Trees on roof terraces and podiums are bringing a new set of challenges. As we are increasingly seeing elevated landscaping in new developments, how can we guarantee the trees on roof terraces are going to thrive?

Thanks to planning requirements implemented by the UK government, every new development must make a substantial ‘green’ contribution. Increasingly, this is taking the form of roof terraces and podium landscapes. But how can we guarantee that the soft landscaping, and in particular trees, are going to thrive when planted up to hundred metres (or more) above London?

Trees play a vital role in a landscaped area. They provide beauty, shelter, shade, perspective and are, in summary, essential components of a landscaping scheme. Can you imagine a landscaped area without them? Nor can we! So here are a few points to consider when planting trees on roof terraces and podiums.

What to consider when planting trees on roof terraces

The high exposure of roof terraces

At height, trees on roof terraces face increased exposure to wind pressure and suction. Wind speeds can be unbelievably high at the top of modern sky scrapers, and risk of an incorrectly planted tree uprooting is very real. The selection of suitable tree species, adequate roof substrate and, above all, the local site conditions with regard to wind pressure and wind suction are essential. Securing trees on roof terraces and podiums uses an anchoring system with tension straps, which are themselves weighted with a deadweight or secured to a metal grid framework. The anchoring system is an effective way of securing the tree without damaging the waterproofing layer, and also plays an important part in supporting the tree in early growth whilst the root framework is establishing itself.

Getting the soil right

The correct soil type is vitally important to ensure the success of trees on roof terraces and podiums. Large quantities of soil obviously mean a lot of weight, especially when saturated, so consideration to load bearing is important. We work with a number of specialist suppliers, and recommend a lightweight blend developed especially for this application. These soils are a careful balance to ensure they maintain water retention and structural and nutritional integrity. Lightweight soils weigh in at around 850kg per m3 when unsaturated, compared to 1300kg per m3 for standard soils. Often using void former reduces the need of large volumes, but requires care to ensure the maintenance of adequate soil depths to enable healthy root formation.

Is there enough sunlight?

The amount of direct sunlight trees and plants receive should also be a consideration when creating a planting scheme. Some roof terraces or podiums can be relatively confined, and often the surrounding buildings extend above them. Depending on the building’s location, this can mean the area spends long periods of time in shadow. In this case, specifying hardy, shade-tolerant species is important and landscape architects should be mindful of this. Plants ‘measure’ shade by using pigments that can detect far red and blue light and modify their growth habit in proportion to the degree of shade.

Irrigating trees on roof terraces

Trees will always require water, and this is no different when on top of a building. For larger roof terraces and podiums, an automatic irrigation system is often fitted. These programmable systems are installed with drip lines running through the soil build-up and ringed around trees. Alternatively, systems such as GreenBlue Urban’s RootRain range, ensure water reaches the roots directly during manual watering.

Speak to the specialist landscaping contractors

Valley Provincial have grown to become a leading podium and roof terrace landscaping contractor. We firmly believe that elevated spaces can be just as usable as ground-level spaces, and we know an integrated, well considered planting plan goes a long way towards this. Contact us today to speak to an expert in trees on roof terraces and podiums.