Green roofs : A Solution to Multiple Problems
- Justine Gratton
- Mar 28
- 9 min read
Did you know that urban building rooftops cover up to 25% of a city's total surface area? These vast impermeable surfaces, while essential to our infrastructure, have unexpected impacts on our environment. They contribute to urban flooding and put pressure on our sewer systems, especially during torrential downpours, which are becoming more frequent due to climate change.
What if these same rooftops could be part of the solution?
Imagine if rooftops could absorb rainwater, regulate temperatures, and provide a sustainable green space. Green roofs could be the solution to these pressing environmental challenges.
In this article, we will explore:
What a green roof is and the different types available.
The numerous benefits they offer for both the environment and buildings.
Their applicability to different types of constructions, whether residential or commercial.
By the end of this article, you will have all the information needed to evaluate whether a green roof is suitable for your building and if you can install one yourself.
Understanding Green Roofs
A green roof, also known as a vegetated roof, is a roofing system that is partially or entirely covered with vegetation. But it’s not just about planting flowers on a roof! These systems consist of multiple layers designed to support plant health while protecting the building structure.
A typical green roof includes :
Waterproof Membrane:
Protects the roof from water infiltration.
Drainage System:
Removes excess water while retaining the necessary moisture for plants.
Substrate:
A growing medium, often made of lightweight soil or a specialized mix, that supports vegetation.
Adapted Vegetation:
Plants that are resistant to local climate conditions and require minimal maintenance.
Green roofs can transform a simple impermeable surface into a thriving ecosystem, enhancing building durability while providing numerous ecological benefits.
Now that we understand what a green roof is, let’s take a closer look at the many advantages they offer—for both building owners and the environment.
The Benefits of Green Roofs
Green roofs do more than just enhance a building’s aesthetics—they provide tangible benefits that improve both the environment and the structure itself.
Here are the main benefits of a green roof
1. Improve Air Quality
Green roof plants capture carbon dioxide and fine particles while producing oxygen. This microclimate helps purify the surrounding air, a crucial benefit in polluted urban areas.
2. Reduction of Urban Heat Islands
Green roofs help mitigate urban heat islands by absorbing less heat than conventional surfaces. Vegetation cools the surrounding air through evapotranspiration, creating a more comfortable urban environment.
3. Property Value and Aesthetic Enhancement
A vegetated roof is not only visually appealing but also increases property value. In some cases, it may even qualify for subsidies for ecological projects.
4. Thermal Insulation and Energy Savings
A green roof acts as an extra layer of insulation, reducing heat loss in winter and keeping buildings cooler in summer. This results in lower energy costs, making it an excellent option for budget-conscious and eco-friendly property owners.
5. Extended Roof Lifespan
By shielding the roof from UV rays and extreme temperature fluctuations, a green roof significantly extends the lifespan of the underlying waterproof membrane.
6. Sustainable Stormwater Management
Unlike traditional roofs that directly discharge water into sewer systems, green roofs absorb a significant portion of rainfall.
They:
Reduce the risk of urban flooding
Ease the burden on municipal infrastructure, especially during heavy rainfall
Enable water reuse for irrigation of rooftop plants
Can a Green Roof Be Installed on Any Building ?
The short answer is: yes, but under certain conditions.
While green roofs are an incredibly versatile option, their installation depends on several key factors, including the building structure and the type of green roof chosen.
Green roofs are generally designed for flat roofs, as this configuration allows for even weight distribution and simplifies the installation of the necessary layers. Although adaptations are possible for other roof types, this article focuses on flat or low-slope roofs, which are the most practical and common choice for green roof projects.
New Constructions : The Ideal Scenario
Buildings in the design phase offer maximum flexibility. By integrating a green roof from the planning stage, architects and engineers can adapt the structure to support the permanent load associated with green roofs, including:
The substrate and vegetation.
Retained water during rainfall.
This approach helps avoid future reinforcement work and optimizes project costs right from the start.
Montreal's Approach to Green Roofs
The revised regulation of the City of Montreal has, for several years, required that the roof structures of new non-combustible buildings over three stories high and larger than 250 m² be designed to support a Type 1 or extensive green roof—even if the green roof is not installed at the time of construction. The same requirement applies to building extensions that meet these criteria.
More recently, some boroughs have implemented even stricter regulations, requiring a green roof on all new buildings over 1,000 m², except for residential buildings. This initiative specifically targets large impermeable surfaces. These measures reflect Montreal’s commitment to sustainable development, tackling urban heat islands and improving stormwater management.
Retrofitting Green Roofs on Existing Buildings
For existing structures, it is crucial to conduct a structural assessment by a professional. This analysis determines whether the roof can support:
The weight of the different layers (membrane, substrate, vegetation).
Additional loads from rain or snow.
In some cases, structural reinforcements may be required, which could increase the overall project cost.
Types of Green Roofs
Not all green roofs are the same. Depending on needs, budget, and building characteristics, there are three main types of green roofs: intensive, extensive, and modular (tray-based) systems.
1. Intensive Green Roofs
An intensive green roof is a true green space, often accessible and designed like a garden.
This type of roof allows for the addition of:
A diverse range of plants, including shrubs and trees.
Landscaping elements, such as pathways or benches.
Intensive green roofs typically require a minimum of 300 mm of substrate to support the root growth of larger plants. This option is more expensive and requires a structure capable of bearing significant loads. Additionally, it demands regular maintenance to ensure plant health.
2. Extensive Green Roofs
Extensive green roofs are designed to be lighter and require minimal maintenance. They have a thinner substrate and use hardy plants, such as sedums, which require little water and care.
Key Advantages:
Ideal for residential projects or existing buildings.
More affordable than intensive green roofs.
Perfect for roofs that are rarely or not accessible.
3. Modular or Tray-Based Systems
Modular systems consist of pre-fabricated trays or modules containing both the substrate and vegetation. They offer:
Simple and quick installation.
The flexibility to remove or replace sections easily.
An ideal solution for individuals who want to be actively involved in the project.
Each type of green roof has unique advantages, but all require careful planning.
If you’re considering a green roof project, an important question arises:
Can you install it yourself, or should you hire a professional?
For More Information
For more specific information related to green roof planning, you can refer to the following links:
Technical Guide for Green Roof Construction – City of Montreal
Technical Criteria for Green Roof Construction – Régie du bâtiment du Québec (RBQ)
If you're considering a green roof project, an important question arises: Can you install it yourself, or should you hire a professional?
DIY or Professional Installation ?
The idea of installing a green roof yourself can be appealing for both economic and ecological reasons. However, feasibility depends on several factors, including the type of green roof and your building's configuration.
To maximize your chances of success, consider a modular solution. These pre-assembled systems offer great flexibility and significantly simplify the installation process.
Leave the waterproofing to a professional – this step is crucial to ensure durability and prevent water infiltration issues. Once the membranes are in place, you can take over and install your modular green roof system.
Steps for a DIY Modular Project :
Assess the Roof Structure
Before starting, make sure your roof can support the weight of the materials and plants, even during heavy rainfall.
Select Suitable Materials
Pre-vegetated modular systems and hardy plants, such as sedums, are ideal for this type of project.
Installation Process
Once the waterproof membrane has been installed by a professional, you can follow these steps:
Add a drainage layer to manage rainwater.
Install the modular or vegetated trays.
Ensure the systems are securely fastened and adapted to local climate conditions.
These steps are achievable with basic tools and some preparation. They help reduce costs while allowing you to take an active role in creating your green space.
When to Hire a Professional ?
In some cases, it is strongly recommended to entrust the installation of a green roof to an expert:
For Intensive Roofs :
Their heavy load and complex design require professional expertise to ensure the structural safety and durability of the system.
If Structural Modifications Are Needed :
If the current structure is insufficient, reinforcement work will be necessary to prevent any risk of collapse or damage. The intervention of an engineer or specialized contractor will therefore be required to properly assess and strengthen your roof.
To Ensure Waterproofing :
The installation of the waterproof membrane is a crucial step that must be carried out by a professional. A mistake at this stage can lead to water infiltration, which can be costly to repair.
For Regulatory Compliance :
Complying with local regulations, such as those in effect in Montreal, may require the involvement of specialists.
For Large-Scale Projects :
Green roofs on commercial or institutional buildings, or projects requiring complex coordination, benefit from the experience and resources of a specialized team.
Whether you choose to install it yourself or hire a professional, it is essential to understand the costs associated with a green roof. Let's now examine how much you might spend and the factors that influence the budget.
Green Roofs : Simplicity and Lightness
Installing a green roof is an investment that can vary significantly depending on several factors. Understanding these costs is essential for planning your project and assessing its return on investment.
Factors Influencing the Cost
Several factors determine the cost of a green roof*:
Types of Green Roof
Intensive :
Higher cost, ranging from $30 to $50 per square foot, including heavier plants, a deep substrate, and landscaping elements.
Extensive :
Lower cost, approximately $15 to $30 per square foot, thanks to its lightweight substrate and low-maintenance plants.
Modular :
An intermediate option, around $10 to $20 per square foot, featuring pre-assembled trays or modules.
Considering that a conventional elastomeric membrane roof costs between $10 and $25 per square foot, the additional cost for greening can range from $10 to $40 more per square foot, depending on the type of green roof chosen.
Structural Work Cost
If your roof requires reinforcement to support a green roof, this work can add 20 to 50% to the total budget.
*Pricing information gathered from various Quebec roofing contractor websites.
Return on Investment & Available Grants
Although the initial investment is higher than that of a traditional roof, a green roof provides long-term financial benefits:
Reduced energy costs
Potential savings of 15 to 30% on heating and cooling bills.
Extended lifespan
Protection against UV rays and temperature variations can double or even triple the lifespan of the waterproof membrane.
Available grants
In Montreal, incentive programs such as RénoPlex encourage the installation of green roofs, helping to reduce costs for property owners. Through this program, homeowners can receive a subsidy of $8 per square foot ($88/m²) for green roof installation.
After exploring the costs and benefits of green roofs, let’s focus on a particularly interesting solution for those seeking a simple and efficient installation: modular systems. These pre-assembled or easy-to-install products offer great flexibility, allowing you to create a green roof tailored to your needs.
Here are some examples of modular products available in Quebec.
Modular Green Roof Products in Quebec
Modular systems for green roofs stand out for their simplicity and flexibility, making them a popular choice for both residential and commercial projects.
Pre-Vegetated Trays Characteristics :
Lightweight substrate adapted to Quebec’s climate
Resilient, low-maintenance plants
Integrated drainage system
Pre-vegetated trays: Pre-rooted plants for an instant green effect after installation
Trays made from recycled plastic materials
Simple, quick, and flexible installation
Removable design for easy roof maintenance or repairs
These modular products make green roofs accessible, even for residential or small-scale projects. With their unique features and ease of installation, they pave the way for a broader adoption of green roofs in Quebec.
Examples of Modular Products Available in Quebec
1. Tundra box Sopranature de SOPREMA (Qc) - Vidéo
2. Ohasis Tech (Qc)
A modular system entirely made in Quebec!
3. SkyScape Pré-noyau de Holcim Elevate (On)
4. Hydropack (NY, États-Unis)
Small or Large Buildings : Green Roofs at Your Reach
Green roofs are no longer limited to large commercial or institutional structures. Thanks to technological advancements and the availability of modular solutions, they have become an accessible and affordable option, even for small residential buildings. These buildings, which are subject to more flexible regulations according to the Régie du bâtiment du Québec, offer a unique opportunity to contribute to ecological transition at a lower cost.
Whether you choose an extensive, intensive, or modular green roof, the solutions available in Quebec make these projects achievable for everyone.
Accessibility at the Heart of Ecological Transition
Whether for a small home or a large-scale commercial structure, green roofs help to:
Reduce urban heat islands
Improve stormwater management
Lower the overall environmental footprint
Take Action !
First step: Consult a professional to assess the structural feasibility of your project.
Get informed: Explore local grants, such as the RénoPlex program, to reduce your costs.
With accessible products and adapted solutions, it's time to rethink your roofs as ecological and sustainable spaces that will benefit your community and the environment.
Take action!
Choose a modular or extensive system suited to your budget and start building a greener future!
