Fyra trädgårdar på bjälklag : en jämförande fallstudie

Abstract: This thesis discusses issues relating to the design and construction of roof gardens. The focus is on whether roof gardens constitute a specific habitat which demands plant material with certain characteristics, and on how the design of a roof garden can be affected by the underlying joist construction. The result is based on a comparative case study of four projects in Malmö and Copenhagen.Each project has been studied through visits, reviews of the construction drawings and interviews with the landscape architect behind the project. Subsequently the projects were discussed, analyzed and compared with each other. The study shows that the habitat in the four cases differs greatly. Some gardens are sheltered from wind while others are exposed to it; some are exposed to sun while others lie in shadow for most of the day; some gardens contain soil depths similar to those in a regular garden while others have shallow soil substrates. I have therefore drawn the conclusion that roof gardens cannot be classified as a specific habitat. In fact, the joist construction only marginally affects the habitat created in a roof garden. The exception is when the roof has a low load-bearing capacity. Since the soil is the heaviest component in a roof garden, such roofs often contain shallow soil substrates. The shallower the substrate is, the easier it dries out. However, weak roof constructions are unusual in modern buildings. In a roof garden, plant material with certain characteristics should be chosen. The most important factors to take into consideration are the expected weight and height of the plants, drought tolerance and the plant's ability to adjust to a new environment. Plants with aggressive root systems should be avoided. In terms of the design of roof gardens, the study shows that the joist construction affected the design most when it had a low load-bearing capacity. When that is the case, soil depth and, consequently, plant material is limited. The design can also be moderated if heavy objects have to be placed on the strongest areas of the roof. If the roof has a high load-bearing capacity the design is not affected as dramatically. Rather the joist construction adds a couple of extra elements that must be taken into consideration when designing the garden. Some examples are raised planting beds, stairs and access ramps, and garage driveways that protrude in the garden. Other conclusions drawn from the study are that specialized products for roof garden construction, such as root barriers and light-weight soil substrates, are not always necessary, that surrounding and underlying buildings should be designed with the roof garden environment in mind, and that semi-extensive green roofs are a sustainable alternative to traditional roof gardens.