Ask the Expert – Growing green aspirations

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For growing numbers of self-builders, green roofs are the right choice for topping off a home. But what are the key practicalies that you need to consider – and are they suitable for all projects? Julian Thurbin of Wallbarn has the answers

WHY INSTALL A GREEN ROOF?

Many self-builders love the individualism, look, and eco-benefits of green roofs. As well as being an increasingly vital habitat for insects and birds, they have many positive environmental impacts, including managing heavy rainfall by prolonging the time between rain falling and enter- ing the drainage system, improving air quality, reducing the Urban Heat Island effect in towns and cities and offering sound and thermal insulation benefits.

WHAT ARE THE BASICS TO CONSIDER?

There are two primary considerations – is your roof pitch suitable, and can your home (or the building taking the green roof, for example an extension, garden room or leisure suite) support its weight?

In terms of roof pitch, most green roofs are installed on flat roofs (generally a maximum pitch of around 15°), allowing the use of a traditional ‘roll-out’ system or newer ‘modular’ approach. That said, they can be installed on more complex projects (for example convex roofs) or pitches of more than 15°, but this does generally limit system choice to ‘roll-out’ only.

When it comes to load bearing requirements, a structural engineer should be consulted to ensure the structure can support the weight of a green roof, which may weigh up to 100 kg/m² when saturated. For new-build projects or extensions, the required structural integrity can be ‘designed-in.’

Once you’ve established viability, the next decision is choosing between a roll-out green roof system and a modular solution.

ROLL-OUT VS MODULAR – WHAT’S THE DIFFERENCE?

Until a few years ago, roll-out (or carpet systems) were the established method for laying a green roof. This approach entails installing each element individually – drainage layer, root barrier, substrate and sedum plants. For large roofs, or those with different depths or a pitch higher than around 15°, this traditional method remains popular.

Modular systems are a newer development, combining all elements of a green roof, including plants, in pre-prepared trays or cassettes that lock together during installation. They are essentially an established green roof in a box, delivering an ‘instant’ result quickly and cleanly, and removing potential for error. They also allow access to the roof deck post installation – by lifting out trays as required, which can then be returned into position.

CAN I DESIGN AS WELL AS INSTALL A GREEN ROOF?

Modular green roofs lend themselves to design and installation by competent self-builders, and system manufacturers should be able to assist with questions and provide technical support. The basic build-up is a geotextile fabric laid onto the waterproofed roof, followed by the green roof cassettes which are clicked into place on top. Cassettes can be cut to fit the space precisely and roof edges finished with aluminium edging and pebbling (see below paragraphs).

If a roll-out solution has been chosen we’d advise engaging specialists to carry out the work from specification to installation.

ARE THERE ANY TECHNICAL ISSUES TO CONSIDER?

Fire regulations require a hard border between vegetation and walls, and around features such as rooflights for all green roofs. This is often achieved using an aesthetically pleasing pebble border.

The fire performance of green roofs is also being discussed more frequently; leading manufacturers to offer systems with industry fire classification B Roof (T4) EXAP – from physical independent testing to CEN/TS1187:2012 (Test 4) and CEN/TS 16459:2019 (Test methods for external fire exposure to roofs). Look out for a classification of no fire penetration of the green roof system in a one hour physical fire test and ask for certificates.

WHAT IS SUBSTRATE?

This means the material the plants grow in; its composition and depth varies from system to system and can heavily impact the roof’s long-term performance. The GRO Green Roof Code (www.green- rooforganisation.org) lays out recommended minimum depths for different systems, starting from 60 mm. We think that 100 mm is the ideal depth for a modular green roof, allowing optimum space for healthy roots. The Green Roof Code also discusses the content of substrates and advises that they should comply with BS8616:2019 Specification for performance parameters and test methods for green roof substrates. An independent laboratory test showing that the substrate complies with BS8616 should be provided by the roof supplier.

WHAT PLANTS CAN I HAVE ON A GREEN ROOF?  

Traditionally sedum plants have been the go-to for green roof manufacturers, because these super hardy succulents are drought-tolerant and more than capable of thriving in harsh weather conditions. They form a year-round carpet of greenery with spring/summer flowers and (brilliantly) absorb carbon dioxide. 

For self-builders and home renovators wanting something more unusual, there’s the option of native British wildflower mixes, which deliver a more natural finish with greater seasonal variation in
appearance. There’s a growing trend for this style of green roof, with some manufacturers reporting that their wildflower mixes are now outselling pure sedum systems. The best systems offer a mix of wildflowers and sedum to ensure all-year greenery.

CAN I ORDER MY GREEN ROOF IN ADVANCE?  

Yes if you are installing a modular system, probably not if it’s a roll-out solution. Modular systems, because they come completely self-contained within trays can, if necessary, be unpacked and left on the ground until required. Elements of a roll-out system can be stored but the sedum layer must be unfurled within 24 hours of delivery to ensure healthy plants. For this reason, it is also important to consider the time spent in transit of your green roof, modular or roll-out. If left too long on transport crates – we advise a maximum of 48 hours – enzymes begin to break down the plants.

DO I HAVE TO LOOK AFTER MY GREEN ROOF? 

Green roofs are low maintenance but not no maintenance. The plants will tolerate extreme conditions (Some green roofs are over 75 years old, and thrive worldwide in a wide variety of climatic conditions) but you should expect to carry out twice yearly maintenance, including applying a slow release fertiliser for best year-round results. In extremely dry conditions it may be necessary to water the roof – or fit a simple irrigation system as part of the overall installation. 

Julian Thurbin is managing director at Wallbarn