Intensive Green Roof vs Extensive Green Roof

Green roofs are roofs that are covered with vegetation and soil that reduce the heat island effect and manage stormwater runoff.

They also help to mimic natural habitats, improve air quality by absorbing carbon dioxide, and provide noise and heat insulation for the building.

There is literally no limit to the ways that green roofs can be designed by incorporating different vegetation, soil depths, irrigation systems, and even climate control systems to ensure that the plants are thriving.

However, there is one aspect of green roofs that dictates the entire design which is the depth of the growing medium.

The depth of the growing medium determines which plants can be grown, the structural load to be considered, the amount and type of irrigation required, and the drainage system design.

This is where the distinction between an intensive green roof vs extensive green roof becomes important. Now, let’s take a closer look at what both of these terms mean and how they affect the design.

Intensive Green Roofs

Intensive green roofs are also known as deep green roofs that have significantly deeper substrates, allowing for far more freedom in designing and growing more sophisticated and elaborate gardens onto concrete structures.

With intensive green roofs, the substrate depth is usually more than 15cm. 

Intensive green roofs require more structural support since they have a deeper substrate and they need to be irrigated on a regular basis.

This will add an additional expense when it comes time for the building owner to maintain their green roof, but intensive green roofs offer more variety in terms of plant life that can grow on the rooftop garden.

Extensive Green Roof

Extensive green roofs are also known as shallow green roofs that have a much shallower substrate depth.

The substrate depth is usually less than 15cm. This type of roof is typically used to better manage stormwater runoff, reduce the heat island effect of the building by providing vegetation on top of it, and provide insulation for noise.

Extensive green roofs can typically support less vegetation because of their shallower substrate depth, but they do not need a lot of irrigation since it does not absorb as much water from rainfall or snowmelt due to their shallow soil depth.

These roofs are much lighter and less complicated than intensive green roofs.

They typically consist of soil or substrate that is only about a few inches deep and the plants are usually lightweight annuals, perennials, and herbaceous plants which can thrive in shallow soils like succulents, cacti, or hardy grasses.

Despite the fact that extensive green roofs are far less complicated than intensive ones, they can still be quite beautiful and eye-catching for building occupants, passersby, or even neighbors to enjoy!

The weight of extensive green roofs will often be less than intensive ones because they are typically shallower and do not contain as many plants or soil.

Extensive green roofs can add to roof load as much as 20 pounds per square foot, but the actual weight will depend on the substrate used.

Should you choose an intensive green roof or an extensive one?

The answer to this question will depend on your design goals for the roof.

However, here are some factors to consider when choosing either an extensive or intensive green roof:

Additional Weight

Both intensive and extensive green roofs will bring additional weight to your roof. However, an extensive green roof will typically be lighter than an intensive one since it has a shallower soil depth and less vegetation.

A typical intensive roof will add 200–500 kg/m2 to the roof, while an extensive one will add 60–150 kg/m².

Therefore, intensive roofs are probably best for commercial buildings with abundant roof space and stronger structural support.

While extensive roofs will work better for homes and other buildings where space and weight constraints are more of an issue.

Capital Costs

Extensive green roofs have lower capital costs since they do not require as much substrate depth or irrigation systems compared to intensive ones.

Also, plants are generally less expensive for extensive green roofs since they are typically smaller plants that thrive in shallow soils.

A typical intensive roof will cost 540 $/m2 while an extensive one will cost 130–165 $/m².

Accessibility

Intensive gardens can be more accessible for people to enjoy whereas extensive roofs are mostly inaccessible due to the low-growing plants.

If you want to make the roof accessible for people to enjoy, then an intensive green roof is a better option.

Planting Options

There are more options for planting species with intensive green roofs since you can plant larger trees, shrubs, and other perennials.

However, extensive roofs typically have less variety of plants due to their shallow soil depth.

If you want to grow fruits or vegetables on your roof, then an intensive green roof is the best choice since you can plant deeper and have better soil for larger plants.

Maintenance Needs

Both intensive green roofs and extensive green roofs require regular maintenance to keep them healthy.

However, since the soil depth of an extensive roof is shallower than an intensive one, it will need less irrigation and fertilization.

Water Retention

Intensive green roofs retain more water than extensive ones. An intensive roof will retain 70–130 l/m² while an extensive one typically retains 27–45 l/m² of rainwater.

Conclusion

If you are looking for a low-maintenance green roof with different planting options, then an extensive green roof is the best choice.

On the other hand, if you are interested in building accessibility or more soil depth for larger plants on your rooftop garden, then an intensive one might be better suited for your needs.