Green Roofs at Steeper Pitches

Last week whilst we were at Ecobuild we had an email from an architect asking

“Any pointers for how to construct green roofs at steeper pitches i.e. 45 degrees?”

It’s a surprisingly common question, so we thought we’d share our response with you on the blog.

Here’s an example of a green roof at the Meydan Shopping Centre in Istanbul, designed by Foreign Office Architects and installed using FDT Rhepanol hg green roofing membrane. The 54,000m2 roof has pitches of up to 70 degrees in places.

Steep Green Roofs – what to consider

With steeper pitched roofs, there are two primary design issues: structure and planting.

1. Structure

With a pitched green roof one needs to bear in mind the management of shear forces in the substrate beneath the planting layer. There are a number of solutions available; which one you choose depends upon the depth of substrate and the pitch and slope length of the roof.

It is often possible to restrain the shear forces with intermittently positioned shear barriers. These may be supports that run parallel to the eaves and ridge details. The spacing, material and gauges are calculated on a project-by-project basis. Ultimately, the structural engineer is responsible for the structural design of the roof and its coverings, but we would, of course, provide technical support. Such measures would typically be detailed and integrated into the waterproofing package.

Subject to the roof’s structural design, it is often possible to use geogrid type reinforcement products to retain the substrate. These solutions are, however, dependent upon the capacity of the ridge detail, in particular, (but sometimes at the eaves too) to act as a fixed point against these shear loads.

2. Plant Selection

There are three issues that are particularly relevant to plant selection on pitched roofs:

1. Erosion – it is often advised that a pre-cultivated blanket is specified to overcome this issue
2. Shade – whilst the orientation of the building will be a factor, at steeper pitches, it is expected that the plants will be subject to longer periods of darkness/shading (and therefore less exposure to sunlight)
3. Drought tolerance – it has been shown in several studies that plant-available moisture (all other things being equal) will be more constrained in a pitched roof application than would be the case in an equivalent flat, or low-pitched, roof.

Species selection will therefore be important to the long-term health of the plant layer. One would expect to see the plant layer specified with species such as (but not limited to) Sedum Album, Sedum Spurium, Sedum Floriferum and Sedum Sexangulare.

Pitched green roofs are an unusual variant and its important to get the details right. If you’re considering using a pitched green roof on your project, get in touch and we’ll take a look for you.