Raised Bed Gardens On Rooftops - Things To Consider

Roof gardens have always been popular, particularly with urban dwellers, who may not have the luxury of space.  A roof garden can cover the entire rooftop with turf and plants, and these are most commonly seen in commercial or public buildings.  They are expensive to set up and maintain, and not everyone can install them due to weight restrictions.  Another common type of roof garden is made up of containers and raised beds.  It is the most affordable option, since containers can be made up of all sorts of materials, including reused old junk.  Containers are easy to set up, require little planning, and can be easily moved around until the right location is found.  Raised beds, on the other hand come with a few restrictions, and there are a few things to consider before thinking about that vegetable patch on the roof.

Image from uncommonground.com

The Pre-Planning Phase:

Before even planning to install raised beds, it is important to note that if the roof is owned by a landlord, permission should be sought.  Raised beds are heavy structures that require a lot of work to set up.  It is not as easy to remove them if the landlord or other tenants object.  The building code also needs to be observed.  It is possible that roof gardens are not allowed on rooftops due to fire hazards.  Permission may also be required to lift all the materials on to the roof.  Lifting tonnes of soil up to a four storey building is no easy task, and requires some careful planning and organization.

The Planning Phase:

Once permission is obtained, or unnecessary, the first thing to consider before building the beds is weight.  Assuming that the raised beds are not going to be built out of concrete blocks, soil, especially wet soil, can get very heavy.  A rough estimate for dry soil is around 1,200 Kg per cubic meter, or 2,100 pounds per cubic yard. Depending on the desired size or quantity of raised beds, the rooftop can be put under a lot of strain.

Soil density and weight varies, and the best way to calculate the potential weight is to measure a sample of the soil that will be used, both dry and wet.  Then calculate according to the inside measurements of the raised bed.  Apart from the soil, the weight of the raised bed itself, the weight of the plants that will grow in it, and any other structures that already exist on the roof need to be considered.  If the rooftop has a protective membrane installed it will not take too kindly to harsh materials being dragged on it.  An additional layer could be installed to protect the membrane, and this will also add to the weight on the roof.

A nursery or landscape designer should know if the roof can withstand the weight, possibly following an on-site inspection.  Alternatively, the building’s architect will be able to offer accurate guidance.  The architect should also guide you on providing the rooftop additional structural support if necessary. It is important that  an assessment is done properly as structural damage is a real possibility, and can be quite costly to repair, especially if the roof is not your own.    

Another consideration that should go into the planning phase is the kind of plants that will be grown in the raised beds.  In the case of vegetables or other edible plants, such as herbs, care must be taken to ensure that the material that the beds are made of will not run into the soil, possibly adding unwanted chemicals and toxins that the plants can absorb.  It could be bad for the plants, but also bad for human consumption.    If the beds are made of wood, it should be water resistant, so that it doesn't deteriorate with repeated watering. 

Image from upper-living.com

The Construction:

Raised beds are essentially large containers.  They can be made up of anything from wooden boxes, concrete boxes, barrels cut in half, or even old bath tubs.  Raised beds are raised off the ground, and they should have sufficient clearing to allow for proper drainage and air circulation.  If the beds are not raised off the ground they can cause serious water damage to the roof of the property, as water sits at the bottom and seeps into the stone.

The Environment:

A rooftop, as with a garden, is exposed to the elements.  There are, however, certain characteristics that may be unique to a roof garden, not necessarily applicable to normal gardens on land.  When selecting the plants for the raised beds, whether the plants need sun or shade will determine the position of the beds. 

Sun is unavoidable on a roof top, however, unless shade is provided by means of man-made shelter.  Heat on a rooftop is also unavoidable.  Apart from heat that is reflected by the sun, the heat coming out of the building, in the form of thermal heat is also present, and can be unbearable in the middle of summer.  Proper air circulation underneath the raised bed is essential.

Wind is also a problem for roof gardens.  Depending on the position of the building, the position of surrounding buildings and the altitude of the roof, wind can be a killer for raised beds, just as it is for container plants.  Since raised beds are normally higher than containers they may not have the shelter of the roof walls, and may be more exposed to wind.  If wind is a risk, it may be worthwhile to invest in shelter, either man-made or by means of other hardy plants that can be grown to act as wind breakers. 

Image from livinghomegrown.com

The Gardening:

When the raised beds are ready, filled with soil and plants, and sheltered from the elements.  Something may become obvious very fast is water supply, or the lack of it!  Depending on the size of the raised beds, it is ideal to have a water connection or some sort of water storage on the roof.  It can get very tiring going up and down stairs and lifts filling up buckets of water.  If there is no water connection, a rain barrel could be installed on the roof. 

Space permitting, it would be also a good idea to have a storage unit for all the gardening equipment.  Items like watering cans, fertilizer, soil, and gardening tools can be stored on the roof in a small closet and available when needed.  Better than to realize that you forgot your fertilizer, and you have to go down three storeys to get it!

The Conclusion:

I think rooftop gardens are a great idea for those who do not have the space for a garden. Raised beds or containers can look great on a rooftop especially with proper design.  If the idea is to create a peaceful spot, then it is possible to add a comfortable seating area where one can sit and relax whilst enjoying the view.  The sky is the limit in terms of what to you can turn an empty roof space into, it all depends on the budget, and some proper planning.  The latter, at least, we have covered!

Image from apartmenttherapy.com