Because there is a structure below the landscape, not soil, it cannot be overstated - that an improper installation may not become apparent until the plants are stressed, in decline or even dead.
A rooftop or over structure landscape can have soil depth of less than a foot to five feet or more. It consists of several layers: root barrier, waterproof membrane, drainage layer, soil, plants and irrigation.
Below is a description of what are essential considerations of over structure landscapes. It explains what can go wrong why you need to have a contractor who is knowledgeable about urban landscapes to design, install and maintain your above ground landscape.
Leakage is one of the worst problems. Unfortunately, it is not always noticeable until the water has already leaked into your building. The waterproofing liners have a lifespan and it is important that your landscape maintenance contractor pays attention to any potential leakage. Waterproofing is done by a contractor who specializes in waterproofing.
Contrary to what you might think, the right kind of soil for an over structure landscape is soil with a high mineral content so it does not break down. These ideal soils are not readily available and must be specially designed and blended for best results. Improper soil types can result in a drop in soil grades which can expose utilities, irrigation pipes and waterproofing liners. Conversely, in a typical landscape you want soil that has a higher percentage of organic matter which will decompose - contributing to fertility of native soils. Consequently, specifying the proper soil for an on-structure landscape installation is a critical step to ensuring long term success.
It is important to keep in mind a few points: You don't want to water too deeply and you will need to minimize runoff. For many over structure landscapes spray irrigation is the best solution unless high wind or blockage from plant material is a factor. Spray irrigation enables more accurate management of moisture in the top few inches of soil Drip irrigation typically creates a deeper moisture profile leaving upper soil layers dry which promotes root development at the bottom of planters within drainage layers. Over structure irrigation is challenging so it is important that your contractor be knowledgeable about appropriate irrigation technology and performing proactive irrigation checks and repairs. For maintenance, irrigation checks should be done more frequently than with in ground landscaping.
Water needs to be able to flow unimpeded through soil towards the drain outlets. Features to expedite this water movement to drains should be included in any renovation or new construction. It is another reason that this type of landscape needs to be closely monitored. A drain can easily get clogged and it would not necessarily be noticeable until a tree dies or flooding occurs. When we build over structure landscapes we include either french drains or composite drain layers to be sure water will move through the soil to the drainage outlets quickly.
Avoid deep rooted plants or those with aggressive rooting habits.
Overall, it is better to plant smaller, slow growing trees.
As an example, trees to avoid planting would be Poplars and Liquid Amber which are both fast growing with aggressive roots.