Green roofs are an aspirational choice for many self-builders and home renovators wishing to finish their project with a gorgeous living roof that changes with the seasons and attracts a host of insects and birds.
Demand for green roofs is booming, with many manufacturers – Wallbarn included – reporting their strongest sales to date in 2021. It’s a green revolution driven by consumer commitment to sustainability and a desire for more natural products and a more natural finish.
The benefits of green roofs are well-documented but we find that most homeowners are interested in two key areas – the ‘feel good factor’ of having a living roof and their ability to support biodiversity, creating habitats for flora and fauna (most notably pollinators; a study of green roofs in London recorded more than 20 species of bee foraging on them).
In addition, they are a valuable asset in managing storm water run-off by extending the time between rain falling and entering the drainage system and have been proven to improve air quality by absorbing pollution and dust particles (up to 0.2kg per m2). In towns and cities they reduce the Urban Heat Island effect by absorbing heat rather than radiating it back into the atmosphere and offer sound and thermal insulation benefits. There’s also evidence that adding a green roof helps with gaining planning permission, can make a building sell more quickly and add value.
With so many benefits it’s easy to understand why green roofs are so popular. And product development means that you now have a greater choice of how your roof looks. Sedum has long been the go-to plant of choice but some producers are offering British wildflower mixes which deliver an even more natural finish with greater seasonal variation in appearance.
For those considering including a living roof in their plans there are two key considerations:
- Load bearing ability of the building
- Choosing a traditional roll-out green roof or modular system
Load bearing
This is the first consideration for any green roof, modular or roll-out. A structural engineer should advise if the structure can support the weight. For new-build projects or extensions the required structural integrity can be designed in.
Traditional or modular?
Until a few years ago the established method for laying a green roof involved installing each element separately, so drainage, root barrier, substrate and sedum plants. For large expanses or roofs with different depths or a pitch higher than around 15° (for example a convex design) this traditional approach remains popular.
The alternative is modular systems which combine all elements of a green roof, including plants, in pre-prepared trays that click together. They are, essentially, a green roof in a box and deliver an instant finish quickly and cleanly with little on-site waste. Quality control is high because all elements of the system are prepared off-site in a controlled ‘factory’ setting, ensuring optimum plant health and long-term growth. The best systems are ‘grown on’ for at least nine months and quality checked before dispatch, ensuring a fully established green roof with mature, well-rooted plants arrives on site. Specification is straight-forward and investment in the product recouped from on-site time and labour savings.
Modular solutions offer three distinct advantages:
- Competent self-builders and renovators can install them; just lay the supplied geotextile fabric onto the roof membrane/finish and position the green roofing cassettes, clicking them into place. It’s a one-stop-shop solution that’s clean, quick and straight-forward.
- Where site access is an issue modular trays can be carried by hand through buildings, upstairs and passed through windows to reach inaccessible roofs, all without mess or disruption.
- If access is required to the roof deck post-installation it is straight-forward to remove and re-install individual trays.
Technical considerations
For both traditional and modular green roofs there are important technical considerations. Fire regulations require a hard border between vegetation and building walls and around features such as rooflights. The roof also needs to be relatively level and generally no more than a 15° pitch.
Fire performance
This is an area being more frequently discussed in relation to green roofs. Leading manufacturers offer systems with industry fire classification Proof (T4) EXAP gained through 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 no fire penetration of the green roof system in a one hour physical fire test and ask for certificates.
Substrate
Not all green roofing systems offer comparable substrate depths (substrate is the growing medium for the plants). The GRO Green Roof Code (www.greenrooforganisation.org) lays out recommended minimum depths for different systems, starting from 60mm. We believe that 100mm is the ideal depth for modular systems, allowing optimum space for healthy roots. The Green Roof Code also discusses the content of substrates and advises that they should comply to BS8616:2019 Specification for performance parameters and test methods for green roof substrates. An independent laboratory test showing that the substrate complies to BS8616 should be provided by the roof supplier.
Delivery
With any green roof system it is important to consider time spent in transit and the need to unpack the system within 24 hours of delivery to ensure optimum plant health. If left too long on transport crates – we advise a maximum of 48 hours – enzymes begin to break down the plants.
Maintenance
Green roofs are low maintenance but not no maintenance. Sedum plants are hardy and will tolerate extreme conditions – there are green roofs worldwide thriving in a wide variety of climactic conditions, some of them 75 years old or more. In exceptionally dry conditions it may be necessary to water the roof; some clients choose to install an irrigation system and we recommend twice yearly maintenance including applying a slow release fertiliser for best year-round results.