How to design a roof garden, according to a top garden designer
Designing a roof garden, where plants wouldn’t naturally choose to grow, is a completely different experience from designing a garden on the ground where lots of plants happily grow. The key things to consider are, can you get the plants up there, will they survive the environment, and will they get too big for the space?
One of the mistakes I think people make with roof gardens is that they aren’t ambitious enough. If you think about a successful garden, there will be lots of plants making patterns, shadows and creative views. You want to create the same effects on a roof terrace, so a few pots on the edge of the terrace just won't do, and will always look like a linear, unrelaxed array of plants. For the Plimsoll roof terrace we intersected the space to create long views and garden rooms. We also used layered planting with planters of different heights. I think this is one of the elements that was most successful.
There are many practical things to consider for a roof terrace, most importantly how much weight it can take. You can calculate the weight of your planted area in cubic meters of saturated soil as this is the heaviest element. It's surprising how shallow the soil needs to be in order not to exceed the weight limit. The rest of the planter can be filled with a lightweight product like LECA. Containers should be good quality 3mm steel, fully galvanised so they don't rust. They can be powder coated with a mellow colour that blends in rather than stands out–it's the plants you want to feature after all. Corten steel is also a good practical choice if you like the style.
Roof terraces often have a suspended floor which means that water can pass through the floor surface and onto the drainage system below. This means that stone or porcelain are practical as a floor choice. I try to choose porcelain that isn’t too bright as the sun light bounces off and makes it glare. Composite deck board is long lasting and low maintenance. Its definitely worth choosing the best quality you can get: I like 'Terrafina' by Kinley Systems.
It's important to have the garden set up ready for enjoying as soon as the weather permits. You need some seating and a tiny bit of shade. One of the things you don't always think about is wind, so you need to find a spot which is fairly sheltered if possible. The other thing that is worth thinking about is that roof terraces and balconies are often viewed from inside and so it's a good idea to bring plants right up to the windows so you feel part of the garden without even stepping out.
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It's best to choose Mediterranean plants that are used the harsh environment of a dry hot and windy space. Do select things with small leaves that allow the wind to pass through. I often find that lavender, santolina, grasses and herbs do well. Mounding Ceanothus is good for early colour and for breaking the line of rectangular pots. Short irises are also a good choice. Don’t forget to choose plants that will spill out of the planters or dangle down so you break the straight lines, making it all look quite natural.
There are often restrictions on the hight or weight of what you put onto the terrace so it can be hard to create privacy. The best way I have found is to get plants that will grow nice and big. Pittosporum tenuifolium varieties seem to do this quite well and are wind tolerant. You can plant them small and they will quickly make a large shrub. Also pines are a good choice for evergreen screening.