The maintenance of green roofs is perceived to be one of the greatest barriers to their installation. The maintenance schedule should be considered during the design process, when the demand for maintenance input can be anticipated. There is no such thing as a 'no' maintenance roof, green or otherwise. All new commercial buildings are required to have roof and gutter checks twice a year and an extensive green roof can be designed to align with this requirement. The maintenance required will depend on the outcome desired by the client; this may range from weekly checks during summer on an intensive roof garden, to quarterly or even twice yearly checks on the most extensive green roofs. However, it is strongly advised to include the cost of the post installation maintenance programme within the budget and tender documents.
Biodiverse roofs and those designed to be low-maintenance will still require visits once or twice a year to clear gutters and drains and remove any unwanted debris or litter. Extensive sedum roofs may require a more intensive maintenance regime, with some manufacturers recommending weeding three times a year and application of fertiliser once a year. Semi intensive roofs (unless a biodiverse type) and intensive green roofs designed with aesthetic appeal or the functionality of a roof garden, will necessarily dictate a more intensive maintenance regime, as would be required in most gardens. Maintenance usually occurs in late winter or early spring. Where breeding birds are expected such works should be undertaken before the nesting season begins. Roofs near to deciduous trees also need maintaining in late autumn to clear leaves.
b) Extensive <100mm Low Nutrition Substrate
•Drainage outlets and inspection chambers cleared of vegetation, as with any roof. •Ensure outlets and shingle perimeters clear of dead and live plants. •Generally they are allowed to develop into dry meadows.
c) Semi Extensive - 100mm to 200mm low to medium nutrition substrate
•Drainage outlets and inspection chambers cleared of vegetation. •Ensure outlets and shingle perimeters clear of dead and live plants. •Removal of undesirable vegetation 6 monthly.
d) Intensive - 200mm + medium nutrition substrates and top soils
•Drainage outlets and inspection chambers cleared of vegetation. •Ensure outlets and shingle perimeters clear of dead and live plants. •Intensive care of lawns, hedges etc. •Replacement of failed plants exceeding 5% of plants installed. •Replenishment of any areas of settled substrate.
e) Biodiverse - very low to low nutrition
•Drainage outlets and inspection chambers cleared of vegetation. •Ensure outlets and shingle perimeters clear of dead and live plants. •Low vegetation is common and none is general undesirable.
f) Post Installation
Many contractors will include, or at least suggest and offer, an agreed period of regular maintenance. This is especially important whilst the roof is establishing. Some extensive green roof system suppliers will have their own approved contractors who will monitor progress immediately following installation and after the first full growing season (usually between 12-18 months) before finally signing over maintenance duties to the building manager. There are an increasing number of maintenance contractors who have received specialist training in green roof care from organisations such as GRO (The Green Roof Organisation) and BALI (British Association of Landscape Industries)
a) Undesirable Plants
All efforts should be made to prevent drains becoming blocked and the growth of unintentional vegetation which could be detrimental to the intentional planting regime, biodiversity aims and the building fabric (Buddleia, for example, should be removed while still immature).
Sedums are not very competitive plants and after a warm and wet summer, areas of weeds can appear on an extensive roof. The weeds can be cleared. And bare areas left to naturally regenerate with sedum or other naturally colonising vegetation such as grasses. The sedum can then be seen merely as a successional stage that will lead to a more biodiverse roof.
An alternative is to place cuttings of sedums on bare areas, cover with substrate and then water. After 3-4 weeks, the cuttings will become established. This should be carried out during spring or autumn to ensure climatic extremes such as frosts or hot sun will not inhibit growth.
Access to a water point is essential during the establishment stage of most green roofs. However, on an extensive green roof a permanent irrigation system is not necessary. On intensive roofs such as lawns in rain shadowed locations this may be required. The deeper the substrate used the more moisture it can hold to ensure plant survival during dry periods.
The preferred option is not to fertilise extensive green roofs, as species diversity may be reduced and the use of fertiliser will result in increased nutrient levels in storm-water runoff which will negatively affect local water quality. Where fertiliser is required to maintain the health of particular plant species, fertiliser application should be kept to a minimum and should be in accordance with the advice of the supplier. Where rainwater is harvested from a green roof, fertiliser should not be applied.
d) Fire Breaks
Vegetation breaks/barriers have an important safety function and prevent the spread of fire. All vegetation barriers at up-stands, roof penetrations and fire breaks must be maintained at their original width and cleared of any encroaching plants.
e) Drain Heads and Outlets
All drainage points must be checked every year and cleared out if necessary to ensure optimum performance. Excess water must be able to leave the roof, to avoid ponding and overloading.
f) Health and Safety During Maintenance
Where maintenance will be undertaken within 2m of the edge of a green roof, fall protection must be provided. It is important that fall protection systems are themselves maintained once a year. There are examples of green roofs on low-rise buildings that are now being designed so that the complete roof can be accessed from a cherry picker to reduce the risk of accidents.
a) Build-Up Awareness Ensure anyone working on the roof is briefed as to the build-up of layered components. b) Tools Tools must be carefully chosen so as not to interfere or damage anything below the substrate. c) Programme Works should be programmed in order to minimise the amount of traffic across the green roof after installation. Repeated walking on a limited area of green roof will result in substrate compression and damage to vegetation.