Page 72

RCI December 2017

RAINWATER SYSTEMS Going to ground – issues when roof drainage hits floor level? By Simon Mawson, CA Group and Malcolm Wearing, CRM Rainwater Drainage Consultancy Metal rainwater systems offer an unrivalled ability to cope with extraordinary weather – materials are strong and gutters can be deep with larger holding capacity and improved flow rates. All these features mean a greater ability to cope with and endure our increasingly volatile weather conditions, including surface water flooding. The Metal Gutter Manufacturers Association’s (MGMA) new guidance document titled, Guidance on metal rainwater systems and underground drainage looks at this subject in detail. Understanding how a site will respond to surface water flooding and drainage is now a key factor for local authorities in consideration of planning 070 DECEMBER 2017 RCIMAG.COM submissions for property development. All involved in the building process must consider the site at the initial planning stages to identify drainage options and risks. There is, however, a disconnect on many building sites, where the rainwater pipes from roof level meet the ground. It is generally assumed that complications arise at gutter level, and not at ground level, which can lead to problems. Where does the responsibility lie? The first issue is that ground level is often the line where responsibility changes; the architect or the cladding company will have designed the above ground drainage, and the civil / structural engineer the below ground. That in itself should not cause an issue, but because it is a different team, there is always the chance something will get lost in translation. Thus, good coordination between these two disciplines, bearing in mind that the underground often precedes the above ground in terms of installation order, is essential on all sites, from a small house to a 10,000m² industrial building. But it is more than just miscommunication, there are fundamental issues with the way the design is undertaken, which can lead to a mismatch in capacity. Most internal gutters these days are designed for a storm with a return period of between 90 to 135 years. In other words, the rainfall designed for will be so large it would only be expected to occur once within this period – “But it is more than just miscommunication, there are fundamental issues with the way the design is undertaken, which can lead to a mismatch in capacity” Poor detailing in SuDS tank (geotextile not trimmed away at pipe entry) limiting flow Many sites now have underground storage tanks which, if poorly detailed, can restrict discharge from roof drainage systems


RCI December 2017
To see the actual publication please follow the link above