Modular Green Roofs: [Ultimate Guide]
As our climate changes, we are constantly looking for ways to reduce the impact that we have on it. One way to do this is by investing in green roofs. Green roofs reduce the amount of stormwater runoff and heat island effect while improving energy efficiency and biodiversity.
A green roof can be made with a variety of plants, growing mediums, and installation methods. However, not all green roofs are created equal.
One of the most important decisions you will make when building a green roof is the installation method. Building your green roof you will mainly have two options.
You can either go with a traditional green roof that you install on-site or you can go with a modular green roof that comes pre-planted.
Both installation methods have their pros and cons which I am going to explain in this post. Let’s start with traditional green roofs.
Traditional Green Roofs
Traditional green roofs are the most common types of green roofs. This type of green roof is installed on-site starting from the bottom up. They can be extensive or intensive roofs depending on the project’s goals.
Installing a traditional green roof start by laying out a waterproof membrane on top of your roof decking, followed by a drainage mat, and finally the growing medium. Once the growing medium is in place, you can start planting your plants.
Since you are installing everything on-site from the bottom up, this type of installation allows for ultimate flexibility when it comes to the selection of plants, growing medium, and drainage solutions you may use.
Traditional green roofs are labor-intensive, require planning and careful execution, and are not always the best option for all projects. They also require more maintenance and are not very forgiving of potential mistakes you may make during the design or implementation.
Because once installed, a traditional green roof is pretty much set in stone. If you need to make significant changes, it can be a costly and time-consuming process.
Advantages of Traditional Green Roofs
- Inexpensive
- Flexible when it comes to the plant selection
- Can be installed on any type of roof
- Easy to customize
Disadvantages of Traditional Green Roofs
- Labor intensive
- Requires a lot of maintenance
- Not forgiving potential mistakes made during installation
Modular Green Roofs
Modular green roofs are a pre-planted growing medium that is sold in modules. Plants are either fully or partially pre-grown in the modules and then shipped to the job site.
Modular green roofs are pre-engineered which makes the installation process a lot easier. There is no guesswork involved with their planning and installation. Because the manufacturer takes care of the design and engineering, you do not need much technical knowledge to install a modular green roof.
Your selected plant species are already in the modules that have been pre-grown to the perfect size and in the right soil mix. This means that you do not have to worry about things like soil selection, soil testing, or planting depth.
The most important downside to modular green roofs is that they are more expensive than traditional green roofs.
Because there is another company involved in the planning, design, and engineering of modular green roofs, the cost is passed on to the consumer.
Advantages of Modular Green Roofs
- Pre-engineered
- Easier installation process
- No guesswork involved
- Plants are already in the modules and have been pre-grown
- Instant greening
Disadvantages of Modular Green Roofs
- Not flexible in terms of plant selection
- Requires careful transportation
- A company is involved in the planning, design, and engineering of modular green roofs which increases the cost.
What are the types of modular green roofs?
The word “modular green roof” doesn’t refer to a single product design or type. It is an umbrella term that refers to all pre-planted green roofs that come in modules.
Therefore, depending on the company that you buy from, the modules may be of different shapes and sizes. They can even be made of entirely different materials.
However, for ease of explanation, we will divide modular green roofs into three categories. These are the following:
Tray systems
Tray systems are the most common types of modular green roofs. They are made up of square or rectangular modules that fit together like puzzle pieces.
These tray systems come with trays that contain soil and plants which sit on top of your roof decking. The trays are then connected together to create one large green roof.
Because they arrive as complete or partial grown units, tray systems are very easy to install. All you have to do is lay them out on your roof, connect the trays together, and add the finishing touches.
Tray systems are very suitable for the smaller green roof or non-professional installation. Since the growth medium is encased in trays, tray systems are also very easy to transport. They also stand wind uplift or erosion during the critical time period immediately following installation.
If the property owner wants to spot-check a point (for instance a possible leak) under the tray system, the specific tray(s) can be easily taken out without causing any interruption to the entire deck.
This makes tray systems very forgiving of user errors during installation. Because the plants are in the tray, they can be removed or replaced if needed.
The downside to vegetated trays is that they are not very aesthetically pleasing. If you would stack up all the trays right next to each other, the roof may look like a green checkerboard.
However, this isn’t a particular problem to worry about. Most green roof trays can be covered with an aesthetic-looking siding that is used to hide the tray edges.
The siding wraps around the sides of the trays and can be custom ordered to fit the specific look of any roof.
Mat Systems
Mat systems are similar to vegetated trays in that they use a pre-grown growing medium. However, the plants are not in a tray but instead are woven into a mat. The mat is then installed on top of your roof decking and covers the entire surface area.
One advantage to mat systems is that they are very aesthetically pleasing. The woven nature of the mat makes it look like a carpet and since it covers the entire surface area, there will not be any checker-board patterns.
The downside to mat systems is that they are not as forgiving as vegetated trays if mistakes are made during installation. If you have to remove or replace a section of a mat system, it can be very challenging and time-consuming.
Sack Systems
Sack systems are the least common type of modular green roof. This is where you have a sack that contains soil and plants which sit on top of your roof decking. The blocks are then connected together to create one large green roof.
You can think of block systems as tiling a roof but with individual sacks. Sacks are usually made of soil that has been pre-seeded with grass, flowers, or other plants. The sacks are then united together to create a modular green roof.
Since the sacks are all the same size and shape, they can be installed in any pattern you want. If there is ever a problem with a certain sack, it can be easily replaced without having to remove the entire green roof.
The downside to the sack systems is that they are the most expensive type of modular green roof. Because each block is highly customized and a lot of engineering goes into the design phase, it can cost quite a lot of money.
What are the key design components of modular green roofs?
There are many design components to consider when planning your modular green roof.
Intensive vs Extensive Roof
One of the first decisions you have to make is whether you want an intensive or extensive green roof.
If you plan to have larger plants with deep root systems, you want to go with an intensive roof. However, if you plan to have smaller plants with shallow root systems, you want to go with an extensive roof.
If you want to plant trees and shrubs a modular green roof may not be the best option. However, you can build your intensive green roof using deeper individual trays with a deeper substrate volume to accommodate larger plants.
There are tray systems up 50 cm in depth which can easily provide sufficient root space for large plants like trees and shrubs.
System Weight
Another important consideration is the weight of the system. The heavier the system, the more load it will put on your roof structure.
Most modular green roofs weigh around 10-20 pounds per square foot, However, if you plan to plant larger and deeper rooted plants, this weight can be as much as 40 pounds per square foot.
If your roof is an older structure with limited load capacity, it is best to go with a lighter system. But in all cases, you are highly recommended to have a structural engineer examine your roof to ensure it can handle the weight of a modular green roof.
The slope of the roof
The slope of your roof will also play a role in the type of modular green roof you choose.
The steeper the gradient, the more material, and engineering go into designing modules in order to accommodate different slopes without compromising the structural integrity of the roof.
Most modular green roofs are designed for slopes less than 20 degrees. For higher slopes, it is best to get in touch with your modular green roof company and/or structural engineer to discuss the feasibility of installation on your roof.
Maintenance
One of the most important considerations is how much maintenance your roof will need. Required maintenance will vary based on the type of plants you choose, the depth of your substrate, and whether or not you have a green roof mat, tray, or sack system.
Generally, you will need to water your plants regularly during the hot summer months, and may also need to fertilize them. Check with your modular green roof company for a more specific maintenance schedule.
Transportation
Depending on the size, weight, and depth of your modular green roof system it may require transportation by truck and even crane. This will also affect the installation time and cost of your project.
The more complex the design, weight, thickness of substrate, or size of your modular green roof the longer it may take for transportation and installation to be completed.
Cost
The last but certainly not least consideration when choosing a modular green roof system is the cost.
Modular green roofs can be expensive especially if you will have specific requirements like plant selection, or a high slope installation.
Generally speaking, intensive green roofs are more expensive than extensive ones. Because they require thicker substrates and deeper planting trays for larger plants with deep roots systems.
For modular green roofs that need to be installed on steeper roofs, you will likely pay more for the installation.
Fire rating
Fire rating for green roofs is not a standard feature to look for. However, depending on where you live the code may require it.
Most modular green roof companies will have a fire rating for their products, but you should check with your local building department to see if you need to meet a certain fire rating for your selected product.
Ideally, green roofs should be built to withstand the spread of fire by taking the following measures:
- Increasing the non-combustible content of the growth medium
- Reducing the organic content of the growth medium
- Keeping the system from drying out.
Because extensive rooftops are not typically irrigated, the fire risk is addressed by the specification of the build-up and fire breaks, as well as by minimizing the organic content.
The GRO Green Roof Code requires a substrate depth of 30mm and an organic content of 20%. Succulent plants are also recommended as they retain water and thus lessen the risk of drying out the substrate.
The ANSI regulation also refers to fire-resistant vegetative systems as “succulent-based” and “grass-based,” with the growing substrate including at least 80% inorganic matter in both cases. The non-vegetative portions of the roofs must have ASTM E108 Class A systems.
Check with your local municipality and/or insurance company for guidance on what types of certification or prequalification they may require for your modular green roof system.
Environmental Impact
Modular green roofs have a positive environmental impact by absorbing rainwater, providing insulation for the building below them, and preventing stormwater runoff. They also provide habitats for birds and insects.
However, they can still be an environmental concern due to the materials used in their construction like plastics and PVCs.
Make sure to prefer modular green roof products that are made of sustainable materials like recycled plastics, cork, bamboo, and wood fibers if possible. Doing so will minimize the environmental impact of your roof.
Conclusion
Modular green roofs can be a great alternative to traditional methods of rooftop gardening. Because they don’t require a lot of labor or time to plan and install multiple layers of soil and plants.
Therefore, they are perfect for people who want to have a green roof but don’t have the time or resources to install a traditional one.
Since they are typically pre-engineered and tested, they eliminate the need for soil testing and design. This can save you time and money on your project.
However, if you would want to have the ultimate flexibility in choosing plants and planters, traditional green roofs may be a better option for you. Because modular green roof systems are pre-assembled and typically cannot be altered.
Finally, no green roof is truly maintenance-free. They require some tender love and care to ensure that they stay healthy over the years.
Therefore, you should expect to be responsible for watering your roof, monitoring its growth and health, as well as removing dead plants from the trays. Just make sure to consider what type of maintenance you are willing to do prior to installing a green roof on your building.
