Home » Myth: ‘Irrigation makes green roofing cost prohibitive’

Myth: ‘Irrigation makes green roofing cost prohibitive’

by Matt Downs

Fact: As the team at GRO explain, a well designed and well established green roof has little need for irrigation…

With the cost of running buildings constantly on the rise, developers and facilities managers may be reluctant to invest in a living roof on the basis that it could cost too much to maintain. In this article, the Green Roof Organisation aims to dispel the myth that green roof irrigation is prohibitively expensive.

Water is a precious resource, and as our climate changes we need to ensure that water supplies are managed carefully and cost effectively. But is there a cost effective way to minimise water usage that can still maximise the benefits derived from plants on living roofs? The answer is a resounding yes, and it lies in sustainable green roof design.

Best practice green roof design minimises irrigation costs

Good green roof design considerably reduces, and in some cases may even negate, the need for irrigation on established living roofs. The GRO Code of Best Practice offers plenty of advice on designing living roofs that are virtually self-sufficient, and therefore cost-effective in terms of water usage.

Substrate Depth: The deeper the layer of growing material, the more water it will be capable of retaining. This is a crucial feature for rainwater attenuation schemes but is also highly advantageous for the plants. Conversely, extensive green roofs with a shallow substrate layer may indeed cost less to install, but they are more likely to struggle to support even the most drought tolerant plants through an extended period of drought without the use of irrigation.

Plant Palette: It is essential to select a plant palette that is suitable for conditions on an individual roof. And that palette needs to be re-evaluated on a regular basis in order to make sure it remains fit for purpose. It is also important to remember that green roofs are living systems and plant palettes will change over time in response to the local environment.

Rainwater Harvesting: Just as a good gardener uses water butts to collect and store rainwater to use during the summer months, given a sensible budget a green roof designer can suggest ways of capturing and re-using water from a living roof. Ideally potable water isn’t used on a roof and rainwater provides a great alternative. This may include storing water at ground level and pumping it back onto the roof via an irrigation system. Or by actually storing water on the roof and applying it directly back to the plants through sub-surface irrigation. 

Resisting the urge to over-nurture green roof plants

Choosing the right plants is crucial for the performance of a living roof. However, so is managing those plants. It is also important to manage the expectations of people connecting with a living roof. “Green” roof needn’t mean that it is verdant for twelve months of the year. Seasonal changes should be accepted and embraced. Only irrigate when absolutely needed!  

Whilst it’s tempting to keep the vegetative layer as well fed and watered as you would a favourite houseplant, green roof plants need to be ready to face tough conditions. Harsh frosts and torrential rain can quickly turn into drying winds and blisteringly hot sunshine. Over feeding and over irrigating the plants can result in soft, sappy vegetation which is vulnerable to disease and sensitive to changes in growing conditions.  

Whilst it sounds harsh, it’s better if the plants on an extensive green roof are somewhat deprived of nutrients and water. That way they will be better able to cope with drought when it does come; treat them mean to keep them keen.

Having said that, it’s important that plants are nurtured during the establishment stage. One way to minimise the need for irrigation is to wait until autumn before seeding, planting or laying vegetation matting. Yes, you will need to keep an eye on things – the substrate should not be allowed to dry out – but it’s more likely that Mother Nature will bring rain to help with the watering.

The above image shows the negative impact of an irrigation leak on a green roof.

Best practice for irrigation systems

It’s always advisable to install an irrigation system for any living roof so that it can be deployed when establishing new plants. For shallow substrate roofs and for sloping roofs, an irrigation system is considered absolutely essential. However, using the irrigation system judiciously will keep costs down. 

Simple water saving tips include: keeping irrigation systems well maintained and fixing leaks as soon as they occur; applying water at night when it is less likely to be lost to evaporation or via sub surface or leaky pipes; designing the irrigation system so that you can choose to water selected areas rather than the whole space; and, if you are going to irrigate, it’s better to give the roof a good soaking once a week rather than a light sprinkle once a day. 

The benefits of green roofing outweigh the cost of well managed irrigation

A well designed and well managed living roof should have minimal need for irrigation. But when water is needed, it’s as well to think of it as an investment. After all, the sum total of benefits afforded by a living roof the whole year round far outweighs the cost of irrigation.

www.greenrooforganisation.org

For more on this important issue, head to www.greenscapemag.co.uk to view Dr Tom Young’s article on drought tolerance and green roof irrigation in the Winter 2022 issue of Greenscape.

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