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RCI March 2018

Project1_Layout 1 07/05/2013 RAINWATER SYSTEMS Recommendations for eaves gutter installation When it comes to the correct height to install an eaves gutter, there is varying advice on the most appropriate options. In this article, Carlton Jones, director of the Metal Gutter Manufacturers Association (MGMA), examines the factors that need to be taken into account when installing eaves gutters, and outlines the pros and cons of each option to help specifiers make an informed decision on the best choice for their application The spread of water as it leaves the roof edge can vary considerably depending on the rainfall intensity, type of roof surface, and the pitch of the roof. BS EN 12056: 3-2000 (Gravity Drainage System Inside Buildings (roof drainage, layout and calculation) recommends that eaves gutters should be fitted in such a position that they intercept the flow at the roof edge, and that gutters are fitted centrally under the roof edge and close beneath it. Gutters can be installed level or with a nominated gradient of 1:600. (Detailed information on BS EN 12056: 3-2000 can be found in MGMA Information Sheet No 03 at www.mgma.co.uk). The standard also states that ‘gutters are to be securely fixed to prevent them being dislodged by ladders or strong winds’ however, there is no mention of protecting the gutters from the velocity impact of sliding snow. BS EN 2056: 3-2000 suggests that snow guards should be fitted where sliding snow may cause injury to people or damage structures below. The gutter should not be positioned at a level, which causes rainfall to overshoot the gutter that is, too low, or where it could be damaged by the high velocity impact of sliding snow. Projection of water flow Different types of roof surfaces will create either more or less projection of discharge at the roof edge; for example, water will flow faster down a standing seam metal roof than a stone tiled roof. Currently there is no formula available that can establish the projection of water flow from the edge of different roof pitches, heights and roofing materials to enable the installation of the correct width of gutter. BS 5534:2014 (Slating and tiling for pitched roofs) states that roof tiling should project a minimum of 50mm from the vertical face of the fascia board. The projection should align with the centre line of the gutter, and therefore, this will establish the minimum width of gutter. However, there is no guidance as to the height of the gutter relative to the roof edge. Changes to Part L of the Building Regulations have resulted in more highly insulated roofs, and this has the potential to cause problems in heavy snowfall, particularly where the gutters are installed at a high level. When the thaw sets in, the snow will slide down the roof taking with it anything in its path, namely gutters fitted too high under the roof edge. Prior to the installation of a highly 088 MARCH 2018 RCIMAG.COM High level gutter: Advantage: Protection against wind driven rain Disadvantage: High risk of damage by sliding snow Low level gutter: Advantage: Less risk of damage by sliding snow Disadvantage: The lower the gutter is installed in relation to the roof edge, the higher the risk is of wind driven rain and overshooting rainwater A straight edge can be placed along the pitch of the roof to determine the position of the fascia bracket insulated roof, the snow would merely melt as it fell onto the roof surface. The correct position and height of the gutter needs to be carefully determined, for example, whilst a lower level fitted gutter is less susceptible to sliding snow, it could result in wind driven rainwater going between the back of the gutter/ roofline, which can lead to water damage to the building fabric. A gutter fitted at a higher level will alleviate this problem. Each option has its benefits and drawbacks, and more often than not, the building design, gutter type and fixing, will dictate how the gutter should be installed. Architects, installers and building owners should discuss the options available and to ensure that all parties agree on the best possible solution. There are several factors which will determine the final position of the gutter for example, fascia board depth, bracketry and the overall aesthetics. Finally, on completion of an installation, it is recommended that all gutter outlets should be blanked off and the gutter filled to the overflow level. After a period of five minutes, the installation should be checked for leakage. Discharging the flood test water into rainwater pipes will identify any leaks in rainwater pipe joints. Any joints that fail should be taken apart, all sealant cleaned off, and then re-sealed and re-tested. The MGMA has created an eaves gutter design flow chart (see MGMA Guidance Document No 18) to enable the user to perform calculations to BS EN12056-3:2000. By entering simple information relating to building dimensions, geographical location and proposed gutter style, the flow chart will provide guidance on gutter specification. The flow chart is intended as a guide for straightforward eaves gutter applications. Use a spirit level against the lowest point of the roof tile to determine the position of the fascia bracket Guidance document GD16 Eaves gutter installation recommendations can be easily downloaded from the MGMA website at www.mgma.co.uk.


RCI March 2018
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