Green roofs are a sustainable alternative that improve thermal comfort by reducing heat islands. In addition, they are excellent for mitigating the environmental impacts of urban development.
However, to ensure safety, certain steps must be followed, and generally, they are constructed as follows:
1. Slab
To ensure proper drainage and prevent water stagnation, the slab on green roofs must have a minimum slope of 2%. Don’t forget that the slab’s strength must be calculated by a civil engineer to prevent future damage.
2. Waterproofing of Green Roofs
A primer should be applied to the slab, followed by the installation of waterproofing membranes and a suitable coating. These membranes are applied like paint; once dry, they form an elastic, waterproof film that seals the areas where water could pool.
3. Drainage
A layer of gravel or expanded clay with Techdreno lines is then applied, covered by a Bidim geotextile blanket to allow drainage from the green roof. This is essential, as it directs water flow to the gutters without causing a substantial increase in weight there.
Note: The Bidim Wavin ExtraDren product combines waterproofing and drainage in a single product. Thus, the base layer can be up to 5 times thinner, accumulating less weight and resulting in a much lower cost. It is the right choice for this type of expansion.
4. Vegetation Layer of Green Roofs
We’re almost at the end of the project. In this phase, the layer consists of soil and the vegetation that will grow on top of it. You need to pay attention to the height to ensure there’s enough space for the roots of the chosen plants to grow.
Here’s a tip: choose native species. The plants should have shorter roots, and be careful with those that have “taproots” (which grow straight down and are concentrated) to prevent them from damaging the system.
To maintain the green roof, regular visits from a specialist are also essential to keep the plants healthy and the roof safe.
Types of green roofs
The choice of green roof type determines the substrate thickness, the weight on the slab, and the possible plants. Therefore, structural calculations are essential:
| Type | Substrate | Saturated weight | Vegetation / Maintenance |
|---|---|---|---|
| Extensive | up to ~15 cm | 60 to 150 kg/m² | Hardy grasses and groundcover plants; low maintenance |
| Semi-intensive | 15 to 25 cm | up to ~200 kg/m² | Herbs, small shrubs; medium maintenance |
| Intensive | 25 cm to 1 m+ | 180 to 500 kg/m² | Shrubs, flowers, and even trees; high maintenance (garden-style) |
Note on weight:
once saturated with water, the substrate weighs much more—so the slab and drainage must be sized for saturated conditions.
Benefits of a green roof
Beyond aesthetics, green roofs deliver concrete benefits:
- Thermal comfort — can reduce indoor temperature by an average of 6°C, reducing the need for air conditioning;
- Rainwater retention — captures part of the precipitation and relieves the burden on storm drains, reducing flooding;
- Sound insulation — the substrate and vegetation dampen external noise;
- Reduction of heat islands and increased urban biodiversity;
- Waterproofing protection — the layers protect the membrane from UV rays and temperature fluctuations, extending its lifespan.
Need more help with your project? Talk to our consultants—we’d be happy to help ensure your project’s success. We offer the most comprehensive drainage solutions and are distributors and partners of Bidim Wavin.
