By: Darcy Larum, Landscape Designer
Densely populated, large cities can cause what is known as an urban heat island effect. Tall mirrored buildings reflect light and heat, while also restricting airflow. Black asphalt on roads and roofs absorb sunlight and heat. Pollution, fuel emissions and other byproducts of civilization add to the buildup of heat that can surround a city. Essentially, a large metropolis can become a much warmer climate than rural areas around it. Read on to learn more about how to grow a green roof garden.
What is a Green Roof?
Green roofs, also called vegetative roofs or rooftop gardens, have existed for centuries as an effective way to keep a home warmer in winter and cooler in summer. Sod roofs have been popular since ancient times in places like Iceland and Scandinavia.
These days, green roofs are still valued for effectively reducing heat and cooling costs, but also because they can reduce water runoff in areas with high amounts of precipitation, improve air quality in polluted urban settings, create habits for wildlife, increase usable space in the landscape, and help reduce the urban heat island effect.
Green roof garden designs are usually one of two types: intensive or extensive.
- Intensive green roofs are rooftop gardens where trees, shrubs and herbaceous plants are grown. Rooftop gardens are oftentimes public spaces, usually have specialized irrigation systems and may incorporate courtyards, paths and seating areas.
- Extensive roof gardens are more like the ancient sod roofs. They are created with shallower soil media and usually filled with herbaceous plants. Extensive green roofs can be done on a very small scale, such as a birdhouse or dog house roof, but they can also be made large enough to cover a home or building’s roof. If you’d like to try creating green roof gardens, you may want to try it out first on a small structure.
Creating Green Roof Gardens
Before starting a DIY green roof garden project, you should hire a structural engineer to make sure the roof can support the weight of a green roof. Also, make sure to get any building permits required by your city or township. Green roofs can be created on flat roofs or a sloped roof; however, it is recommended that you hire a professional to install a green roof if the pitch is more than 30 degrees.
Green roof kits can be ordered online. These are generally a system of planting trays that can be attached as needed and ordered in custom sizes. You can also make your own planting box frames with 2 x 6s and 2 x 4s. Green roofs cost approximately $15-50 per square foot. This can seem expensive at first, but in the long run green roofs save you money on heating and cooling costs. In some cases, grants for green roof projects may be available through the U.S. Environmental Protection Agency.
Taking accurate measurements is the first step in creating an extensive green roof. This will help you know what to order if you are ordering a green roof kit. If you plan to build a green roof yourself, measurements will help you know how much pond liner, wood, draining media (gravel), weed barrier and soil media you will need.
Green roofs are a system of layers:
- The first layer consists of two layers of pond liner or rubber roofing.
- The next layer is a drainage layer, such as gravel.
- Weed barrier is then placed over the gravel layer and a moisture blanket is laid over the weed barrier.
- More drainage can be added with a layer of wood chips or the final layer of soil medium can be laid. It is suggested that you use a lightweight soilless growing media to keep the overall weight down.
In extensive green roofs, xeriscaping plants are often used. Plants need to have shallow roots and be able to tolerate times of drought and high precipitation, as well as intense heat, high winds, and possible pollution. Good plants for extensive green roofs are:
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Published: May 14th, 2020 at 9:32 am
You Will Need
- Thick, weather-treated timber, 12–15cm in width (for frame)
- Small wood blocks (two-by-four cut-offs work well)
- Water-impermeable sheeting
- Weed matting
- Exterior wood screws
- Pollinator-friendly plants
- Woodchip or compost
Lay the water-impermeable sheeting over the roof, cutting it to size with a slight overhang. Fasten to the side of the shed using a hammer and tacks.
Saw four lengths of timber to match the dimensions of your roof. Screw them to the sides of the shed. Drill drainage holes (2cm) at the lower end.
Put a small block of wood in each corner, screwing through the frame to secure them. This adds strength to the structure.
Pour a layer of gravel into the frame, 2–3cm deep. This improves the drainage of the green roof, which helps reduce soil erosion and aerate plants.
Cut a piece of weed matting large enough to cover the roof and the inside of the frame. Lay it over the gravel and attach to the frame using tacks.
If you’re worried that soil may be too heavy, place potted plants on the gravel instead and surround them with woodchip to reduce weight on the roof.
Raise the Roof
Whether you have a green thumb or not, there are plenty of advantages to a personal garden. Beside the simple pleasure that comes with growing your own fruits and vegetables, maintaining a garden can improve your health and promote sustainability. And for people who think they don’t have the space to create an incredible garden, it’s time to look up.
Sure, the upfront costs and workload may be more intensive, but with the proper preparation, installing a rooftop garden can put a land-challenged green thumb to use.
If you remember only one piece of advice before installing a rooftop garden, remember this: Dirt is heavy and wet dirt is even heavier.
When outlining a rooftop garden installation strategy, first investigate the maximum structural load capacity of the roof. This is where consulting a structural engineer is necessary. Improper planning may cost you dearly in the long run, so make sure to calculate the maximum weight capacity your roof can handle before installation.
Once you’ve established the area that will be converted into a garden, there are some additional environmental factors to consider. Before outlining the shape of your garden, consider the type of sun and wind exposure on the roof.
Obviously, sunlight will be an important factor for growing any type of plant, so make sure you have a good idea of the daily and seasonal sun exposure and shade variations in your particular location.
Also, sun exposure dictates which plants, flowers, herbs, and veggies you can grow. Typically, rooftops receive a lot of direct sun, so if that’s the case, there are many options because most vegetables grow best in full sun (at least six hours of unobstructed, direct sunlight). However, if you’re dealing with a large, adjacent building that blocks sun to your rooftop, more shade-tolerant plants and veggies—such as kale, lettuce, and spinach—will fare better.
Generally speaking, roofs may experience more windy conditions than conventional gardens. Give some major thought into wind protection, both for the garden and passersby below.
A rooftop garden with good airflow is great for most plants, but extremely windy areas can dry out the plants or break the stem system before they can mature. Of course, in addition to broken plants, the last thing you really want is for materials or tools to blow off into the street below.
Before installing an elaborate gardening system, invest in a secured border area or fence to protect it. Consult your local city building department for any necessary permits and regulations regarding fence construction and height regulations.
Drainage and Waterproofing
Once you have planned out your garden’s size and dimensions, the next step is to waterproof and prepare a proper drainage system.
Cutting corners on rooftop drainage and waterproofing is a big mistake. Not only will you have a soggy garden with drowned roots, but you could set yourself up for serious structural damage.
First, install a tapered layer of insulation to the rooftop to channel excess water into a storage tank system or drainage on the ground. This will, of course, depend on your living situation. If you own your building and can do as you please with the property, you may want to consider draining the water into tanks to use as a part of a greywater irrigation system.
On top of the insulation, add a layer of paneling or secure flooring to use as a hard surface base. Deck paneling is one viable option. The objective here is two-fold: provide a base for the garden and keep roots from making their way to the actual roof below, which can cause damage to your home.
On top of the hard surface paneling, it’s wise to add a weatherproofed rubber layer. A pond liner is easy to find, but you may want to splurge on a sturdier option such as an EPDM rubber membrane.
Whatever type of waterproofing layer you use, make sure to choose something with a high puncture resistance. This will provide more stability as you move around the garden.
When installing the weatherproof layer over the roof, extend the layer over the roof’s edge just a bit (a good 5 inches) to prevent rainwater from leaking underneath the waterproof layer.
Rooftop Garden Containers
While occupying the entire rooftop with dirt is enticing, you may want to install special gardening trays to give you more flexibility when moving about the garden. Although you may want to play in the dirt as much as possible, there will be times you’ll want to do a little bit of maintenance without stepping in the soil.
In order to organize the garden (and avoid messy boot prints), consider special gardening containers. Look for lightweight containers that come with integrated drainage systems. These containers are fairly straightforward to install and make removal easier should you move or need to repair the rooftop.
Alternatively, there are companies that deliver trays fully loaded with soil specifically designed for green roofs. Modular planting systems, such as those designed by The Green Grid, can be quite useful—especially since their systems usually come with specific instructions and customer service. However, pre-fab garden containers can be costly.
Repurposing Garden Containers
Instead of converting the entire base of your roof into a garden, you could repurpose discarded materials such as old bathtubs or sinks into creative planters. Even worn-out wheelbarrows or plastic kiddie pools can get a new life as a gardening container.
If you choose to go this route, you may not have to worry as much about your roof’s structural capacity. However, for the sake of healthy plants, you’ll still have to create a drainage system within the repurposed container. Keep in mind that although drilling a few simple drainage holes in any container may keep the root system properly drained, you will still have to protect the roof from the constant water drippage.
To properly protect the roof from water damage, you’ll want to install some sort of water collection system underneath any gardening containers so dripping water does not penetrate the rooftop.
Depending on the size and number of containers in your garden, you have a number of options for protecting the roof. If you just have a few containers and little water drainage, you could place some plastic containers underneath and manually empty the water periodically.
Or, to keep your workload at a minimum, install a collective water gathering system. If you’re feeling extra confident in your DIY skills, you could construct a system to collect both excess drainage water and rainwater to be used in irrigation of your garden. But keep in mind that if you’re going to be using rainwater for irrigation of edible crops, there is some controversy about the potential health effects.
Rooftop Garden Crops
When you begin to design your rooftop garden, think about what types of plants you will grow based on your rooftop’s environment and materials you’d like to use.
Generally speaking, rooftop gardens receive a lot of sun exposure, so look for plants that enjoy direct sun and heat. Most herbs enjoy full sun, so a nice, fragrant herb garden is a great idea. Additionally, if you’re looking to make a fully edible garden, you’ll want to choose plants that grow low and have a thick root system to avoid potential wind damage. Root veggies including carrots, beets, and rutabagas are good low growers. Watermelon, spinach, most leaf lettuces, cherry tomatoes, zucchini, cucumbers, and peppers typically grow well in full sun and make ideal crops for rooftop gardens.
Rooftop Garden Soil
With a good garden design in mind, it’s time to plant!
Planting a rooftop lemon tree orchard would require a lot of intensive structural preparation. But growing a sweet herb or veggie garden bordered with some beautiful perennials is much simpler.
As a general rule, standard green roofs need to hold at least four inches of soil and withstand about 15 to 20 pounds per square foot. This is a general estimation because with a deeper soil capacity, you’ll have more options for planting. However, four inches is sufficient to grow a wide range of flowers, veggies, and herbs.
For most types of rooftop gardens, you may want to consider using a specific type of planting medium instead of common potting soil. Unless you have a lot of space to create your own compost system, a lightweight soil specifically made for green roofs is ideal. Specially made rooftop garden soil generally contains a highly engineered blend of minerals and organic matter that is genetically designed to be lightweight, retain water, and ensure proper drainage. This is key for a garden environment with lots of direct sunlight and little shade.
For the most part, planting a rooftop garden is very similar to most basic, in-ground gardens. It takes a lot of planning and work to get a garden to thrive, and rooftops are no different. Once you have a sound structural plan set up that allows for proper drainage, you’ll be ready to plant any number of plants, flowers, herbs, and veggies.
If this article has you interested in starting your own rooftop garden, you might want to check out these custom sinks to use as planters. — http://www.custommade.com/gallery/custom-sinks/
Where to start?
To design a roof garden, the first thing to do is to ask an expert for advice to evaluate the maximum weight that the roof is able to withstand. This is because the soil tends to double in weight with rainwater, resulting in a water saturation which, in some cases, can even exceed 200 kg/m². In addition to the thickness of the substrate, the maximum load is also determined by the type of plants to be cultivated and the furnishing solutions that may be inserted.
If you are passionate about garden design and you want your garden to be always looked after, we advise you not to overload it with excessive weights, opting for essential accessories that allow you to optimize the walkable area without adding too much weight to the environment.
10 great plants for a living roof
Rooftops are tough place for plants: they’re vulnerable to intense heat, cold, wind and drought, plus they can’t support a lot of weight, so the plants need to grow in just a few inches of soil (actually, ultra-lightweight soilless growing mediums are typically used). Thus, it’s plants that grow naturally on desert cliffs, alpine crags, and other such inhospitable places that are used for green roofs. Fortunately, these include many truly stunning species—some exquisitely beautiful, others absolutely bizarre and even a few that are edible or otherwise useful. Green roof plants fall into four general categories:
Related: Need a Rooftop Farm? Call this Company
These are the mainstays of any green roof and should form the bulk of the planting, unless provisions are made for the roof to support soil deeper than the 3 to 4 inches that is typical. These tiny succulents thrive with virtually no water or soil. They are available in a kaleidoscope of colors, giving a broad palette with which to design your living roof.
Sedum spp. Also known as stonecrop, because the succulent foliage resembles smooth, polished stone, sedums are the royalty of living roof plants. There are literally hundreds of varieties, found growing in cliff-side cracks and crannies around the world and were the first species employed in the green roof industry. With so many distinctly colored varieties available, you can paint a beautiful picture on your roof.
Sempervivum spp. Called houseleeks (because they were used as a traditional Scandinavian rooftop plant) by some and hen and chicks by others (the mature rosettes “give birth” to tiny replicas of themselves as they spread), sempervivum means “evergreen” in Latin, indicating that your roof will be attractive year-round with this type of succulent. Like sedums, they stay low to the ground and come in many colors.
Delosperma spp.are spreading succulents grown for their daisy-like flowers, which bloom throughout the growing season. There are white, yellow, red and purple varieties and most have the habit of changing their shade of color as the flowers fade, creating a monochromatic effect
Aenoium arboreum is a variety of houseleek that grows as a tiny tree (usually less than two feet tall) that looks like it would be more at home on Mars than planet Earth. It’s not a spreading ground cover like the other succulents in the list, but it can create a bit of vertical variation in your roof garden. The variety or (black head, in Dutch or German) will create plenty of interest with its color as well—it’s such a deep purple that it’s almost black.
Related: How to Make Your Own Green Terrarium
Most grasses would fail on a living roof, unless they were watered constantly in the summer. However, there are a few that have what it takes to withstand the conditions. To be honest, most green roof grasses are not considered as such from a botanical perspective, and are more accurately termed “grass-like plants”. Like the succulents, they are good for covering a lot of territory and create a pleasing contrast when combined with succulents. Many seed themselves, making your rooftop garden a self-replenishing landscape.
Armeria maritima is not at all related to what grows in lawns, but the foliage appears as a tidy green clump of grass. In nature, it grows in ocean-side cliffs and dunes (hence the name maritima), making it well adapted to rooftop conditions—especially those by the sea. It is also called sea thrift and, unlike any grass, it is crowned with pink or purple flowers in summer.
Carex nigra is technically considered a sedge and is often used on living roofs, because its roots require less soil than most other grasses, or grass-like, plants.
These are used more sparingly and benefit from a bit deeper soil than the other species listed here. This can be accomplished by mounding the planting medium here and there to create little wildflower hummocks. Use them for a taller accent in sporadic locations in your living roof design.
Aster alpinus is an aster from alpine regions, meaning it is no stranger to intense weather and thin soil. Nonetheless, it produces brilliant sprays of deeply saturated purple flowers with yellow button centers, which attract hordes of butterflies.
Achillea millefolium is commonly known asyarrow a wildflower that, unlike asters, will spread across the surface of your living roof as a ground cover. This powerful medicinal plant has ferny, aromatic foliage and tall flower stalks capped with broad concave blossoms which make great landing pads for butterflies. Yarrow has the added benefit of tolerating light foot traffic.
Related: The Biggest Hospital in North America to Feature a Green Roof with Medicinal Herbs
This is where a living roof crosses over to become an herb garden. Many of the most common culinary herbs happen to grow in dry, rocky places, making them ideal candidates for a green roof. The varieties listed here are low-growing, wide-spreading groundcovers the other key trait for a living rooftop carpet.
Thymus vulgaris is the standard garden variety of thyme that creeps along just a few inches tall and, like yarrow, can tolerate being walked upon. It makes a luxurious aromatherapy bed for rooftop sunbathing and, of course, can be harvested on demand for the kitchen.
Origanum vulgare is common oregano. Like thyme, it is native to the rocky hills of the Mediterranean basin and it can bring that special flavor to your rooftop if you choose to plant it. It’s also a ground cover, growing 4 to 6 inches tall.