Green Up Your Roof!
Posted on 30/04/2014
Roofs – flat and uninteresting on the shed, garage or extension (and out of sight, out of mind above the house)....until they need repairing or replacing. This is when the financial costs of roof felt, timber and insulation rear their ugly heads, along with questions about their environmental impacts. So, which option to choose: felt and bitumen? Rubber? Or perhaps a green roof, which can be sustainable, longer-lasting and perhaps save maintenance and energy costs over time?
So, what is a ‘green roof’?
Also known as a ‘living roof’, ‘turf roof’, ‘biodiversity roof’ and several other terms including the wonderful ‘sod roof’,a ‘green roof’ simply means a roof under vegetation. More accurately, according to the Green Roof Centre at the University of Sheffield, 'Green roofs are vegetated layers that sit on top of the conventional waterproofed roof surfaces of a building. ... usually a distinction is made between extensive, intensive and biodiverse or wildlife roofs'.
This means in practice that some roofs need more maintenance than others, but costs and benefits also vary. An intensive green roof will need more soil and thus a stronger base, but could support more plants, including trees. For most purposes an extensive or semi-extensive green roof requiring less support and maintenance may be more interesting. This would be built of 'free-draining material that support low-growing, hardy, drought-tolerant vegetation' resistant to the exposed conditions (wind, rain, heat) found there. A wildlife roof would be designed to encourage a particular habitat, for example, bee friendly. Green roofs can be built on sloping roofs, although they seem to be most common on flat or gently sloping roofs, and an intensive green roof is probably out of the question on any steeply pitched roof.
What are the benefits of green roofs?
A key benefit is improved insulation and therefore reduced energy use. It’s worth noting, however, that the roof’s thermal properties might be impacted by its water holding capacity, so in a damp winter the improvements may be less noticeable. A further important benefit of green roofs is reduced rainwater run-off thanks to soil absorption (how much depends on the roof type). This can help reduce flooding during extreme weather, when existing water management systems may be overwhelmed. It is also possible for recycled household water to irrigate a green roof in hot and dry spells.
Additional benefits include ‘the mitigation of local air pollutants [including Nitrous Oxide absorption] and the extension of the useful life of the roof membrane', says Simon Rawlinson; a green roof over a waterproof membrane could increase its life-span by 2-3 times (estimates vary).
Benefits to wildlife could include creating habitats for bees as well as targeted habitats for endangered species, notably birds or wildflowers.
What are the costs of having a green roof?
The initial outlay will be more than a conventional roof, but should save money over time. For an installed roof, Ecolocker suggest 'As a very rough indication, in the United Kingdom one can expect to pay around £100 per meter squared of extensive green roof, and around £150 per meter squared for the intensive variety'. Others suggest as little as £20-50 per square metre including normal roofing costs (which on a shed are around £30 for a rubber roof). This variation may be due to rarity in the UK and because to date, many have been installed on commercial rather than private buildings.
The Scottish Green Roof Forum points to the following cost benefits: 'Evidence ... has indicated that savings in energy made from green roof installations has provided payback times in the region of three-to-five years. ... German studies have shown 2 litres/year of fuel is saved for every square metre of green roof' . They also highlight that 'energy savings have been shown to amount to 2.5kWh/m2/yr'. Savings should also result from an increased lifespan of the roof itself, and a green roof could increase the property value as green issues and energy efficiency become more important.
Costs will of course be influenced by whether the roof is purpose built or DIY (The BBC website has a very simple shed roof design). A competent or even semi-competent DIYer might be able to do a lot themselves and there are plenty of kits and suppliers out there, from affordable sedum kits that can be retrofitted onto an existing waterproof roof in good condition and able to bear some additional load, to wildflower roof kits.
There are many installers, although allow plenty of time for evaluation and quotes.
In addition to the initial installation costs, there will be maintenance, much of which you should be able to do yourself. Notably, more intensive gardens will need trimming, weeding and watering, and the drainage needs regular maintaining and cleaning. Living Roofs estimates about £1 per square meter for an extensive roof for additional maintenance costs. There are no estimates of intensive costs but think about weeding, cleaning, maintaining soils, replacing plants, and so on.
If you’re seriously thinking about creating a green roof (and we’re delighted if you are!), it’s worth having a look at the green roof code, as well as visiting the Green Roof Centre site, which features both commercial and DIY case studies.
And finally… planning permission may or may not be required (notably for new build), but it is worth checking with your local council planning department.
Written by Simon Ferrigno, Consultant in Sustainable & Organic Farm Systems
Sources and selected further reading
- Green Roof Guide: www.greenroofguide.co.uk
- Landmark Living Roofs: http://www.landmarklivingroofs.co.uk/
- Green roof courses are run by the Green Roof Centre and also in Brighton: http://www.brightonpermaculture.org.uk/courses-conferences-and-events/courses/green-roofs.html
- Blackdown: http://www.blackdown.co.uk/green-roofs/
- Nottingham Trent University http://construction.ntu.ac.uk/staffwebs/greenroofs/aboutGRandESB.htm
- The Urban Wild Project: http://theurbanwildproject.org/projects.html
- Building.co.uk: http://www.building.co.uk/data/sustainability-green-roofs/3069718.article
- Willmott Dixon: http://www.willmottdixongroup.co.uk/assets/b/r/briefing-note-2-green-roofs-ad-approved-270110.pdf