Installation green roofs
Making the right choice
Making the right choice depends ont three factors: physical, monetary ones and service-thechnical issues.
We can help you answering the following key questions.
- How much extra weight can youor roof structure support?
- How much should you expect to spend for a top-quality green roof?
- How do you maintain the green roof of your dreams?
Once you have the answers to these basic questions, we can address the more specific questions:
-
The roof structure permitting, should the green roof be accessible to the occupiers?
The reply to this question will determine the overall solution. Think about it carefully, as it is a fundamental choice
-
Which plants are most suited?
The roof structure and the maintenance level is required will contribute to determining which plants to use, e.g. sedums are very maintenance-friendly requiring only 1-2 maintenance visits a year. Perrenials and lawns, however, require a higher degree of maintenance in terms of fertilization, irrigation and pruning.
We advise you not to use plants with an aggressive root system (e.g. bamboo) on a green roof. It is better to go for plants which grow slowly. Tall trees catch a lot of wind, deciduous plants slow down the growth of the vegetation ubderneath, the fallen leaves may also block the water drains. All these factors need to be considered.
-
What roof pitch or slope is required?
Flat roofs require a fall or pitch of at least 2%. Sloping roofs can be covered with vegetation up to an inclination of 35° when they are facing north, west or east and up to 20° when they aref facing south.
-
What is the best orientation and location for your green roof?
If you want your green roof to develop ideally, it should have at least three hours of sunshine on a Summer’s day. The shade of surrounding buildings and the reflection of glass can have an adverse impact on the vegetation’s growth, as does wind.
The vegetation most commonly used consists of sedums and mosses. We use varieties of sedums which have been specifically grown for the use on roofs, which theFLL has selected with the greatest care.
Do you have any specific requirements, such as ornamental grass, shrubberies or even trees?
Simply ask us : De Boer Green Solutions has a wealth of experience and we like to be challenged to provide a creative solution.
back to top
Methods and techniques
Whether you are laying an ordinary garden or a green roof, the methods and techniques remain the same.
The most obvious are:
Spontaneous or natural colonization
The cheapest way to develop a planting layer is simply to allow itto grow spontaneously. The disavdantage to this is that it takes a long time for it to develop and the result cannot be predicted. This technique is mainly chosen when developing ‘biodiversity roofs’. The vegetation usuallu consists of various types of plants spread by the wind, for example lichens, mosses, wild flowers, weeds, herbs, etc.
Planting
On intensive green roofs, plants are used so that a ‘mature’ garden is obtained faster. This method is more expensive and consequently used less on extensive green roofs. The development period for sedum and herb roofs will quickly take two years and the level of coverage depends on the number of plants. Compared to other techniques, this one gives the most beautiful results.
You need 15 to 20 units per square meter. The time frame is 5-7 min/m². The best periods are Spring to Autum and the ideal months are May to June
Sowing
This technique is sucessful particulary on simple intensive (grasses and herbs) and extensive green roofs. It is less suitable for real intensive green roofs. To obtain the desired coverage of 95% takes about two to three years from te sowing date.
Sowing is the cheapest mthod after spontaneous growth. It can be realised in the following ways :
- Dry sowing :
- manual/machine application
- only possible with heavier seeds (risk of erosion of lighter seeds: can be washed or blown away)
- Wet sowing (hydroseeding) :
- application with a spray pistol and compressor
- blending the adhesive (cellulose or artificial) with reduced risk of erosion
- more even spread
- faster
- larger areas
- Shoots/cuttings
- multiplication by using living shoots from plants
- cuttings are sown in
- limited storage life after cutting
- development period is the same as sowing
- cheaper than plants
- amount ±50 g/m²
- time : 3-5 min/m²
- period : April – June & September – October
Vegetation mats
Vegetation mats are pre-cultivated mats made from plants (moss-sedum/sedum-herbs/herbs-grass), substrate and a network of artificial or coconut threads.
They ensure a fast result and due to their three-dimensional structure, function as erosion protection, for example on sloping roofs.
However, this solution is the most expensive of all the techniques
The ideal period for applying vegetation depends on the technique :
The most suitable period for planting differs depending on the technique :
- sowing: between the end of April and the end of September
- planting: between March and October
- Vegetation mats : all year round
Of course, the weather conditions also play a role (heat, dryness). In July and August, it may be necessary to provide extra protection for the planting due to the heat and drought.
The life expentancy of a green roof depends on a good design and correct structure plus the quality of the material selected and the essential maintenance. Installation times vary around the world. Check all options and ask a local specialist for advice.
back to top
installation of a greenroof
The installation of a green roof takes place over a number of stages. A brief overvieuw follows below
1. Preparation
During this phase, a number of preparatory works are carried out on the root resistant waterproofing already installed
Note: the root resistant membrane must be applied vertically to cover the substrate level. Once the waterproof/root resistant membrane has been applied, this area can no longer be used a work floor.
The following activities are included in this preparatory phase:
- cleaning the entire roof area
- visiually inspecting the membrane
- checking infiltrations and carrying out a water test (24 hrs)
- repairing any faults
- signing the hand over document for the waterproofing works
- applying extra prottection in the event that heavy loads are expected (pallets, etc.)
- indicating drains and expansion joints
- installing inspection chambres above the drains
- always take account of safety measures during executions
2. laying the drainage & filter layer
The drainage must be laid as a single continuous layer. Depending on the type of drainage, the seams are placed overlapping, wedged tight or simply placed next to each other.
This layer must be cut to size around any breaks in the roof and fitted as well as possible. The drainage mat is cut off horizontally and consequently not put down vertically.
3. Laying paving and/or gravel
Any paving (for paths) and gravel (for vegetation free areas around roof penetrations and roof edges) must be laid before the substrate is put in place. If continuous irrigation is provided, this must also be installed.
4. Laying the substrate
The substrate can be laid in various ways :
- in bags (20-25 kg)
- in big bags (1m³)
- in bulk (25 m³)
It may be dilivered to the roof via crane or pumping installation
The packaging and method of delivery depends on the size and eccessibility on the roof.
- 4 cm substrate = 2 bags/m²
- 6 cm substrate = 3 bags/m²
- 4 cm substrate = 25 m²/bigbag
- 6 cm substrate = 16.5 m²/bigbag
With a roof area in excess of 500m² it becomes more cost effective to deliver the substrate in bulk and to install it using compressor and pump.
Note: to prevent the drainage laye lifting up in windy conditions, the substrate should be installed at the same time as the drainage mat is laid.
5. Planting the vegetation
The vegetation is planted during this phase.
Vegetation-free zones
We recommend to have vegetation-free zones (width 300mm) around the roof details (roof penetrations, rainwater drain-pipes, ventillation, façades, eaves, ...).
A vegetation-free zone can be made with gravel and/or with tiles.
There are several reasons for such vegetation-free zones :
- draining capacity must be as high as possible in the vicinity of the rainwater drain-pipes. Washed and rolled gravel provides precisely that.
- There will be a better resistance to wind pressure near the edges and in the corners
- It is easier to check and, if needed, to repair the roof-system’s most critical zones.
- Using gravel strips makes it easier to reach the required elevation of 150mm
- The vegetation-free zones can also serve as passages or escape-ways
- There is an increased protection against splashing water and the pollution which goes with it.
- Increased fire-security
Elevation
the elevation to the finished roof-garden must be at least 150mm
in orde to limit the risk of damages, the roof details should ideally be square
Rainwater drainpipes
rainwater drainpipes should be accessible at all times. Measures must be taken to avoid obstruction :
- extensive and simple intensive roof gardens: through the combination of a (spherical) grid with a vegetation-free zone
- intensive roof-gardens: through an inspection-chamber system on top of the rainwater drainage
- we recommend not to use gravity-induced vacuum systems such as GEBERIT. These systems are prone to pollution which increases the risk with roof gardens. These systems do not function properly with roof gardens: as roof gardens slow down the removal of rainwater, no vacuum is formed in the draining pipes.
Footpaths should be placed above the drainage.
back to top
previous: Built up next: Maintenance