Understanding Extensive Green Roof Maintenance

There is a common misconception that extensive green roofs, and sedum plants in particular, are always green and that from ground level they resemble grass. This is misleading, as they consist mainly of low growing, drought tolerant sedum plants and may also include other species such as Saxifrage, wild flowers, grasses, moss and herbs.

The appearance of the vegetation within an extensive green roof will change year on year, dependant upon fluctuations in the seasonal weather throughout the period. It should also be expected that more grass and moss will be present during the wetter months, because the conditions will be ideal for these species to exist, although they will tend to die off during the dry summer months, as free-draining extensive substrates will not hold sufficient moisture for them to survive.
It is another misconception that extensive green roofs are maintenance free, this is incorrect and they are best described as ‘low maintenance’ rather than ‘no maintenance’. As an example, the very lightweight sedum blanket systems contain little in the way of natural nutrient, so fertiliser must be applied annually to ensure that the plants become resistant to extremes of weather and temperature. These types of sedum blanket installations contain approximately 8-10 different plant species, some very similar in appearance to others but being more drought tolerant. Not every species incorporated will survive and the more dominant will be expected to prevail over time because they will adapt better to a particular location.
Extensive green roofs that include a substrate growing medium or ‘soil’, where the vegetation is provided either by plug plants, vegetation cuttings or seeds, will generally support a broader species mix, which can include wild flowers, grasses and herbs. An increased amount of dead vegetation will arise from this type of species mix following flowering, which will need to be cut back and removed, both to reduce the bio-mass on the roof and to encourage seed drop from the dead flower heads.
In the early spring the first signs of life returning to the vegetation within an extensive green roof are lead by any grasses present, quickly followed by a general “colouring up” of the sedum foliage, with other species following suit shortly thereafter. The growth and flowering of the individual species within the vegetation mix through the late spring and summer will be dependant upon the weather prevailing at the time, which will also determine which species will be most prominent in any given year. In the winter, sedum plants will appear to shrink back, the leaves will become smaller and turn red/brown in colour as they prepare themselves to withstand the coming winter frosts. This gives most extensive vegetation mixes a generally red/brown hue in the late autumn and winter months, which is sometimes mistaken for the plants being distressed, when in fact they are in optimum condition for the time of year.
General Maintenance
General maintenance is best carried out at least once a year, preferably during the springtime. However, certain tasks which will be dependant upon the location of the roof, such as the removal of weeds, seedlings and accumulated leaf litter from overhanging trees may also need to be done in the autumn.
To keep an extensive green roof in good condition, the following schedule may be used as a guide to plan the work required.
• Ensure that safe access can be gained to the roof and that relevant Health and Safety procedures are followed additionally always seek proof of current maintenance for any man-safe roof access systems prior to proceeding with any roof-top work.
• Remove all dead vegetation and debris from the roof surface, taking particular care to ensure that all chute outlets, gutters and downpipes are clear. Where the species mix incorporates wild flowers and grasses it is recommended that all dead vegetation is strimmed off and the waste material removed.
• Roofs in the vicinity of taller trees will need more frequent maintenance, to remove dead leaves during the spring and again in the autumn to ensure that they do not damage the roof vegetation.
• Remove the lids of all outlet inspection chambers, ensure that the rainwater outlets and downpipes are free from blockages and that water can flow freely away.
• Ensure that any protective metal flashings and termination bars are securely fixed in place.Examine all mastic sealant and mortar pointing for to the flashings for signs of degradation and advise the client of the need to repair or renew as necessary.
• Check that any promenade tiles or paving slabs are securely fixed to the roof surface and in good condition.
• Ensure that all items of equipment and plant on the roof are mounted on suitable isolated slabs and that any fixings used to secure these in place have not penetrated the waterproofing. Advise the client of any defects found so that repairs may be put in hand.
• It is recommended that a roof plan marked with co-ordinates be used to record the findings of the inspection to avoid confusion and provide an on-going record of vegetation performance, which can be reviewed year on year.
The following tasks should be carried out annually: -
1. Plant encroachment.
Any vegetation which has encroached into drainage outlets, walkways and the vegetation barriers (pebbles) should be removed. The vegetation removed may be set aside and used to repair any bare patches if required. If movement/settlement of the pebble vegetation barrier has occurred, additional washed stone pebbles similar to the existing are to be added.
2.  Monitoring the colour and rate of growth.
The colour and rate of growth of the vegetation should be reviewed to establish the health of the plants. Many factors can affect the growth and colour of the vegetation, most plants tend to be greener in wetter, mild conditions (springtime) and where the roof pitch is shallow.
• Sedums flower during the period from May to July, and you will see a mixture of colours – predominantly whites, pinks and yellows, with some purple. The foliage of some species of sedum, such as Sedum Album “Coral Carpet”, blushes red naturally during the summer and autumn, and so the vegetation can take on a more red/brown appearance. This becomes more noticeable once the plants have flowered, leaving remnants of dry brown seed heads. The best visible indication of the health of a plant is if the leaves are fleshy and contain plenty of water.
• When exposed to extreme conditions, sedum plants have a tendency to turn a deep red colour, which is a natural occurrence and is important to help the plant to acclimatize, ready to survive a cold winter or hot summer.
• If sedums are showing signs of distress, but have received regular rainfall, then the most likely problem is a lack of nutrient and a fertiliser should be applied.
• Only a relatively few species of sedum and other plants suitable for an extensive green roof installation will persist in partial and full shade, and they will generally be greener in colour and grow “leggier” in these locations. There will be a significant variance in the growth and colour between the plants growing in full or partial shade and those in full sun and this should be recognised as a feature of the living nature of each individual roof.
3. Weeding.
With the exception of saplings, which should always be removed, weeds in an extensive green roof should only be considered as a problem of aesthetics. If considered excessive, they can be removed either manually or by using a ‘spot weed wipe’, ensuring that care is taken to follow specific instructions regarding the use of any proprietary products. After the removal of weeds and saplings, treat the affected area as if it were a bare patch. All extensive green roof installations will at times include some moss and grass.
Repairing Bare Patches.
Bare patches in extensive green roofs can be easily repaired. Take vegetation cuttings from surrounding areas of abundant growth and place on bare patches, pressing gently into the soil. A light sprinkling of sand mixed with compost should then be dressed over the affected area to improve the uptake of the cuttings. The best results will be achieved if this work is carried out during spring maintenance and the affected area is kept moist for a short period afterwards.
In areas of extreme exposure, or where localised wind-swirl is caused by adjacent structures, it is possible that both the vegetation and substrate will be disturbed by periods of high wind. Should this occur, consideration should be given to how best to secure the installation against similar conditions in the future prior to re-instatement.
5. Fertilisation of very lightweight sedum blanket systems
The very lightweight sedum blanket systems are grown into a shallow growing medium incorporated into the blanket and which contains very little nutrient, so the annual application of fertiliser is crucial to ensure that the plants remain healthy. Fertiliser should ideally be applied during March/April, as it helps the plants to prepare for extreme weather conditions and flowering whilst also allowing the different species to gain sufficient nutrients without competing against each other. An organic fertilizer should be used at the rate of 80gm/m², unless otherwise advised on the packaging or instructions supplied with the product. Areas of up to 30m² may be applied using either a hand held spreader or strewn by hand from a bucket. Larger roofs should always be done using a trolley applicator. It is recommended that the fertiliser is lightly ‘watered in’ immediately after application, to avoid “burning” of the foliage, which may occur if fertilizer pellets settle on the leaves. Dung-based organic fertilizers should be avoided.
6. Fertiliser for either plug planted or hydro-planted extensive green roofs
Use a 6-month slow release chemical fertiliser with an NPK ratio of 15:9:14. Areas of up to 30m² may be applied using either a hand held spreader or strewn by hand from a bucket. Larger roofs should always be done using a trolley applicator, which can be purchased direct from Bauder. Always apply the fertiliser at the given rate written on the bag. This type of product may also be used on sedum blankets.
7. Irrigation
Extensive Substrate Installations
It is generally not considered necessary to irrigate traditional substrate green roof systems, although it is always advisable to ensure that there is a water supply point adjacent to the green roof, both to assist with general maintenance and as a precaution against extreme drought conditions.
Very Lightweight Sedum Blanket Systems
Irrigation systems are sometimes incorporated into very lightweight sedum blanket installations to support the vegetation through periods of prolonged dry weather. These may be found where one or more of the following conditions apply.
• South-facing roof slopes exceeding a 5° pitch.
• Roof slopes exceeding a 10° pitch.
• Exceptionally windy and exposed site locations.
• Sites up to 50 miles inland of the east coast of the UK mainland.
Irrigation should only be activated during prolonged periods of hot, dry weather, or if the sedum plants are showing signs of distress. The irrigation system is best activated for 2-
3 hours at a time, preferably at dawn or dusk to minimize unnecessary evaporation, then once every 4-6 days for the duration of the hot weather conditions, which can be easily managed by using a battery-powered, programmable timer. It should be noted that continuous daily watering is not necessary and will onlypromote weeds and other unwanted plant species.
Whilst every client’s view on what is wanted from their green roof will vary, the basic principal of investing a very small amount in maintenance on a regular basis, to both control lifetime costs and maintain it’s performance, remain the same. So get it right from the outset, when setting out to specify an extensive green roof make sure that it’s maintenance requirement is recognised by the client and included as part of the running costs of the building.