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Design a rooftop garden, my experience

In early September, the landlords of our building came onto the rooftop to fix a leaking concrete cistern. The truth is that it had been leaking since we moved in 7 years ago, but as seepage started through the wall right next to the cistern, it had to get finally fixed.

Although it was making the floor of the rooftop dangerously slippery during rainy periods, I also loved to see ferns growing beautifully on the concrete wall. Their slow thrive was like poetry to me while their roots became with time a beneficial habitat for insects and other mineral, wet-environment, liking plants.

The leaking was big enough to also irrigate all the plants placed under the cistern. This was definitely a factor that I included in my garden design. However now, the situation is radically different. My garden is officially located on a windy, hot (in the summer) and dry rooftop. As mentioned in my previous article, design is crucial for your plants well-being as well as your own peace and time.

Remember that the mindset to adopt for rooftop gardening is working with your surroundings by first acknowledging both sun and wind exposure. That way, you may at first spend some time thinking through how to set up your garden but once this is done, you will spend less time fixing structures and less money buying new plants to replace the dead ones.

I am the kind of person who learns more with visual examples or on the field so enough of the theoretical part, let's have a look at my rooftop map.

Rooftop eye bird view. 1/3 is covered by a roof (green)

As you can see, it is a north facing rooftop, with a third of it covered with a metallic roof (in green). The concrete cistern is elevated 30 cm above the ground and is 150 cm high. On top, four new metallic cisterns were installed which act like a wall that shades the area right under it in the morning but also works as a windbreak.

The potential gardening area is rather rectangular but you can see that there is an extra gardening spot on the top right corner as well as a tiny corridor longing the building on the bottom left corner which needs to be kept clear as it is the only access from outside our apartment for the habitants of the building to reach the rooftop (for emergencies or simply to fix the concrete cistern, for instance).

Winter sun exposure during morning and afternoon

Through the years I have noticed the difference of sun exposure during the winter and summer sun path. Right now, the winter sun path is covering my rooftop with up to 70% of shade in the morning, while the afternoon sun reaches all the way to the gardening area from 12 PM to 3 PM. This information will be important later when choosing the right spot for the plants that I want to grow.

Wind coming from North East is particularly strong during winter

Do you have to deal with a windy rooftop? Well, I would say it is quite common in Taipei. We usually get a strong wind coming from the north in winter while during the summer, well, we have typhoons! In my situation, the wind is also coming for the airplanes landing and taking off as I live next to Songshan airport.

Also, my building is as tall as all the other buildings around which means I don't get any wind protection from them. It also means that I don't get any shade from them either which is great during winter but less so during summer. Both wind and sun are limited by the 1 meter high wall framing the rooftop.

Plants can benefit from it if they are placed right under, especially under the north facing wall. To soften the wind on a rooftop, I think trees and shrubs are the best option. If they are suitable for windy environments, they will cope well with the wind even with a very strong one like they experience during typhoons, for example.

Using pioneer trees and flowering shrubs as windbreak

To be honest with you, I didn't buy any of the trees growing on my rooftop. They are pioneer trees that have grown wildly on my rooftop, and which, while they were still small, I repotted in planters. Some were also found in the street, half dead, waiting for someone to take them away. They now have a happy life on my rooftop.

Another tree grew on the edge of my vegetable planter located on the Western side of the rooftop. For now, it is not causing trouble but I suspect it will grow bigger and take all the space available in the soil with its roots within 1 to 2 years. I will see then what to do with it then; maybe repot it in another container or reuse the wood and leaves if it happens to die after the repotting attempt.

I am using osmanthus and azalea shrubs to thicken the wind barrier on the North-Eastern side as it is the windiest corner. These two plants are super hardy, both are drought and typhoon resistant. Azalea shrubs bloom beautifully and have colorful flowers all winter long while the osmanthus brings an amazing scent in the garden. Not to mention that it is also a pollinator attractant. These shrubs have been on my rooftop since the beginning and I have to confess that I have neglected them a lot. Yet, they are still here, alive and kicking!

Bamboo can be used as windbreak, too, though I have never experimented with it in a garden design. The western side trees are not so useful as windbreak but they provide vital shade during the summer and have become a great spot for birds too.

So here is my playground:

Deciding on where to install my planters

Choosing plants now will depend on their ability to cope with a windy environment, their sun needs and what I want to grow! Another factor to take into consideration on the long term is crop rotation. This is only detrimental if you want to grow annual vegetables like tomatoes and broccolis.

You may have noticed that I have placed the growing areas on the edge of the rooftop. This has more about security matters than the overall aesthetic of my garden. In Taiwan, most of the rooftops can handle 200 kg per square meter and usually the floor is more resistant to weight near bearing walls like the outside walls of buildings.

Also, if there is an emergency, the people living in the building should either get out of the building or go onto the rooftop. Therefore, leaving at least 50% of the rooftop area empty is a necessity.

Considering the weight capacity here, most personal gardens installed on a rooftop would be suitable, but always keep that number (200 kg) in mind while you decide on the size of your planters (including their depth) as well as the plants you want to grow in it, like trees.

When calculating the total weight per square meter, you need to measure the weight of the soil that you have purchased (or made) as well as the water retained within it and the global biomass created by the plants. Materials used to make planters are usually quite light, if not, try to also evaluate their global weight.

If you happen to know a landscape designer or an architect and if you have any doubts about the weight security, ask them. They are usually happy to help you. At least, that is my personal experience.

Stay tuned in music!

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