In the 18th of a series of articles charting the Wild Wanstead project, green roof expert Chris Bridgman explains how to turn your extension roof into a wildlife-friendly garden in the sky
Living green roofs can be installed on most flat or gently inclining roofs. For an average residential extension, the green roof system is just loose laid directly on top of the roofing material as an extra layer. This can be done when building a new extension or retrofitted onto an existing roof.
The green roof system is made up of three main components:
A root barrier that stops the roots of the plants getting through and damaging your waterproof layer (most new roofing felts are already root-proof so this may not be needed).
A drainage layer, which stores water but also lets any rain seeping through the vegetation run off the roof and into the gutters in the usual way.
A lightweight growing medium for the plants – small particles of porous material and organic matter, which takes the place of soil.
Plants are grown on top with a small gravel border around the perimeter. For low-maintenance options, roofs are usually planted up with sedum, meadow wild flowers or a mix of both – but you could add other features like a log pile too. The green roof system for growing sedum is approximately 6cm deep, increasing to 12cm if you want wild flowers (and more if you want a turf roof). The roof will need a bit of attention twice a year – a feed for the sedum in spring and cutting or removing the dead wild flowers, plus checking for any weeds that have snuck in during autumn.
Because adding a green system is an extra layer over your regular roofing material, protecting it from the elements, it is thought that the life of your roof could be extended by as much as three times. The green roof has a cooling effect in hot weather (free air conditioning) and an insulating effect in winter, which can reduce bills. Other benefits of green roofs include restoring gardens lost to development, providing a great-looking design feature and ensuring upstairs rooms have a nice view, supporting biodiversity, mopping up carbon and other emissions, and preventing flooding (because the vegetation absorbs some of the rain).
If you’re interested in installing one, here are the typical steps:
- Touch base with the council about your plans. It is probably easiest to use the pre-application advice from Redbridge Planning Service, which costs £50 for a 30-minute meeting. Redbridge currently doesn’t have a standard policy on green roofs, but a request to install one has never been rejected.
- Pick a suitably qualified green roof installer. I recommend using one that installs the Optigreen system. Always look for a supplier that is a member of the Green Roof Organisation.
- Speak to the supplier. Describe what you want to do and discuss which of their roofing systems would best meet your needs. Find out the weight per square metre of the different options.
- Schedule a structural engineer to visit your property to assess your extension roof and confirm the weight it will be able to take. Pick someone who is familiar with green roofs – your supplier should be able to recommend someone. It will typically cost around £350 for the survey. If you’ve got the technical plans for your extension, that will help the engineer, but this isn’t essential.
- Based on feedback from the structural engineer, pick an appropriate green roof system and liaise with the supplier to book them to visit your property to discuss the roof and its installation. For a good quality system, expect to pay around £80 to £90 per square meter (plus VAT). This would cover all the materials and the cost of installation (based on a minimum of 10 square meters).
- One final thing you need to do before your green system is installed is ensure your roof is completely watertight. If you’re putting it on top of an existing roof, you could either get new felt laid on top to be sure or pay a roofing company to do an integrity test to check for any leaks.
- Once this is done, you’re ready to create your garden in the sky. For a typical single-storey extension, the roof will be accessed by ladder and materials brought to it in sacks.
- If your roof is installed during a dry period, you will need to water at first to help it establish.
- The supplier should provide you with all the information you need about maintaining your roof. If your roof is safe for you to access, you could do this yourself. If not, or you’d prefer to outsource the job, most companies offer a management service.