Green Roof

Landscape with LECA

Leca® Lightweight Expanded Clay Aggregate has been an important ingredient for civil engineers throughout the world in providing the perfect material to solve a variety of engineering issues when developing Green Roofs within a city landscape.

In general, Leca® Lightweight Expanded Clay Aggregate is used to reduce, compensate or prevent settlements, to improve stability of a fill structure or landscape or to decrease soil pressure on a retaining structure produced by backfill. In the same applications, Leca® Lightweight Expanded Clay Aggregate can be used both as drainage intensifying and as moisture retaining for vegetation and also as a frost protection layer.

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Why choose a Green Roof?

The use of rooftops as gardens means that we can create more green areas through property development, so that areas that have not previously been used can get new uses. The result is a direct improvement in the places where we live and work, and thus an improvement in our well-being.

Landscape with LECA
Habitat and biodiversity
Landscape with LECA
Green roofs cool down the surroundings
Landscape with LECA
Building physics
Landscape with LECA
Water management

Habitat and biodiversity:  The environmental advantage of a green roof is, among other things, storage and return of rainwater to the natural water cycle, higher humidity, dust control, improvement of the microclimate and the creation of a habitat for plants and animals.

Building physics:  Protection of the structure that lies under the roof is an important aspect for the architect or owner of the building. Another advantage of a green roof is that the vegetation provides both sound and heat insulation - against summer heat as well as winter cold - and reduces the physical and mechanical stress on the roof structure, because temperature fluctuations and aging of the roof's basic structure are reduced.

Water management:  A well-designed green roof will greatly contribute to limiting the load on the drainage systems during storms, thereby reducing the risk of local flooding. Green roofs store rainwater in plants and growing media and release it back into the atmosphere when the water evaporates. The amount of water that is stored on a green roof and evaporates into the atmosphere depends on the growing medium, its depth and the type of plants. In summer, green roofs can retain 70-80% of the rain, in winter up to 25%. The retention of the water can make a huge difference to urban drainage systems.

Green roofs cool down the surroundings:  " Planting trees and creating green areas will - in addition to creating a beautiful living environment - cool down the surroundings. Projects have shown that by building green roofs, green areas and using the right water elements in London, the temperature has been reduced by 8 °C. Furthermore, the temperature impact will be even greater if the effect from the elimination of dark building surfaces is taken into account .”
Sources: Bjørn Lomborg's book "Cool it" and reference from the authorities in Greater London, 2006.

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Managing Storm Water with Landscaping Design

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Managing Storm Water with Landscaping Design

When Leca® Lightweight Expanded Clay Aggregate is applied to reduce settlements for landscapes, the following design aspects shall be taken into account:

  • Properties of the underlying subgrade and its loading history
  • Allowed rate of settlement and differential settlements
  • Designed life time
  • Groundwater and/or perched water level and possible open water level
  • Distance from ground surface to top surface of structure (height of fill layer)
  • Adjacent buildings.

Easing Structural Pressure

It is usual to take in to account geotechnical evaluation during lightweight structure or fill design in which soil parameters and load are accounted for each construction project individually. The lightweight designed structure or fill can entirely or partly compensate additional loads on the subgrade. Partial compensation can be used where settlements have largely occurred, the settlement period after construction is relatively short, or controlled settlements can be accepted. Entire compensation eliminates settlement periods altogether.

When the groundwater or surface water level is high, it can be good practice to partially reduce the weight of the fill structure to ease the pressure on the surrounding area. The structure shall be designed with consideration to buoyancy connected with rising water levels within the structure, a particularly relevant factor for Leca® Lightweight Expanded Clay Aggregate structures located in vicinity of a water body or flood area.

Limited Buoyancy

Leca® Lightweight Expanded Clay Aggregate fill structures are designed to compensate additional loads produced by a fill structure on a subgrade either entirely or partially. Partial compensation can be used when settlements have mostly happened, the settlement period after construction is relatively short, or controlled settlements can be accepted. Additionally, when groundwater or surface water level is high, it is helpful to partially reduce the weight of the fill structure (limited buoyancy).

Green roofs as architecture

Green roofs as architecture

When green roofs are used as recreational and more flexible areas, there will often be a need to build slopes and variations in the terrain. Here, solutions with low weight are preferable to reduce the extra load and set limitations for the underlying construction. A well-tested, effective solution is to use Leca® 10-20 mm. Before the Leca® materials are laid out, the final surface level is determined using leveling. The materials can be incorporated with blowing equipment and then covered with geotextiles and reinforcing mesh to stabilize the entire structure. The remaining layers can be designed as with normal extensive or intensive green roofs.

Under footpaths and other areas with traffic, it may be necessary to vibrate lightly with a small plate vibrator. The footpath's final covering can be paving stones or gravel, for example.

Ventilation shafts and other technical installations on the roof can be covered with a layer of Leca® lightweight fill, which will transform the roof area into a lovely, green public area or a private garden

Extensive green roofs

Green roofs as architecture

Extensive green roofs are characterized by their low weight due to a limited thickness (typically 5-15 cm for the growing medium) and very special plants, which must be low and hardy, most often alpine, dry soil or succulent plants. Sedum plants are often used. The growing medium, which is typically composed of a mineral mixture of sand, Leca® light aggregate and sphagnum, is designed for the selected plants.

Usually, the plants are watered and fertilized only until they are established. After the first year, maintenance consists of two visits per year for invasive species weeding and safety and membrane inspections.

Intensive green roofs

Green roofs as architecture

Intensive green roofs are characterized by their use as a 'garden' in much the same way that people use a garden at ground level. The increased soil depth (20-200 cm) allows for more diverse plants, the use of trees, shrubs and other landscape architectural aspects and thus the development of a more complex ecosystem.

The requirements for maintenance - especially watering - are greater, and irrigation systems are usually prescribed. We recommend construction and gardening expert advice and an experienced contractor.

Leca® lightweight aggregate can be used in the mixture for the growing medium and as an effective drainage layer. Drainage or retention of rainwater can be optimized by using different types of Leca® lightweight aggregate.

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