THINGS TO CONSIDER

1. Weight capacity

The primary concern when thinking about starting up a rooftop garden is the weight bearing capacity of the roof. The best possible and most cost-effective scenario is to construct a rooftop garden when the roof needs to be replaced or is newly constructed.

• Before you even start designing your rooftop garden, consult with a structural engineer and if possible the building’s architect regarding the weight bearing capacity of the roof. If you live in an apartment building, consult with the superintendent or owner to propose the idea.

• For the structural analysis of the roof, the architect and/or structural engineer will consider such factors as loading strength, condition of the waterproof membrane of the roof, and necessary safety provisions.

• Once the structural capacity has been determined, you will then know the type of garden that is best suited for your roof.

• Your first concern should be the weight bearing capacity of the roof. Most roofs are designed with an average loading strength of 40 lbs. per cubic foot. Wet soil weighs on average 100 lbs. per cubic foot.

• In order to reduce the soil weight, lighter soil varieties should be used such as perlite, vermiculite, peat moss and coconut husk fibre. SEE WHAT YOU'LL NEED - SOIL

• Find out where the building’s structural columns are located, as they are able to support greater loads than other areas. Place the largest planters and containers at these locations and spread the smaller, lighter ones about equally. SEE WHAT YOU'LL NEED - CONTAINERS

• Since different plants require different soil depths depending on their root structure, find out how much soil each plant requires. Plants with shallow, spreading roots can grow in less soil depth than plants with longer (tap) roots. Do not use more soil than you need.

2. Exposure

The second and almost equally important concern to the weight capacity of your rooftop is the exposure to elements. Rooftop gardens are more susceptible to factors such as wind, sun and heat due to their height off the ground. The roof can actually be as much as 5oC hotter than the surrounding land. A four pronged approach is usually the best option to overcome these conditions.

• Choose PLANTS that have the ability to withstand harsh conditions, or provide them with SHADE.

• Provide a thick layer of mulch to insulate and shade the soil. This will minimise evaporation and keep the soil cooler .

• Install a drip irrigation system, on a timer, that will deliver steady moisture to each planting bed. Without this kind of irrigation, many planters, especially the smaller ones, may need watering twice a day. SEE WHAT YOU'LL NEED - WATER

• Choose your CONTAINERS with care. Although you might find that terra cotta pots look better than plastic containers, they are too porous to conserve water under rooftop conditions. Plastic pots seem to work a lot better

3. Wind and sun protection

Wind conditions on a roof can be dramatically different from those on ground level. Generally, the higher up you are, the stronger the wind. Combined with the increased sun and heat exposure on a rooftop, plants could have a hard time surviving unless some precautions are taken.

• Use the sides of the plant’s own container as a windshield for young plants by planting them several inches below the top of the pot.

• For larger plants in very sever conditions, you may have to build a windbreak or use hardier plants themselves as windbreaks. An outside row of tall sturdy plants, such as sunflowers or evergreens, or hardy vines on a trellis, can protect other more delicate plants.

• When selecting plants for a windbreak, look for plants with thick leaves with hairy or waxy surfaces. They loose less water to evaporation than thin, smooth leaves.

• Stake your plants using stronger and more numerous stakes than you would normally use on ground level. These will protect and support the more heavily laden plants. If the plant grows upright rather than in a spreading fashion, it will probably need some support.

4. Access and safety

Access and roof safety should be priority considerations when designing your rooftop garden. As well as physical access for people, there should be water access as well. If your only method of getting to the roof is by ladder, you should think carefully about the task of carrying all the materials needed for building your garden as well as the repeated transportation of water to the roof.

If it is a public building, the Building Code will not allow ladder access for this intended use. In terms of safety, consider installing guard-rails at appropriate heights around the perimeter of the roof where there is garden access. These will not only protect the people visiting the rooftop garden, but will also shield the plants themselves.

5. Roof properties and drainage

Any excess water not absorbed by the plants should be drained into the existing drainage system with only a few slight modifications depending on the complexity of your garden.

Try to keep the flow of draining water free of obstructions (i.e. containers) so water does not build up on the roof and can reach drains and eves troughs. Maintain a gravel ring and filter cloth layer around the roof drains and over flow scuppers.