Green roofing failures: Root causes

Green roofs offer many attractions. For a start you get a host of environmental benefits and an extra plot of grass, on which you can have plants or even trees. Also, by putting a garden on your roof, the weatherproofing layer lasts two to three times longer than one exposed to ultraviolet radiation and thermal cycling.

When they work, green roofs are great, and fortunately most of them do, most of the time. But when they go wrong, the cost of putting them right can cause a serious dent in the maintenance budget. Whether you go for an intensive system with deeper soils and higher maintenance, or an extensive system with a more shallow soil layer and less maintenance, the trick is to design, install and maintain the right system for the building location and geometry. Here’s a guide to the causes of and cures to common problems:

1 Deterioration of plant life This can be avoided by getting the right balance of growing medium and plants for the maintenance regime and weather. Ensure correct soil depth, porosity, drainage and irrigation. On sloping roofs, minimise soil slippage and anchor trees

2 Erosion and uplift These are particularly problematic at corners and edges of the roof especially on higher ones. A solution is to include a band of pebbles or paving at the edge and retain trims at the eaves

3 Deflection of the roof deck This may occur with changes in loading such as alterations to the building, accretion of soil, growth of plants and trees. The additional forces on a roof and the possibility of rain-saturated soils should be allowed for. Construction and maintenance should be planned so concentrated loads are not created by piling up soil or materials.

4 The wrong type of soil There may be cost and environmental benefits in using excavated soil from the site. However, this is only an option if the soil is suitable as a growing medium and is capable of being supported by the roof under wet conditions.

5 Moisture penetration Water has a phenomenal capacity to track through construction. Establishing the point of water entry from manifestation inside the building can be challenging. Before remote testing – such as thermal imaging, electronic leak detection or radio frequency testing – can be attempted, the plants and growing medium may have to be removed. The simplest solution may be to re-roof from scratch. Waterproofing failures may be caused by:

• Plant root disruption. BS EN 13948:2007 provides a test method for determining the resistance of membranes to root growth. But results are only available after two years of testing. The effect of rhizomes such as couch grass or Japanese Knotweed may need to be tested separately. Ensure the waterproofing layer is resistant to root growth or incorporate a root resistant membrane
• Poor detailing at junctions, penetrations and edges. If at all possible, avoid penetrations or concentrate them in one area. Keep the roof design simple and minimise junction variations. All junctions should be planned in three dimensions to give installers a clear guide for what is expected. At upstands the waterproofing layer should finish at least 150mm above the finished roof surface. Allow for drainage at the base of facades.
• Failure of waterproofing upstand. To maintain longevity of the waterproofing membrane, incorporate protection from direct sun exposure.
• Inadequate slope. Flat roofs will result in ponding unless there is at least a 1:60 slope.

More prevention methods

• Specify a complete green roof system from a specialist, manufacturer or installers with a proven track record and only use approved or accredited installers
• Commission an integrity test of the waterproofing layer before building up the green roof strata
• Carry out regular maintenance. Remove unwanted plants and carry out annual servicing of drainage outlets
• Follow up-to-date guidance. For example the German Landscape Development and Landscape Construction Research Facility (www.f-l-l.de) publications which have been translated into English, some of which are freely downloadable
• Further information can be found at: www.livingroofs.org, www.thegreenroofcentre.co.uk.