Horticulture Magazine

Tips For Flat Roof Garden Spaces

woman harvesting vegetables on a rooftop garden

Creating city gardens means making the most of every inch of space, so here are some tips to help you create a roof garden.

If you have a flat roof where you live, either on your own home, or a shared space on top of a block of flats or shared property, this article is for you. A flat roof can be a great spot to create a garden. It opens up potential to grow your own even when space is limited. And also brings many other benefits within an urban environment.

Roof gardens can sequester carbon, clean the air, cool buildings, and, of course, provide an abundance of natural yields. They can be wonderfully rewarding and relaxing spaces to spend time. And can re-green our grey cityscapes.

But gardening on a flat roof takes a little thought. There are a number of things to consider that you could not usually have to consider if you were growing on the ground. Below are some tips to help you create the perfect flat roof garden spaces:

Check That The Roof Can Support The Weight of the Garden

If you want to create a roof garden, load-bearing capacity is key. First and foremost, you need to make sure that the roof you are considering is strong enough to support the weight of a garden.

You need to check its structural integrity, and ideally consult with a structural engineer who can determine whether the roof needs to be strengthened or provided with additional support in order to create the garden you wish to create.

Safety is key, and you also need to think about practicalities. Is there easy access to the roof garden? Can people safely use the garden space?

raised vegetable beds on a rooftop
A rooftop could be the ideal spot for your new vegetable garden

Design and Layout For a Roof Garden

One of the main challenges when creating and maintaining a roof garden is access. It can be challenging to get materials and tools and other things you need up and down stairs. If the roof is suitable for a garden, therefore, one of the next things to consider is what elements need to be included in the design above and beyond the plants and planting areas themselves.

A key tip is to make sure, if possible, that there is somewhere on the roof itself where tools and materials can be stored. This might be a small tool shed. Or, on a smaller roof, perhaps a storage area below a bench seat or other feature in the space. Weight is, of course, a key consideration in many roof gardens. But if you can find a space to store things on the roof itself, this will save a lot of time and effort for those who will be tending the garden.

Creating a composting area is another key thing to think about. Being able to create compost on the roof, or as close by as possible, can help you fill your pots or containers or raised beds more affordable, and in a more eco-friendly way. If it can be on the roof itself, this can be beneficial because it can avoid the need to lug heavy materials up and down stairs.

In addition to thinking about design and layout for planting areas/ containers, you should also think about human enjoyment. It can be helpful to think about where a seating area could be incorporated for garden users to enjoy the views and/or enjoy spending time in their beautiful new garden. Incorporating seating areas in a design can help you develop a holistic and integrated plan.

Choosing a Growing System for a Roof Garden

hydroponic growing on a garden rooftop
Rooftop hydroponics could be one interesting option

On a flat roof that is of sufficient strength and integrity, you can grow food, flowers and other plants in much the same way that you would in a container garden or planter gardens on a patio at ground level.

However, on the top of a building, it is often important to think about weight. This is not something that you would usually have to consider when gardening at ground level. Again, make sure you consult an expert to you have an idea of how much weight your particular roof can support.

Even with weight restrictions, however, there are plenty of exciting growing systems to consider. You could:

  • Grow food in planters or enclosed raised beds.
  • Start a typical container garden in pots.
  • Add vertical gardens, hanging baskets etc. to make the most of vertical space.
  • Grow food hydroponically, in water rather than soil/ growing media. And even potentially consider an aquaponics system, with fish as well as plants.

When choosing a growing system for your roof garden, you need to consider not only weight but also a range of other factors. Space, your own preference, and of course environmental factors all need to be considered.

Understanding Environmental Factors on a Flat Roof

Understanding environmental factors on a flat roof begins with simple observation. Make sure you spend some time on the roof, and think about the conditions that are found there before you start thinking about layout, growing method, and plant choices.

Typically, a roof garden will be more exposed than a garden at ground level. Many roof gardens are windier than gardens at ground level. This is something that you need to think about. And something that you can mediate through good garden design.

Roof gardens are also often less shaded than other sites. While some roof gardens may be shaded by another part of a particular building, or by other taller buildings in the vicinity, many will be in full sun. This is something else that you need to think about when designing your garden.

A full sun site can be beneficial, and you can often grow a wide range of plants. However, one thing to bear in mind is that, especially in a full sun site with wind exposure, things can dry out more quickly. Since plants grown in raised beds and pots often require more water than plants grown in the ground in any location, water shortage can quickly become an issue in some roof gardens. So access to water, and how easy it will be to meet water needs, is another important thing to consider.

The Garden at 120 in London
A roof garden in London

Wind-Breaking and Privacy for a Flat Roof Garden

Thinking about wind-breaking for a flat roof garden can go hand in hand with thinking about privacy for the space. City roof gardens can not only often be exposed, they can also often be overlooked.

Creating wind breaking and privacy screens can often involve the placement of small trees in containers, shrubs, bamboo or tall grasses. Thinking about creating green, lush barriers around at least some edges of the space can help block prevailing winds and give privacy. They can also mean that a wider range of plants can successfully be grown at the centre of the space.

When it comes to planting up a roof garden, border plants for wind-breaking and/or privacy are often some of the first plants to consider.

Creating Shade for a Rooftop Garden

If your roof garden is in full sun, things can quickly heat up and dry out during the summer months. Especially with the city heat-island effect. While this can bring opportunities, it can bring up challenges too.

One key thing to think about is how you might be able to create some shade to reduce plant stress and reduce water needs in your roof garden. Creating shade might sometimes mean adding a man-made structure. But more often, it involves the careful combination of different plants. Trees, shrubs and other taller plants can create shade for other plants or seating areas beneath them.

Harvesting Rainwater for a Roof Garden

One other very important consideration when planning a roof garden is where water will come from. Of course, you should try to reduce water needs as much as you can. You can do this by choosing the right growing method or methods, and by creating wind-breaks and shade.

But you also need to think about where the water comes from in the first place. If possible, try to think about how you can collect and harvest rainwater on your roof. Even placing a few buckets or other open containers on the roof could reduce water needs – where connecting to guttering etc. is not possible.

If water is to come from below, you will need to think about how the water will make its way upwards, since you’ll soon find it impracticable to carry watering cans up and down stairs on a regular basis.

Choosing Containers and Creating Growing Areas

No matter what method of growing you choose to employ, the materials you choose for your roof garden are key. Making or choosing planters or containers for your roof garden will involve careful thought. You will also need to think carefully about how you fill those planters or containers.

Remember, on many roofs, weight is a key consideration. Even where the roof is strong, it can be tricky to carry heavier containers and materials up to the roof. Choosing lighter-weight materials can often be a good idea. In areas where water could be a concern, it is also often important to choose containers/ planter materials which retain water. Reclaimed plastic containers, or wood, can often be better choices than heavy materials like terracotta. However, there is a trade off – and you also need to make sure that things you choose for your roof will not blow away.

friends gardening on a rooftop
Creating a roof garden

Choosing Plants For a Rooftop Garden

Once you have a good idea of layout and method, and have an idea which containers you will use or growing areas you will create, you need to think carefully about plants. When it comes to choosing plants for a roof garden, you have plenty of choices. Many smaller trees, shrubs, fruits and vegetables, herbs and flowers can be ideal for a roof garden. Just make sure you think about the needs of the things you wish to grow, and conditions on your roof before you make your choices for the space.

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