How to Build an Edible Rooftop Garden
I am smitten with award-winning landscape architect, Senga Lindsay’s, book on unique and urban applications of edible landscaping, Edible Landscaping: Urban Food Gardens That Look Great (Harbour Publishing). You will be too when you get a look at this wealth of information from the book from this project:
The Edible Rooftop
No ground to grow your edibles? Think up—onto your rooftop! If you don’t want to allocate valuable real estate to creating an edible garden but still want the pleasure of growing vegetables and fruits, a green roof may be the solution for you.
Although green roofs have been around for centuries, the concept was popularized in Germany about 50 years ago. A green roof is simply a living plant cover on a structure. The most common example seen today is an extensive roof where sedums or grass are grown in soils 4–6 in. (10–15 cm) deep. Now with urban agriculture gaining momentum, a new type of roof garden has emerged—the edible kind! With less than 1 ft. (30 cm) of soil, you can grow most vegetables, herbs and even some fruits on top of your house, shed or garage.
B. Waterproof roof with drainage layer encourages drainage
C. Walkways 2 ft. (60 cm) wide allow for safe movement and bed access
D. Lightweight green-roof soil reduces the load on the roof structure
E. Raised planter beds with open bottoms lined with filter fabric allow for drainage
F. Guardrails line the perimeter of the roof
G. Trellising on the north side supports vertical planting
H. Beehives boost pollination
I. Good drainage system
J. Rain barrels collect water runoff from roof drains
Roof Dimensions: 28 × 24 ft. (8.5 × 7.2 m) 672 sq. ft. (62 sq. m)
Total Area of Edibles: 250 sq. ft. (23 sq. m) of raised beds, 120 sq. ft. (11 sq. m) vertically
The garage structure was upgraded and raised planters of various sizes were added. Using the square foot method (see page 119) and five raised planters (total of 228 sq. ft./21 sq. m) filled with only 10 in. (25 cm) of lightweight green-roof soil, I harvested fresh produce through most of the year. The costs to install my rooftop garden were as follows:
- Lumber, filter cloth and hardware: $500
- Structural reinforcement: $300
- Green-roof soil (8 cu. yd. or 6 cu. m @ $65/cu. yd.): $520
- Seeds: $100 or less (and I have enough seed for the next 5 years!)
- Labour: Free (I did it myself!)
Before You Begin
There are a few considerations to take into account before diving into construction—and they are important ones.
Load on the Roof
The first step is to evaluate your roof’s loading capacity. This is the amount of weight your roof structure can support and includes everything: planter boxes, soil (when wet), possible water storage, weight of crops at maturity, equipment and such temporary loads as people and snow. This may be your biggest expense and will determine whether you are willing to pursue an edible roof or not. Working with a structural engineer is a must!
Contact a structural engineer and discuss what you intend to design; for this, a plan of your garden is required. The engineer will evaluate the possibility of carrying out the project, what it would take in terms of reinforcement, and the influence of obstacles (vents, chimneys, etc.) and/or possibility of eliminating or moving them.
Review your municipality’s regulations. Architects or building designers can often assist you in interpreting building-code requirements for green roofs.
Sun and Wind Exposure
Consider sun and wind exposure. Edible plants require a minimum of six hours, with heliophilous plants such as tomatoes needing at least ten. Study your patterns and hours of sunlight and note adjacent buildings that may create additional shadows. Wind is often stronger at rooftop heights than on ground level and can cause serious damage to plants. Structural windbreakers may need to be designed in conjunction with the building frame and they must be able to withstand wind loads.
Depending on how elaborate you want to go, additional considerations may be:
- Storage or areas for composting
- Rainwater collection system—possibly from adjacent roofs with storage on garden roof
- Electricity for running power tools and equipment
- General security and lock off to limit access
My Edible Green Roof
My edible green roof is bare bones in design and located on top of a two-car garage. I wanted maximum vegetable production but did not want to use up my at-grade outdoor dining areas or wildlife gardens. I also wanted to keep a couple of beehives for honey and pollination but did not want ninety thousand bees buzzing around at ground level.
Creating an Edible Green Roof on a Garage
Prepare Your Roof Structurally
Ensure it meets municipal building and safety requirements for a green roof. Also, check that your roof membrane can support walking and planters—you don’t want to risk a leak! Flat roofs are best.
Build Planters to Hold Soil
Here I use 2 × 4 fir on sleepers, allowing for 10 in. (25 cm) of soil. Gravel on the roof encourages drainage, which is an absolute must, so check that your roof drains properly!
Line Your Planters
Use weed block to contain soil while allowing drainage.
Add Green-roof Soil
Lightweight soil specific for green roofs is placed in raised planters (soil weight: 75 lb. per sq. ft. or approx. 300 kg per sq. m). A bonus is that this mix starts out with no weed seeds.
Plant Vegetables, Herbs and Fruit
Swiss chard, watermelon, iceberg and assorted leaf lettuce, vine and cherry tomatoes, zucchini, cucumbers, peppers and everbearing strawberries are my favourites.
Tip: To grow vegetables that require more soil depth without committing to larger roof loads, use deeper containers in strategic spots (over your roof beam or posts). Always check with your structural engineer on any roof-load issues.
Like this project? How could you not?! Whether your outdoor oasis is a boulevard, backyard or merely a balcony, everyone can get on the fast track to creating an edible and aesthetically pleasing garden with this practical guide. Edible Landcaping (Harbour Publishing) includes advice on the most resilient and rewarding edible plants and detailed plans for fifteen types of urban gardens from green roofs, community gardens and children’s gardens to hanging container gardens enabling anyone with an interest in growing their own food to design a space to perfectly match their environment and needs.