Roof gardens

Roof gardens are popular. Combining uses is a frequent choice, in particular when it comes to urban infill development areas. For instance, the roofs of offices or schools can be converted into park-like gardens with lunch terraces for workers or students. We are also seeing more and more roof gardens at street level: at first sight they may appear to be like any ordinary garden; however, because they are above an underground car park or tube station, they are built exactly like roof gardens. These structures place a number of limitations on the trees because the root space is restricted. For this reason, trees in the second or third size categories that do not have too much leaf mass, but which do have a dense root system, are the most suitable. In some situations, trees in the first size category can also be planted but special measures need to be put in place. Below we have listed a few suggestions for trees that are suitable for roofs and you can read an article about roof garden specialists

Rooftop trees: the technical story

We asked Bertus van den Dikkenberg and Walter van de Werken of the Koninklijke Ginkel Groep what the criteria are for roof trees. ‘You have to start with the supporting power of the roof. This is the most important restrictive factor. If you are working with an existing roof, you will have to adjust your plans to what the roof can carry. In the case of new builds, there are more possibilities, because the construction can take into account where and what trees will be placed on the roof. The bigger the load that the roof can carry, the larger the trees that can be planted. Planners do need to take into account the weight of the mature tree. It is possible to plant (small) trees on a roof with carrying power from around 1000 kg/m2,’ says Bertus van den Dikkenberg.

Different kinds of rooftop greenery

There are different types of green roofing, which can be split into the categories of extensive (sedum roofs), semi-intensive and intensive. This last category, which requires the highest supporting power, not only comprises herbs and shrubs, but also trees. A combination of extensive and intensive roof planting is also possible. In these cases, a tree can be planted on the roof on places supported by a pillar, while the rest of the roof is only suitable for sedum matting. Walter van de Werken: ‘In these cases, there is a difference in height with regard to the structure of the roof garden. Sedum matting only requires a few centimetres of substrate, while you need at least 70-80 cm root space for trees. A tree can then be placed on a mound or in a container.’

Apparent groundwater level prevents trees from drying out

Containers do restrict the growth of trees, which means they will always be a bit smaller than normal. The best results can be achieved when trees are planted in high-quality substrate with the right oxygen and water balance, and where they have enough room, also widthwise, to take root. ‘It is important that a system is selected which imitates the normal water level. A float system ensures that the artificial water balance is kept at the right level, and trees do not dry out. This is very important, because it can all go wrong very quickly. And if it goes wrong, you’re in trouble, because replanting trees on a roof can be extremely complicated.’

Trees in the second and third size categories

When all the technical requirements have been met, there are many possibilities and trees can thrive well on a roof. ‘In principle, a roof garden, if designed well and created in accordance with the guidelines (e.g. FLL guidelines, Groenkeur, BDA notices, SBR guidelines etc.), has the same economic lifespan as the building itself.’ Of course, the choice of tree is an important factor in the success of a project. Firstly, you have to look at the size and sensitivity to wind. ‘Size 2 or 3 trees, which do not have too much foliage and which have a compact root system, are best suited for roofs. The smallest trees can be secured with counterweights and the larger trees will have to be attached to an underground reinforced netting. Size 1 trees can be used in some situations, but do require special provisions. In reality, each roof garden is bespoke, because there are so many factors to take into account.’

Wind factor has a huge effect on trees

The higher the roof garden is, the more important the wind factor gets. In high gardens, but also in roof gardens on ground level which are susceptible to descending wind and whirls, trees that are sensitive to breaking branches should be avoided. Types of trees that are especially susceptible to aphids or mildew in draughty conditions should also be avoided. ‘I also prefer not to use trees with long, straight trunks,’ adds Van den Dikkenberg.

Grey-leaved trees do well on a roof

Luckily, there is plenty of choice. Multi-trunk trees are not only trendy, but also very suitable for roof gardens. Topiary trees (not to be confused with trained trees) are often also a good option. Because they are constantly pruned into shape, they stay relatively small. The tree type obviously has to be suitable for use on a roof. ‘From experience, we know that grey-leafed trees are extremely suitable. They can withstand heat, drought and wind,’ says Van den Dikkenberg, but there are many more options. Our tree specialists have compiled a list of trees for roof gardens. Of course, every roof garden requires its own bespoke solution. You can always ask us for our advice!

Trees for this application