Green roofs can be added to almost any type of roof including flat, saddle roofs, pitched roofs and barrel roofs. Installing it is more complex than you might expect. Much depends on the location involved. DIY installations are possible for small locations such as a shed, pergola, bird house or dog kennel but anything larger, especially if structural changes are involved, will need to be undertaken by a recognised contractor who can provide the essential warranties required by insurers and mortgage lenders. Mistakes or errors involving a roof or a major structure can prove expensive.
Planning Permission may also be required. Much depends on the type and size of building, as well as the proposed type of garden. Domestic settings involving roof gardens may impact on the privacy of adjacent householders while planning permission involving a change of structure for large buildings is mandatory. It is also important to check whether the local authority involved has any specific requirements regarding environment and biodiversity to be included in the final scheme.
Costs
This depends entirely on the type of roof and materials chosen. Sedum matting is the most common since it just has to be unrolled and can be installed fairly quickly.
Design
Take time to consider the style involved – sedum, wildflower, clover, turf, intensive with trees and shrubs, roof patios. When designing the roof, it is essential to consider drainage methods. The planting will take up some rainwater, but there will be a need for surplus water to drain away. Can this extra water be captured for use, perhaps via underground tanks for use in watering the garden during the summertime?
Access and maintenance
Equally important is the question of access and long term maintenance. You need to work out how to get the green roofing materials onto the roof. Will ladders or scaffolding be required? The quantity of membranes, planting materials, sedum matting can weigh heavily when you are moving them from ground level to roof level. Even if it is just a small project putting matting on top of a summerhouse or birdhouse, you will need several pairs of hands to help secure ladders, and hold materials in place while they being firmly fixed on. Electrical points may be required if drills or other mechanical equipment is being used either to construct the green roof or maintain it afterwards.
Once installed, a green roof will need maintenance from time to time. This means going up to the roof and checking condition, replacing plants when necessary and removing leaves in late autumn. During times of drought, some watering may be necessary. Turf and wildflower roofs may require mowing once or twice a year – think about how to get a lawnmower up onto the roof and operate it safely!
Load bearing
It is not just a matter of putting on a waterproof layer and adding some growing materials. Installing a green roof requires a variety of materials, all of which add considerably to the overall weight on the roof. Quite apart from the weight of the materials, heavy rain can add significantly to the overall weight. It is crucial to ensure that the roof is capable of bearing the required saturated weight of the type of green roof being installed.
Typical load bearing figures for a fully saturated roof are approximately:
Gravel 90-150 kg per square metre
Paving slabs 160-220 kg per square metre
Green roof with sedum & substrate 80-150 kg per square metre
Green roof with plants, seating 200 – 10000+ kg per square metre
Even small structures and sheds may need walls reinforcing in order to cope with the extra weight.
The components
The same basic layer components are required for every type of green roof, no matter what size or design. These comprise:
Original roof base
Waterproof membrane
Root protection membrane
Drainage membrane
Growing medium
Planting & vegetation
Long term access
Small areas can usually be retrofitted, but the building structure will need to be strengthened in order hold to extra weight. Such retrofits usually involve a framework being built around the roof to hold materials in place with weight bearing struts underneath.
Suitable waterproof membranes are an obvious priority. These are usually made out of bituminous fabrics, butyl rubber or plastic. A leak test should be undertaken as soon as the membranes have put in place. Create a temporary dam at one end of the roof, and cover any drainage holes. Then fill the surface with water and leave for at least 24 hours. Check for any leaks and deal with them, double checking for leaks through another 24 hours before continuing the installation. It is much easier to deal with leaks at this stage rather than when the entire roof has been completed.
Root membranes are placed on top of the waterproof layer. These prevent the waterproof layer degrading due to UV light while ensuring that plant roots do not penetrate through to the waterproof layer.
The growing medium depends on the type of green roof being planned as soil depths can vary between 200 to 2,000 mm. A shallow depth of around 200mm is generally acceptable for sedums, mosses and wildflowers. Planting shrubs and small trees will require 500 mm plus. Add roof edges slightly higher than the growing medium in order to keep all the green roof materials in place.
Irrigation systems such as porous pipes should be added at this point. This will allow the plants to be watered when necessary, as well as ensuring surplus water drains away into the gutters and downpipes.
Keep all turf, wildflower or sedum mats rolled up until the moment of installation. This will help keep them moist and the plants in good condition. Most large roofs will involve the matting being rolled out and fixed into place. Smaller areas such as the top of a pergola or bird house can often be filled with a selection of plug plants.
Angela Youngman