home > green roofs > design & construction
Design for biodiversity
Where a green roof is being implemented to enhance biodiversity and to meet specific habitat or species objectives, the roof design and system used should be tailored to meet the specific circumstances of the site and surrounding habitat. A detailed assessment should be undertaken including consideration of:
Feasibility – Is the roof area sufficient to provide meaningful habitat? Will the roof structure support varied substrate depths? Will the height of the roof restrict its use by certain species?
Substrate type – Would the target species benefit from the use of a range of substrate types? Are local substrates appropriate for use on the green roof?
Vegetation type – Will a generic/ off-the-shelf system provide appropriate habitat? Could a generic system be enhanced through the use of a local seed mix?
Other features – Would species benefit from the incorporation of logs, boulders or other features.
Design for sustainable drainage
Factors that affect the water retention of green roofs include:
The attenuating affect of a green roof on run off is typically taken into account by allowing a reduced run-off coefficient (the percentage of rainfall that turns into surface water running off the roof) when analysing single storm events.
Green roofs will be most effective from a drainage point of view when they are combined with other SUDS components to form a stormwater management train (a series of drainage structures placed one after the other). For more information please see the CIRIA SUDS website.
Slope limitations and structural issues
Typically intensive green roofs are constructed on slopes of up to 5o and extensive roofs on slopes of up to 30o. Generally, the maximum slope for commonly available extensive green roof systems is 45o. As the steepness of the slope increases so detailed consideration of substrate depth, methods of substrate retention and drainage become more important.
In most cases the introduction of a green roof will have loading implications for the building. Any roof should be sufficiently engineered to carry the roof self-weight (including saturated green roof materials) and intermittent loads from wind, snow and human access etc. Most buildings are designed with some inherent structural redundancy. However, for the retrofit project in particular the available structural capacity of the roof may well limit the depth of substrate that can be used, thus constraining the choice of green roof system.
An assessment of structural capacity is a prerequisite for any green roof project and should be carried out by a qualified structural engineer.
Green roof construction
Constructability – As with any construction project, careful consideration needs to be given to how the materials to be used for the green roof will be procured, delivered, stored and installed. The bulkiest element of a green roof is the growing substrate. For green roofs with a small surface area it may be appropriate to lift the materials to the roof manually. However, for larger projects, a crane or pump will be required to deliver the materials to roof level. Protection of materials whilst in storage will be vital to the success of the project, especially the waterproofing.
Quality – This can be divided into reliability and biological function. To ensure the former, the appropriate materials must be specified and installed by competent contractors, and tested throughout and following construction to ensure installation quality. Avoiding damage to the waterproof layer, whilst in storage, transportation to the roof and following installation is essential. Leaks to the waterproofing can be costly to remedy. The success of the roof in meeting its designed biological function will come down to the appropriate specification of substrate, plant types, planting method; correct assessment of the local climate and microclimate, and appropriate post installation care.
Health and safety – Green roof projects involve working with heavy and bulky materials often at heights. Careful consideration should be given to aspects including safe access to the roof area, working close to unprotected edges, the use of hazardous materials (e.g. hot bitumen), material handling and the risk of dropping materials on people at ground level.
Green roof maintenance
The amount of maintenance required will be dependent on the type of green roof installed (extensive, simple intensive or intensive) and on the clients expectations for the completed roof (e.g. managed or natural environment). Intensive and simple intensive roofs require relatively high levels of maintenance (weeding, addition of fertiliser, and irrigation). In contrast, extensive roofs tend to be designed with minimal maintenance in mind, although typically in the UK a period of irrigation and application of fertiliser will be required in the early stages.
Environmental factors such as rainfall, temperature and light will influence the need for maintenance, but importantly requirements can be reduced by selecting species suited to the particular environment in which the roof is to be built. It is important that maintenance is considered from the very beginning of the green roof project (i.e. concept design).