Green roofs are not a new concept, but they have grown in popularity over the past decade as people aim to achieve more sustainable construction projects and utilise previously unused spaces, such as flat roofs.
There are three main types of green roof systems. The one you choose for your project will depend on the type of roof construction you have and what kind of finished look or purpose you want to achieve.
Extensive Green Roofs
The most common type of green roofs are extensive green roofs. These roofs are completed with hardy plants that are not nutrient hungry and require minimal maintenance. They also require the least amount of planting substrate, which makes them generally lighter options compared to intensive green roofs. Therefore, they are suitable for a wider range of roof constructions including both flat roofs and pitched or sloping roofs.
A sedum roof is the most common type of extensive green roof which has drought-tolerant sedum plants. Because sedum plants are shallow-rooted succulents, they require little water and nutrients and the shallowest soil depth of all the green roof types. They are very low maintenance. Sedum plants can be planted on the roof by applying seeds to the soil or using sedum matting or pre-grown blankets which contain a mix of sedum species.
Pre-grown sedum blankets are typically chosen to complete sedum roofs. These are filled with a blend of perennial sedum plants, so there is no decline in growth over winter. Some colour change can be witnessed throughout the year providing an aesthetically diverse green roof.
All green roofs offer environmental benefits, but some soft landscape extensive green roofs have the added advantage of being able to retain up to 90% of rainfall. This reduces water run-off and reduces the amount of additional water required for irrigation.
Another major benefit of extensive green roofs is that they are suitable for recreational activities. Previously unused or neglected roof spaces can be converted into tennis courts, football pitches or other recreational facilities – this is particularly desirable in built-up areas such as city centres where space is limited.
Intensive Green Roofs
Intensive green roofs provide greater design options due to the wider variety of vegetation that can be used. Both hard and soft landscaping can be incorporated into an intensive green roof, which also makes it the ideal type of green roof for an area that will be accessed by pedestrians.
To accommodate the vegetation, the roof build-up is heavier and almost always built on a concrete roof or podium deck. They require regular maintenance and irrigation and are therefore more labour-intensive post-installation compared to extensive green roofs.
Some people classify a category of green roof called semi-intensive green roofs. They are similar to intensive green roofs, except that the soil depth is typically between 15-30 cm deep, and they cannot sustain as diverse types of plantings as intensive green roofs can.
Biodiverse and Brown Roofs
Another type of green roof is a bio-diverse roof. Biodiverse roofs aim to replicate the ecological environment of the site where construction has taken place or tries to create a natural habitat to support a variety of plants, birds, animals, and insects.
There is a difference between biodiverse roofs and brown roofs. Brown roofs are not planted at the time of construction but left with bare soil to self-vegetate from windblown seeds or bird droppings. They are intended to recreate the naturally-occurring, wild vegetation that exists at ground level. Biodiverse roofs, however, have seeds or plants placed in the soil at the time of building construction.
The requirements of the bio-diverse roof or brown roof may dictate a heavier roof construction, similar to that for intensive green roofs.
Main Differences Between Extensive and Intensive Green Roofs
| Extensive green roofs |
|---|
| Growth medium 3-6 cm |
| Lightweight (30 – 90 kg/m2) |
| Low growing vegetation |
| Low maintenance |
| Low water requirements |
| Usually non-accessible |
| Slopes up to 30 degrees |
| Intensive green roofs |
|---|
| Growth medium 20 cm or more |
| Heavier weight (150 kg/m2 or more) |
| Trees, shrubs, gardens and more |
| Higher maintenance |
| Irrigation usually necessary |
| Designed for human recreation |
| Only used on low slopes |
Sika offers a full range of inter-compatible systems for green roof build-ups, including waterproofing membranes, thermal insulation, drainage boards, vapour control layers, prefabricated accessories, scuppers and drains. Check out the green roof systems that Sika provides.
"When you specify or choose a Sika green roof system, you get more than watertight security - you get peace of mind knowing you made the right choice."