Wildlife Roof Design Guidance
Biodiverse or wildlife roof designed to either replicate the habitat for a particular species or to create a range ofhab itats to maximise the array of species which may inhabit the roof.
Biodiverse and wildlife roof specific design guidance
Roofs aimed at replacing or replicating habitats are becoming more common in the UK. These roofs are sometimes referred to as: biodiverse; brown; rubble; brownfield; eco; habitat or even by the name of the species they are aimed at i.e. black redstart roof.
Well designed roofs of this nature are beneficial to urban biodiversity in the UK. It is also worth noting that a biodiverse roof still provides most of the benefits associated of any other type of green roof of a similar depth.
However, the choice to create a wildlife roof is not necessarily the least expensive option nor should it be considered as an opportunity to recycle un-screened site-won waste on the roof of a new development. Amongst other reasons British Building Regulations outlaw the direct transfer of demolition waste to roof level, without adequate screening for potentially polluting elements within the materials. In addition for a wildlife roof to be genuinely beneficial, it should be designed to support the intended insects, birds and plants to inhabit the roof.
The starting point for any wildlife roof should be:
Which species are to be conserved ?
As with any green roof the growing medium is the key to the success or failure of that roof, and as with any green roof the growing medium should be a designed and engineered mixture of minerals and nutrients to achieve that desired outcome. Care should be taken to select substrates carefully. Demolition waste or topsoil can be very fertile or especially heavy when saturated or may have the seed or propagules of undesirable species within, which may lead to an unexpected level of maintenance requirement.
Many of the guidelines that apply to any green roof also apply to a biodiverse roof, what makes a biodiverse green roof different is the design of the growing medium, the choice of seeds or plants or inoculants and the inclusion of features to attract particular species of plants, birds and insects.
Common to any green roof:
A building structure designed to take the loads of the proposed green roof.
Root resistant waterproof membrane or roofing finish.
Suitable drainage system to allow excess water to leave the roof easily.
300mm gravel or paving un-vegetated margin to all up-stands and roof penetrations.
Up-stands of a suitable height to prevent material being blown off the roof.
There should be a 1m wide, 300mm tall un-vegetated barrier every 40m, on large areas of green roof.
Growing medium should be not more than 20% organic matter by volume.
Designing for biodiversity
Therefore referring back to the original question is important:
Which species are to be conserved? – i.e. what do you want to live, grow and thrive on your roof?
A green roof can be designed to mimic many habitats found in the UK and therefore support many of our rare or declining species.
Before deciding on which habitat to create or species to target, consult the Local Biodiversity Action Plan (LBAP) for the area the roof will be built in. LBAPs for each area in the UK are available on the internet or from your local Wildlife Trust. The LBAP will give you information about priority species that are being supported in your area and may also provide guidance on what makes a good habitat for them. LBAPs apply to habitats, plants, invertebrates and birds.
Mosaics of habitat
If the roof area is large enough it may be possible to create a variety of microhabitats to increase the attractiveness of the site to a wider range of plants, invertebrates and birds. A way of doing this on larger development with different buildings is to have different habitats on different roofs, giving the whole site a mosaic of habitats. Putting too many habitats into one roof can be a problem if insufficient space is available to create a viable area of each habitat type.
Features to attract wildlife
The specification of natural features can be problematic because the construction industry is used to dealing with items that are regular and uniform. Often many of the materials which go to make up natural features are on site before any work begins. Even existing sites which are being redeveloped may have some of the following items, which can be stored, ready to be placed on the roof after installation of the main components.
Dead trees/tree limbs – stripped and laid on the roof to provide perches for birds (they may need securing or weighing down).
Logs – trees sawn into logs of no more than 600mm in length, laid on top of each other to provide nesting for insects and perches for birds. Stacked to no more than 350mm high.
Clean/cleaned bricks – full and half bricks dumped in a pile to provide habitat for spiders and insects. Stacked to no more than 350mm high.
Stones – use whatever the local stone cobbles or rocks to create mounds for insects to live in and birds to peck through. The stones should be no more than the size of a fist or in the case of flat stone about the size of a plate. Stacked to no more than 350mm high.
Sand – ordinary sharp sand or builders ballast can be used to create either sand beds or sand mounds for species of insect and especially bees which nest in sand. Mounds should not be higher than the top of the nearest upstand.
The greening or vegetating of roofs designed for wildlife is dependant on the nature of the habitats that are aiming to be created. Many wildlife roof designers have opted to allow self-colonisation to occur on the whole roof or part of the roof, thereby, allowing a more natural and locally appropriate habitat to develop.
With the exception of those grown specifically for the purpose, pre-grown vegetation mats are normally unsuitable for use on roofs where wildlife and biodiversity is the primary objective. This is because the small number of species used limits diversity and the mats limit colonisation by plants and be of limited value for many invertebrates, especially those that burrow into the soil.