If you are considering building a vegetable garden on the roof, you will first have to consider the structural integrity of the building.
Flat rooves are not usually designed to carry a lot of weight, apart from the roofing materials. If you are going to build a vegetable garden on your flat roof, how much do you think all the materials for it will weigh?
The load bearing capacity of your roof (how much weight it can safely bear without collapsing) must be taken into consideration. Is your flat roof strong enough to carry the weight of your vegetable garden? Will you have to add strenghtening beams to it before you can start building your grow beds and so on? How much will the soil that you are going to use weigh?
Wet soil, the kind of soil flowers and vegetables grow in, weighs a lot more than dry dust or compost as it comes in the bags. Wet soil actually weighs more than water on its own.
To reduce the weight of your vegetable garden, as well as eliminating a lot of the maintenance work and expense involved, you should consider building a deep water culture aquaponics system, instead of using soil beds to grow your vegetables in your rooftop vegetable garden.
Aquaponics is the low-tech, but very scientific technique of growing plants in nothing more than fish water. It does not use soil. So you can significantly cut down the weight of the grow beds by replacing them with hydroponic deep water tanks of around 2 feet in depth.
These do not have to look ugly, and can conform to the space you have got up there.
However, you will also have to include an equal volume of fish tanks, since in aquaponics you grow fish and vegetables together in the same recirculating aquaculture system. No fish, no fish water.
Whether you grow edible fish to eat, or just ornamental fish such as koi carp is entirely up to you. But you need at least 250 gallons of fish tanks to 250 gallons of hydroponic tanks. That is 500 US gallons of water.
500 US gallons of water weigh in at exactly 4,172.63 POUNDS. However, the equivalent volume of soil will weigh more. Soil, depending on moisture content, weighs in the neighborhood of 75 to 100 pounds per cubic foot, so assume maximum moisture, and you can immediately estimate how much more this will weigh than water. 500 US gallons is 66.84 cubic feet. Multiply that by 100, and you get 6684 pounds. That is 2511,37 pounds MORE than that same volume of water weighs.
So your aquaponic system will weigh a lot less than just the same area under soil beds. This alone makes it more suitable for rooftop vegetable garden construction.
Also, an aquaponic system will grow you up to 40% more food, conservatively speaking, than your soil garden. This is because you can grow your plants at up to half the spacing between them that you can in soil. They also grow up to twice as fast in fish water, without any loss of taste or plant health.
So you get more plants, quicker, in aquaponics. Fish water just happens to be miracle grow organic fertilizer.
Of course, as they get big enough, if you are rearing edible fish, you get fish to eat or sell off. If you are growing koi, you get valuable large koi to keep as pets or sell on to other koi carp enthusiasts. Aquaponics gives you two products, fish and vegetables, from the same amount of space that a soil vegetable garden would only give you one, vegetables.
To justify the expense of constructing a rooftop vegetable garden, especially if it is a commercial one, you need to maximize the productivity of the space. This is especially true in urban settings where high mortgages and rents come into the costs of your vegetable garden rooftop space. Aquaponic systems are far more productive for the space occupied, with less hassle and expensive inputs such as labor and fertilizer.
You don’t have to buy any fertilizer with aquaponics. The fish provide all the fertilizer you will ever need.
You aquaponic system is also largely automated. With a normal raised bed rooftop vegetable garden, you have all the normal weeding, watering, and bending over to do. This is not necessary at all with aquaponic systems. There is no weeding, watering or back breaking work to do. The crops lift out on their floating rafts onto a trestle table for harvesting purposes. Weeds cannot grow on polystyrene rafts. The fish and pumps do the fertilizing and irrigation for you, automatically. Friendly insects do the biological pest control as they hunt down the greenfly, etc.
You may, however, need to build a greenhouse over your rooftop vegetable garden in aquaponic systems. This is because the best, most productive aquaponic systems can only produce crops and fish year-round in temperate climates if they have some protection from the weather, especially in winter. Greenhouse conditions also give you more control over your biological pest control measures, and make it more pleasant to work there in bad weather.
By controlling the climate in a greenhouse, you can ensure that your rooftop vegetable garden in your aquaponic systems can reliably perform all year round, and supply your family and/or local community with fresh food grown locally.
This ensures the food security of your loved ones and neighbours, which is currently threatened by climate change and the unstable world economy.
Because you can grow fish as well as vegetables on your local flat rooftop, (providing the building is strong enough to carry the weight, as above), you are bypassing a lot of costly food delivery processes, such as the cost of transporting your food halfway round the world to the store. This means that you can grow food more cheaply locally than imported food.
One of the ways to reduce rooftop vegetable gardening costs is to use recycled energy from the building to heat your greenhouse and water. Heat exchangers can be fitted to take waste heat from heating and cooling systems and transfer it to your greenhouse air and water systems, such as your aquaponic systems.
There are many ways to make a rooftop aquaponic vegetable garden energy efficient without breaking the bank. Solar greenhouse designs can be downloaded from the web. Wind turbines and solar panels can be installed along with the right kind of storage batteries, that will save you a lot of electric bills in the long term. The batteries mean you can run pumps and heaters at night as well.
Insulating your floors and tanks can also save energy.
The list of improvements and features you can build into a rooftop greenhouse is pretty long, and many of them actually can be constructed using recycled materials from the trash.
Did you know that one of the most advanced insulated greenhouse designs uses bubble wrap for the greenhouse walls?
When did you last see a skip full of discarded bubble wrap? Or wood from old forms for concrete building moulds?
You see what I mean.
Perhaps it is worth getting that flat roof to which you have access properly surveyed for weight loading?
Aquaponics Global Anthology 1 is available for instant download and to print out here: [paiddownloads id="1"]