Anthony Barnett (pictured), Technical Marketing Manager at ABG Geosynthetics, addresses this outdated green roof myth…
When asking fellow GRO members about the most common green roof myths that we can address in this column, high up on the list is the misconception often held by surveyors and developers that the cost will be prohibitive. However, when compared to many traditional roof constructions this is not the case, and once the sustainability and energy efficiency benefits are factored in, they can actually add value to the property price; so I’d say the question should really be ‘can my development afford not to have a green roof?’
Conservative cost estimates for a biodiverse or extensive green roof installation usually come in around the £75 per sq m mark – depending on the specific roof configuration and planting. In reality, projects are often able to be delivered significantly cheaper, and this is before the additional energy efficiency benefits, as outlined later, have been factored in.
Currently the components of a blue or green roof are not considered as part of the roof build-up when calculating thermal performance, so the insulation design and specification is the same as for a traditional roof design. Despite the UK building regulations currently making no allowance for a blue or green roof, research shows that the introduction of layers of drainage, growing media and vegetation have a positive impact on the thermal performance of a roof. The thermal mass reduces the amount of solar heat entering a building and the various layers act to absorb the heat of the sun. This reduction in transmission of solar gain into the building significantly reduces the air-conditioning and cooling demand in the warmer months.
Green roofs also reduce the urban heat island effect, whereby heat from the sun is absorbed by the hard surfaces within a city during the day and then radiated back during the night. This creates a hotter city microclimate with urban temperatures many degrees warmer than the surrounding countryside, requiring a higher energy demand to cool city buildings through the night. However, the evaporation of water from a green roof’s soil surfaces and the leaves of plants creates a natural cooling effect on the surrounding air. The many layers within a green roof system also prevent solar radiation from increasing roof surface temperatures, and therefore subsequently radiating any heat back at night.
As a result, many waterproofing manufacturers offer an extended warranty for green roof surfaces owing to the additional longevity of the membranes which are protected against the daily temperature fluctuations and expansion and contraction effects. For example, exposed membranes could reach 50-60ºC in the summer months and below freezing in winter. A green roof acts as a buffer to these fluctuations and reduces the severity of maximum and minimum temperatures, thereby extending the life of the membrane.
The energy saving and positive impact on local biodiversity ensures green roof installations directly contribute towards achieving the required BREEAM rating of a building, including from the following ecological categories:
- LE4: MITIGATING ECOLOGICAL IMPACT – To minimise the impact of a building development on existing site ecology
- LE5: ENHANCING SITE ECOLOGY – To recognise and encourage actions taken to maintain and enhance the ecological value of the site as a result of development.
- LE6: LONG TERM IMPACT ON BIODIVERSITY – To minimise the long term impact of the development on the site’s and surrounding area’s biodiversity.
The uplift in a project’s BREEAM credentials made possible by a green roof often enables cost savings to be made in other areas of the building’s design, whilst still achieving the desired BREEAM ‘Very Good’ or ‘Excellent’ rating overall.
By helping to mitigate the ecological impact of the wider development, and especially when used on urban sites where space for ground level landscaping is limited, green roofs help developers and local authority planning teams to achieve the 10% gain in Biodiversity Net Gain score required for planning approvals.
In summary, before the mitigation of flood risk and wider ecological and aesthetic benefits of a green roof system are considered, there are a number of practical energy efficiency and waterproofing longevity improvements that help to add value to the property and offset the cost of the roof from being prohibitive to developers and local authority planners.