Water retention extensive green roof

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Acoperiș verde extensiv cu retenție de apă

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.

Extensive green roofs

Sustainable solutions for any climate

Extensive green roofs are a sustainable choice for buildings in any climate zone — whether in cold northern regions, areas with heavy rainfall, or locations with prolonged dry summers. These systems are designed to support biodiversity, including hardy species such as mosses, flowers, perennial grasses, and sedum, capable of surviving in extreme environments.

A key aspect of a living roof is its ability to retain water within the substrate, ensuring plant survival during drought periods and reducing the load on the drainage network during heavy rainfall. Moreover, the constant humidity maintained by the system promotes greater plant diversity, enhancing ecological value and contributing to the natural cooling of the roof, thereby reducing the urban heat island effect.

Advanced green roof systems with water retention

Green roof systems with increased water retention are specially designed to maximize rainwater absorption and can be installed on any type of building, regardless of climate conditions.

Professional services for green roofs

Personalized consultancy for the design and implementation of green roofs and green terraces
Turnkey execution and long-term maintenance for both extensive and intensive green roofs
Use of durable construction materials and specially cultivated vegetation rolls for all types of green roofs

Available green roof types

Ultra-light green roofs or light green roofs for buildings with low load-bearing capacity
Intensive green roofs for parks, relaxation areas, and leisure zones
Paved terraces for pedestrian or vehicle use
Green roofs with solar panels for increased energy efficiency
Vertical gardens for interior or exterior walls
Green terraces for offices and commercial centers