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Water retention extensive green roof

10 min read
Water retention extensive green roof

Extensive green roofs can be found all around the world. No matter the geographical location or weather conditions, buildings can be landscaped with green vegetation. Even in severe cold conditions or far north we can admire ecological rooftops, having a green, yellow or brown moss blanket. We could go on with other examples too just to demonstrate that extensive green roofs strictly adapted to the requirements of every building, could bring really spectacular results.

There are cases when we have to consider the absolutely need of water retention.

In the areas where rainfalls are quite frequent or where the drought periods are too long, or – in most of the cases – these conditions alternate, this aspect must be very seriously considered. It could happen that in a relatively short time the evacuation system of the roof and the sewage system to be overloaded.

Such situations can be avoided, if we have a maximized retention system. The substrate and the drainage course can stock a great amount of water, without overloading the sewage network. However, in drought areas, the rainwater is retained in the substrates of the green roof, assuring the necessary moisture for plants.

There are also other types of green roofs with water retention. By evaporation, the high humidity of the structures offers a pronounced cooling effect too. We have to consider the ecological value too, because the great humidity assures better conditions for a greater variety of flowers, together with grasses, perennials, sedums and moss.

This type of extensive green roof can be arranged on flat surfaces having an inclination between 0° and 5°, but also on gently pitched roofs with 5­ -10° of inclination. Building differences are not too pronounced, but they have a very important role in the seat exploitation, so must be treated separately.

For pitched roof with 0-­5° inclination, the following structure can be applied; it’s like an extensive green roof with small changes:
  • At the base of the structure is used a membrane against roof penetration (if the waterproofing is not FLL certified).
  • The next layer is for the mechanical protection of the membrane; it is a geomembrane made of polyester fibres or a mixture of mechanically consolidated fibres, polyester and regenerated polypropylene, having the capacity of absorbing 6 l/m² of water.
  • Drainage boards looking like egg casings form the next layer. They are made of expanded polyester or polyethylene of high density (HDPE). Their height is up to 75 mm, retain water and also evacuate water toward gutters. They can retain 7­-21,5 l of water per m² and their own weight is about 0,6­2 kg/m².

If the building structure allows, it is recommended to choose the option of the greatest water retention capacity, but with heavy weight on the surface.

  • The first layer is a 1-2 mm thick polyethylene cover, forming a filtration layer which prevents clogging and filling of drainage course with fine particles.
  • The next level is the substrate composed of a mixture of minerals with a 50% of water retention capacity. This substrate contains recycled and natural minerals, split face natural tiles, perlites, pumice, volcanic clay, peat fibres, biodegradable fibres and water binding organic substances. Usually this mixture is available in granules of 1-40 mm and its thickness is about 90-200 mm, again taking into account the total weight on the surface of the structure.
  • The vegetation layer is formed of plants resistant to high temperature variations, extreme weather conditions, frost, drought, wind, acid rains etc.

Ideal plants for extensive green roofs are sedums and wild grasses, combined with other plants and flowers. These gardens can also achieve beautiful landscapes and high decorative values. There are known several methods of sowing or planting extensive vegetation, but the most effective is the one with vegetation rolls. Using these rolls, we can save money and time (see section Extensive vegetation).

Extensive green roofs with water retention built on pitched surfaces of 5°-15° inclination, can be landscaped the same way as flat surfaces, having only one important difference: the vegetation layer must be put directly over the drainage course, without using the filtration cover. This is how stability is ensured against erosion or slipping. Water retention is really important in case of pitched roofs, because their drying trend is more pronounced.

Such an extensive green roof will have a water retention capacity up to 44 l/m² and a surface weight in water saturated condition about 130 kg/m².

Our company offers consultancy and turnkey projects for green roof and terrace design, landscape and long-term maintenance in order to obtain the best results. There are special solutions to build green roofs even in sloping surfaces, regardless the materials used for the roof.

We use reliable building materials, plants especially developed for rooftop gardens and high quality vegetation rolls. Every landscape work offers the best cost/effectiveness/safety balance and every project is adapted to our customers’ aims. This can be even a retention green roof for extreme weather conditions and for any geographical position.

We are also able to landscape extensive green roofs on top of industrial halls, inside or outside vertical gardens for office buildings, and intensive green gardens with special relaxing zones, landscape green roofs with pedestrian traffic for shopping malls and many other ecological constructions.

To increase the ecological value of the building, photovoltaic panels can be also installed.
We are happy to help you with green projects, system solutions, estimates, and consultancy in the field.

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