Drainage and irrigation

Good drainage ensures that the green roof, wall or facade does not compromise the structural integrity of the building and that plants are not adversely affected by waterlogged substrate. A drainage system must effectively remove surface and sub-surface water from the roof or wall. 


Laying irrigation pipe at Minifie Learning Centre green roof. Image: Junglefy

Laying irrigation pipe at Minifie Learning Centre green roof. Image: Junglefy

While green roofs, walls and facades are able to reduce the flow and assist in retaining stormwater run-off, it is still necessary to have drainage systems that can cope with extreme rainfall events or flooding caused by other factors on the site. See here for more information on drainage systems. All roof drainage systems (green or otherwise) should be designed to handle the most intense 60-minute duration rainfall that has a one per cent probability of being exceeded in one year (in Melbourne, this is a 48.5 mm rainfall event). Green roofs will not directly increase drainage needs on a roof, however design must ensure that excess surface run-off can be discharged readily through the roof drainage system. The components that make up the substrate will affect the flow of water through a green roof profile.


Irrigation is critical to the success of most green roofs, walls and facades, and many green walls have a particularly high water demand. It is strongly recommended that non-potable water sources for irrigation are explored, particularly for systems/designs that are water intensive. The likely water demand of vegetation can be estimated by developing a water budget based on multiple characteristics of the green roof, wall or façade, including

  • calculating total water needs based on the ‘landscape coefficient’ or ‘crop factor’ values
  • evaporation data
  • effective rainfall
  • the substrate water storage.

A number of horticultural publications provide more information on these issues, including Growing Media for Ornamental Plants and Turf (Handreck and Black), Water Use Efficiency for Irrigated Turf and Landscape (Connellan) and Water Use Classification of Landscape Species– a method for estimating the water requirements of landscape plants, based on species, vegetation density, and microclimate (University of California Cooperative Extension and the California Department of Water Resources).


Sustainable design of green roofs, walls and facades should incorporate key principles of water sensitive urban design (WSUD), particularly to manage stormwater in the landscape, rather than into the stormwater drainage system. For green roofs in particular, it is desirable to maximise water retention in the system for as long as possible, and to send reduced volumes of high quality (low nutrient) run-off off the roof. Re-use of irrigation water is useful in green roof, wall and facade installations, where large volumes of water flow through the system. Treatment to disinfest and treat run-off water is needed in these situations as continuous recycling of untreated water can spread soil-borne disease and/or lead to the build-up high levels of nutrients. 


In many green wall installations, water reticulation and reuse is standard practice.

More specific advice on drainage for green roofs is available in here.