RAINWATER DRAINAGE | ROOFING GOING WITH THE FLOW Traditional roofs in Japan were designed to deflect rainwater away from a building and these types of roof forms are an elegant and sustainable alternative to the pipework and guttering used now. Jake Cherniayeff says a performance-based design approach can achieve similar drainage strategies on modern buildings Art Gallery of New South Wales Sydney Modern Project has a large glass entrance canopy that allows rainwater to freely drain off the roof into the stormwater system via a purpose-made fabricated steel raincatcher located in the landscaping below downpipe to 16l/s. In some instances, this is not appropriate for a building, so the hydraulic design may choose a performancebased pathway for compliance using other recognised standards or calculation methods. This same performance-based approach can be applied to allow a roof to drain freely, like the traditional Japanese roof structure. A canopy at the Art Gallery of New South Wales Sydney Modern Project is one example (left and below right). The reasons for doing this may be an architectural vision, cost benefits, materials reduction, or a landscape strategy to return rainwater directly to the earth. When looking to adopt this design philosophy, there are a few considerations that need to be addressed all of which go back to our overarching requirement to keep water out of the building. Wind-driven rain and water ingress T he general purpose of a roof on top of a building or internal space is to keep the weather out, with windows and doors providing an opportunity to let the weather in mainly daylight and fresh air. Many traditional roof-construction techniques consist simply of a roofing material over a structure that allows rainwater to collect and run off the side of the building into the landscaping. This traditional technique is still common in countries such as Japan, where, often, less is more in architecture, and the built environment strives for minimal impact on the natural watercourse. Through early advancement of building techniques, some countries adopted gutters and downpipes in the early 19th century. These were used to convey roof rainwater in a controlled manner to points around the building. Gutters and downpipes can be used around the whole roof perimeter, or local to building openings and awnings where free-draining rainwater might cause a nuisance. Today, gutters and downpipes are necessary to collect rainwater in a central point for harvesting and re-use lets park this thought for now. In Australia, our National Construction Code provides the overarching requirement for building rainwater designs to keep water out of the building during a one-in-20-year and one-in-100-year storm. It then refers us to AS3500.3 Plumbing and drainage Stormwater drainage for a guide to achieving this overarching requirement; by following this code, you may produce a deemed to satisfy design. AS3500.3 is not a one-size-fits-all approach to every building. For example, the charts within this standard limit the flow to any single Wind has a huge influence on the path of travel for a drop of rain. The dynamic relationship between wind, rain and buildings is complex and challenging to predict without detailed site-specific wind analysis by a wind engineer. In most cases of building design, wind and rain are intensely affected by the immediate surroundings and topography, and their estimation is limited to environmental data available at that location. Despite myriad research into winddriven rain within the built environment, its behaviour is still ambiguous. Also without a body of sound equations available to apply to roof and faade drainage designs, understanding rain behaviour will be on a case-by-case basis. We know the behaviour is influenced by the local wind climate, the velocity of rainwater as it leaves the roof, height of the roof and distance before a water stream separates into droplets, rain droplet size, and intensity. Wind experts suggest the angle between the roof and the maximally deflected stream of ejected water is 30. Using simple trigonometry, we can then calculate the horizontal deflection at ground level. 28 December 2023 www.cibsejournal.com CIBSE Dec 23 pp28-29 Roof drainage.indd 28 24/11/2023 17:55