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Roof Drainage ENSURING EFFECTIVE FLAT ROOF DRAINAGE – KEY CONSIDERATIONS Brian Bell, Head of Technical Services at Marley Alutec, looks at the key issues that need to be considered in order to ensure the correct design of rainwater outlets on a development, including the relevant standards and how best to approach calculations, material options, layout and specification, as well as technical support. Designing effective rainwater management and drainage is essential to ensure the longevity of a flat roof system and protect the building from damage. If water is allowed to pool on the surface it can place additional pressure on the flat roof system, causing damage and leading to water ingress. Regulations and standards Guidance and standards for rainwater drainage is provided in Approved Document H of the Building Regulations for England and Wales and Building Standards Part 3 in Scotland – specifically section H3 of the Building Regulations and Part 3.6 of the Building Standards. Furthermore, the National House Building Council (NHBC) standard 7.1.5 also provides guidance on the design of drainage for flat roofs, balconies and terraces. All of these documents require that flat roof drainage is designed in accordance with BS EN 12056-3 – Gravity Drainage Systems Inside Buildings. Design considerations and calculations Decisions around the design of the drainage system should be based on factors such as the level of expected rainfall, the design of the building and roof area, and the level of risk from extreme weather in the area. Rainfall intensity The geographical location of the building is the main determining factor and can be calculated using data provided in the National Annex for the UK found in BS EN 12056. The calculation of rainfall intensity is based on the predicted runoff from two minutes of a summer storm event and is measured in litres per second per square metre (l/s/m²). Roof area The total roof area that needs to be drained must also be calculated. For a flat roof this is simply the plan area of the roof plus that of any roof that drains onto it. Also, while it is often not factored into the calculations, BS EN 12056 requires that 50% of the area of adjacent vertical surfaces is also included in run-off calculations. This is defined as any wall as well as any roof with a pitch above 70 degrees. Building life and design category The drainage strategy is also influenced by the level of protection against water ingress that is required. BS EN 12056-3 provides four design categories based on the return period of intense weather events – the likelihood that a storm of a given intensity will reoccur within a certain period of time. This is essentially a cost versus benefit determination based on impact that rainwater exceeding the capacity of the drainage would have on the building. To calculate this correctly the intended design life of the Top to bottom: Marley Alutec cold roof outlet; inverted paved terrace roof outlet; green roof outlet. building must also be defined. Under Category 1, roofs are designed based on a one-year return period, meaning that the proposed flow rate of the drainage will be exceeded on average once a year. Therefore, it is only suitable where the flood water can overflow from the roof and fall clear of the building. Also, the roof structure must be capable of withstanding the additional temporary loading. For Category 2, 3 and 4, the roof drainage is designed to cope with storms of greater intensity that occur less frequently with the designed life of the building as a basis. For Category 2 the safety factor is 1.5 times the building life. So, for example, if the building is expected to have a 60-year lifespan before major refurbishment, then the roof should be designed for a return period of 90 years. In effect there is a theoretical 1 in 90 chance that within a given year a weather event will be severe enough to exceed the drainage capacity. Category 3 offers a higher level of security and places the safety factor at 4.5 times the design life. Therefore, with Continued on page 46 44 TC SEPTEMBER <strong>2021</strong>
STAY SAFE with GRP rooflights from Filon Safety is at the front and centre of our product policy: Our technical expertise has contributed directly to UK rooflight non-fragility classification over decades, with Filon personnel playing key safety roles in influential trade associations including NARM (National Association of Rooflight Manufacturers) and ACR (Advisory Committee for Roofsafety). Our unique, patented Fixsafe system is the only solution that allows profiled rooflights to be safely installed from within the building envelope, without the need to access fragile roof areas. We offer a range of fire grades to provide compliance with Building Regulations. Our Supasafe triple reinforced rooflights provide high impact resistance, spanning and load-bearing capabilities. We’re the UK’s longest established manufacturer of profiled rooflights by continuous process, with a successful track record of excellence in design, product performance and customer support. As you’d expect, we’re also currently employing Covid-secure working practices throughout our operations. Filon: truly a ‘safe pair of hands’ for specifiers. Telephone 01543 687300 Email: sales@filon.co.uk www.filon.co.uk
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September 2021

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