Rooftop Greening Taking Advantage of Plant Characteristics and Not Requiring Care

March 12, 2001

Rooftop Greening Taking Advantage of Plant Characteristics
and Not Requiring Care
Load on Buildings Reduced Using Thin Layer Planting Base

Takenaka Corporation

Takenaka Corporation (Head Office: Osaka; President: Toichi Takenaka) has developed "Thin Layer Rooftop Greening System," by using a thin mat as a planting base, which reduces the live load on buildings.
Succulent plants of the Sedum class (*) are used. These are robust plants that do not require care and therefore there is a reduction in maintenance costs. The thickness of the planting base and the load are about one-third that of the lawn usually used in rooftop greening, so there is a great reduction in the load on the building. This method can be used not only for new buildings, but also for existing buildings for planting over the entire rooftop without having to be concerned about load limitations.

(*) Sedum class:
This is a variety of small succulent plants that can be found over a wide area of tropical and temperate regions in the northern hemisphere, and is the generic term for the perennial plants in the Crassulaceae family. This variety has been widely used around the world in gardens from long ago. In Japan there are over 20 native varieties, including Sedum erythrosticum, Sedum lineare, Sedum erubescens and Sedum kamtchaticum. These varieties are characterized by their outstanding health, are strong against heat, cold and dry conditions and more importantly can be grown in barren land or thin earthen layers. Because they can be left unmanaged, they are said to be one of the most suitable varieties for rooftop greening.

Background to promotion of rooftop greening

In recent years the heat island phenomenon causing temperature rises in cities has become a problem. Rooftop greening has been attracting attention as a method to improve city environments. With revisions of the greenery standards of the Natural Environment Conservation Ordinance approved by the Tokyo Metropolitan Government in April 2000, guidelines were introduced encouraging 20 percent of rooftop areas to be planted. From April 2001, companies that fail to meet these guidelines will face fines of up to 200,000 yen and increases in ratios of building volume to lot will be applied to the buildings with rooftop greening. Furthermore, in addition to the Ministry of Land, Infrastructure and Transport's "Ecological Building Project," the cabinet has decided to implement reductions in fixed assets taxes for buildings with rooftop greening. These types of policies are expected to increase throughout Japan, and a sharp increase in rooftop greening is expected along with the revisions in the Tokyo ordinance.

About the "Thin Layer Rooftop Greening System"

By using specific plants such as those of the Sedum variety, the "Thin Layer Rooftop Greening System" is a new type of rooftop greening system featuring a reduction in the weight of the planting base, a reduction in future maintenance and therefore a reduction in maintenance costs.
The characteristics of Sedum varieties such as: (1) strong against dry conditions, (2) can be grown in barren and thin earthen layers and (3) strong against hot and cold conditions enable them to be grown in severe environments where other plants cannot grow. By taking advantage of these characteristics, the planting layer can be made 25 millimeters thick, and the drainage layer underneath 15 millimeters thick, totaling a mere 40 millimeters. Even if a water retention layer is put between these two layers in cases where more moisture is required, the total thickness is still only between 50 to 60 millimeters, and under 40 kilograms per square meter, it is lightweight. The weight and thickness specifications are around one-third when compared to using lawn. The mats are made of a three-dimensional polypropylene textile. The openings in the planting mat are made larger to enable ceramic soil to be poured in more easily. The openings in the drainage mat, which is built onto the filter layer, are made smaller to give the mat more strength. The spindly growth of the Sedum is prevented and the growth of weeds is controlled by keeping soil short of nutrition and dry. This maintenance-free planting base means a reduction in maintenance costs.

Cross section diagram of the structure (40mm example)

Features of this development

[Feature 1: the narrow thickness reduces live load and the load on the building]
By making all the layers thin, from the top planting layer, to the bottom waterproof and root prevention sheet, the weight is about one-third compared to using lawn, less than 40 kilograms per square meter, greatly reducing the overall live load. This method can be used not only for new buildings, but also for existing buildings for planting over the entire rooftop without having to be concerned about load limitations.

[Feature 2: by using management-free plants, there is a reduction in maintenance costs]
Due to the synergism of the planting layer and the drainage layer, the system is constructed to maintain dryness and oligotrophic conditions. This controls the growth of weeds and maintains a growth environment only suitable for Sedum varieties. There is virtually no need for sprinkling, fertilizing or weeding, so there is a reduction in maintenance costs.

[Feature 3: a safe fixing technology that can also be easily removed]
The fixing technology is extremely important to cope with wind pressure when it comes to rooftop greening. By making the root prevention and waterproof sheets, which protect the building surface, and the bolts that hold all the mats and sheets together out of PVC, the system can be easily and firmly put together on-site. The bolt that is put through the planting mat can be installed with one touch using a push ring. When the waterproof sheet needs to be checked or a mat replaced, the bolt can be easily removed. Furthermore, this mechanism enables it to be used to hold the edge and to adjust the height. It is dry, and anybody can easily install and adjust it.

[Feature 4: reduction in work]
Because all the layers are sheet and mats, the system can be easily constructed. By adjusting the height of the gap covered by the push ring, height adjustment of the edge that up to now took a long time can now be easily done. On average, only a short work period of one week is necessary to complete a rooftop greening job.

Construction method

The root prevention and waterproof sheet is laid and the bolts welded.
The edge of the planting part is installed and the height adjusted.
The drainage mat is laid. (If required, a water retention layer is laid over the top.)
The planting mat is laid.
All the mats are fixed using push rings.

Depending on the condition of the building and the client's requirements, the best rooftop greening method should be chosen. Takenaka will continue development of rooftop greening technology to provide our clients with a diverse range from which to select.