

3. Green roof systems absorb rainwater and help reduce the volumes and rates of surface run off responsible for flash floods; there is less water entering the sewers and water treatment plants. Dirty and surface water can therefore be released slowly back into water courses and work towards addressing the new Sustainable Urban Drainage (SUDS) legislation. As rainwater passes though the drainage substrate and growing medium of the green roof systems drainage, small particles are trapped providing ‘clean’ rainwater harvesting opportunities.

4. With green roof construction predominantly laid on bitumen waterproofing layer, the coverage of the plants acts to protect this layer and treble the lifespan of the underlying roof membrane.

5. The plants on green roofs contribute to recycling carbon dioxide and producing oxygen into the atmosphere. The plants also absorb noise and act to reduce pollution by trapping micro airborne particles

Green Roof Construction

Careful attention must be given to the design and construction of green roofs to ensure sufficient water storage and plant nutrition to support the vegetation all year round. Most commonly, green roofs are made up of a number of distinctive layers.

1. A structurally sound deck such as a timber frame is required to hold the green roof elements on the roof. Roof slopes can be anything from a 0° to 35° slope with special design requirements for pitches over 35°. Occasionally, additional supports and may be required to ensure that the building can take the load.

2. Green roof systems waterproofing such as a 300 micron pond liner acts a suitable damp-proof membrane and vapor control barrier.

Green Roofs - Wholebuild UK

3. A filter sheet stops fine soil from leaving the roof and acts as a root barrier membrane whilst allowing water to pass through and free drainage to occur.

4. A Geotextile membrane acts as thermal insulation and an absorbent moisture retaining layer. Types vary but suitable alternatives include blankets, woolly fleece, towels or cardboard.

5. A base layer of free drainage substrate should consist of aggregate such as crushed concrete/brick, limestone chippings, gravel or clay pellets.

6. A growing medium consisting of sand or soil, preferably both. This layer is typically about 65mm and 150mm deep and depends on the roof structure, the types of plants and the climatic conditions. Lightweight growing mediums, typically vermiculite can be incorporated to provide a lighter green roof.

7. The plants required for green roofs are self-sustaining and create biodiversity. Preference is given to those which combine qualities suitable for them to thrive depending on such factors as temperature and rainfall. Suitable options for green roof systems include mosses, wild flowers, succulents, herbaceous plants and grasses. The client’s desired finished green roof will determine the substrate and the substrate depth; alternatively the substrate will determine the types of plants used in green roof construction. Plug plants, hydroseeding, plug planting or sedum mat can be used to create green roofs.

Green Roof Maintenance

Green roofs require minimal care, irrigation and maintenance. The only requirement for a green roof is an annual inspection of the drainage system and the removal of weeds and any windblown debris.

Roofing Contractors

Note: Green roof suppliers normally provide guarantees for their waterproofing if installed by an approved contractor.

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