- Green Roofs

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The aim of a roof is to protect us from the effect of the weather. An ordinary roof fulfils those requirements perfectly. But other ways are also possible, for example by choosing a ‘green roof’. On a green roof, the coverage mainly consists of living plants. In fact, the name “green roof” is a little misleading, after all, this kind of roof does not really have to be green. And nor do the plants growing on it. For example, in the United Kingdom one talks of ‘brown roofs’.

Naturally, plants solely as a roof covering are not sufficient. The basis used for the plants is crusially important. A correct basis with a suitable structure, a suitable substrate and the correct draining facilities are needed.

In fact, a green roof could also be realised in an entirely natural way. The only problem is that nature takes its time about it. In practice, it will easily takes ten- to –twenty years before you get the right effect. If you would like to hellp nature along, choose the landscaped variety of green roofs.

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  • The aim of a roof is to protect the building from the effect of the weather, especially the humidity.
  • An ordinary roof fulfils those requirements perfectly. But other ways are also possible, for example by choosing a ‘green roof’.
  • A green roof looks attractive. This visual aspect has a positive effect on the mood and physical health of people.

On a green roof, the coverage mainly consists of living plants. In fact, the name “green roof” is a little misleading, after all, this kind of roof does not really have to be green. And nor do the plants growing on it. For example, in the United Kingdom one talks of ‘brown roofs’.

Naturally, plants solely as a roof covering are not sufficient. The basis used for the plants is crusially important. A correct basis with a suitable structure, a suitable substrate and the correct draining facilities are needed.

In fact, a green roof could also be realised in an entirely natural way. The only problem is that nature takes its time about it. In practice, it will easily takes ten- to –twenty years before you get the right effect. If you would like to hellp nature along, choose the landscaped variety of green roofs.

There are two big categories of green roofs. The German FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V. or Landscape and Landscaping Development Research Society) distinguishes green roofs based on the required level of maintenance:

  • intensive greening: creates a roof garden which is comparable to a garden at ground level, both in use and in the required maintenance;
  • extensive greening: the green roof is low-maintenance but access is limited or even ruled out

In between these two categories, there is also semi-intensive greening.

One cannot say that all green roofs are roof gardens. The opposite does hold true: all roof gardens are green roofs. A “roof garden” is a green roof which has the characteristics of a garden, but which is situated on a roof.

Why opt for a green roof?

  • a green roof is so much prettier than a  conventional roof;
  • environmentally-friendly: green roofs reduce the pressure on the sewage system, they reduce the risk of flooding, they have a cooling effect, etc.;
  • economical: investing in a green roof will help you save money. Not only will the waterproofing system on your roof last twice as long, you will also benefit from government grants.

Several factors influence the quality and the lifespan of a green roof. For instance, if certain components are wrongly selected, dominating plants may start overgrowing the green roof.

Climate conditions can be important, too. Sedum requires at least three hours of sunlight during the Summer. When periods of drought last for more than 5-6 weeks, irrigation is needed.

Other species thrive better in shadowy areas or can withstand longer periods of drought.

Design and maintenance require attention, as well. Do bear in mind the following:

  • choose the right substrate. A wrong choice of substrate, or the wrong composition, increases the likelihood of undesired vegetation developing. The share of organic matter must be adapted to the vegetation;
  • determine the water-retention capacity needed for the vegetation. Too high a water-retention may prove disastrous as undesired vegetation would develop lavishly;
  • execution of roof details. You would want to minimise the risk of (unwanted) vegetation damaging roof details.

As soon as the vegetation has been put on the roof, you need to fertilise it using an organic fertiliser, and you need to irrigate it adequately.

Whatever the type, all green roofs require maintenance.

Extensive green roofs: low maintenance

Contrary to other types of green roofs, extensive green roofs require only little maintenance. However, that does not mean that no maintenance is required at all. During the first year, we recommend to control extensive greening each quarter. From the second year onwards, one yearly inspection should suffice. In the first two years, maintenance can take place in Spring and in Autumn. Afterwards, one maintenance session per year, in Spring, should be enough.

  • remove parasitic plants with roots and all;
  • replace dead plants and sow bare spots;
  • check on erosion in the substrate (on sloping roofs);
  • remove waste, such as dead leaves which have piled up;
  • in case vegetation mats would have shifted: put them back in place;
  • check all roof details.

Intensive green roofs: same level of maintenance as an ordinary garden

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Drainage

Our DBGS DRAIN 8 is a three-dimensional mono-filament structure which consists of two filter fleeces with a mat of polymeric entanglements with large cavities in between. The layers are thermally bonded to one another.

Because of its specific construction, DBGS DRAIN 8 fulfils three roles:

  • it drains any excess water;
  • it protects the underlying waterproofing membrane from mechanical damage;
  • it filters the small substrate particles, avoiding any clogging of the drains.

Water-retaining layer

In order to achieve sufficient water retention, we sometimes put water-retaining geotextile or mineral wool on top of the drain mat. Whether this is needed or not, depends on the chosen vegetation, on the maximum installation height and on the roof load.

Growing medium

  • This is the layer in which the vegetation roots. As such, it is the vital component of the green roof system. It consists of either a mineral substrate or a water-retaining layer.
  • As the plants’ nurturing ground, the substrate most not only have the right physical, chemical and biological composition, it must also have proper ventilation and water management.
  • We determine the thickness of this layer based on the intended vegetation. The larger the plants, the thicker the substrate layer will need to be.

Vegetation layer

In the case of intensive greening, there are hardly any limits: flower beds, a lawn, shrubs and even trees. Extensive greening is limited to sedum, moss and sometimes herbs.

Attention: plants which root aggressively or plants with tap-roots are not suited for green roofs.

When considering which type of green roof to install, you first of all need to consider the physical and technical characteristics of your building. The maximum installation height and the maximum load of the roof structure will determine the thickness of the substrate and, thereby, the choice of vegetation.

The latter also depends on the roof’s orientation and on the level of maintenance you are looking for.

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Wouldn’t it be wonderful to turn your roof into a real living area ?

  • This is perfectly feasible. Indeed, unlike extensive green roofs, intensive green roofs are meant to be walked upon. In fact, you can use them for various recreative purposes.
  • An intensive green roof requires maintenance similar to the maintenance of a traditional garden. Of course, the building’s design needs to cater for the thick substrate layer.
  • But, then, the “land” used for your garden comes for free. A big saving, bearing in mind the high and ever increasing price of land.
  • De Boer Green has developed two standard systems for intensive vegetation. We can easily adjust them to your project’s specific requirements.

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Semi-intensive green roofs
Although this “light roof garden” does not have a very thick substrate layer, it does offer lots of possibilities to create an aesthetic unity.
By opting for a semi-intensive system, you combine the benefits of extensive green roofs and the benefits of intensive green roofs:

  • a wider range of plants can be used (ground cover plants and ornamental grasses);
  • accessible;
  • requires little maintenance;
  • less structural demands than with intensive systems.

Regular irrigation is, of course, needed. Our range comprises species and varieties which are adapted to the specific environment of your roof:

  • ornamental grasses: Festuca spp., Penisetum spp, Imperata spp., … ;
  • ground cover: Vinca spp., Hedera spp, Pachysandra spp., …

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Extensive green roofs

These low-maintenance vegetation systems are suited for (low pitch) roofs (1-35°) with a normal exposure to the sun, rain and wind and which are seldom walked upon.

Advantages:

  • requires little maintenance;
  • limited choice of vegetation;
  • not meant to be walked upon, except for maintenance;
  • the least demanding from a building-technical perspective.

Functional characteristics

  • aesthetic: permanent vegetation, which changes with the seasons and with the sunlight;
  • florescence from May to August, depending on the species;
  • slow-growing plants, requiring little maintenance, pruning or mowing;
  • plants which resist well to drought: watering is not needed, unless just after planting or in the event of exceptional drought (longer than 6 weeks);
  • acts as a rain water buffer (depending on the system up to 34 liters per m² and 50 % of annual rainfall); in the case of heavy rain, less water goes into the sewage system, and the water that does, does so much slower;
  • protects the waterproofing membrane against UV rays, thus significantly increasing the membrane’s lifespan;
  • protects the building from the sun during Summer: this natural cooling reduces the energy consumption of the air-conditioning system;
  • retains the dust particles in the air;
  • insulates against air nose and contact noise (rain or hail);
  • enhances bio-diversity (birds, bees, butterflies,…);
  • maintenance is limited to removing unwanted vegetation, fertilising and, if needed, extra sowing. Once or twice per year, depending on how the vegetation develops.

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Why opt for a green roof?
What should motivate you to go for a green roof?

Economic Benefits

Green roofs...

  • improve water management (retention capacity, less risk of flooding,...);
  • cool (especially during Summer, your building will remain cooler; you will, therefore, save on air-conditioning costs);
  • are sound insulating;
  • extend the lifespan of the waterproofing membrane (thanks to the vegetation, the membrane is less exposed to UV rays, to the weather, etc.);
  • save space and use otherwise unused space;
  • promote recycling (good to know: most components of our green roof systems result from recycling).

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Environmental benefits

Green roofs...

  • improve the air quality;
  • the air is much purer on a green roof. That’s normal, considering that the vegetation produces extra oxygen and filters the fine dust particles;
  • reduce the warming in built-up areas;
  • have you ever heard of the “urban heat island effect”? Even if you haven’t, you will certainly have felt it: in cities, temperatures are higher and the air is dryer. That’s where the vegetation on green roofs comes in: it lowers the temperature and the evapotranspiration makes the air less dry. Indirectly, lower temperatures reduce the consumption of energy, the use of fossil fuels and the emission of CO2. In short: a better air-quality;
  • increase biodiversity in cities;
  • the more green roofs there are, the more vegetation there is and the more animal diversity there is. Green roofs bring nature back into the cities.

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Aesthetic Benefits

Green roofs...

  • make for a pretty investment;
  • add an extra floor to your building;
  • are eye-catchers which is also commercially appealing;
  • it’s clear that plants appeal more to people’s imagination than a plain grey or black roof would ever do.

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