074 RCI 0517

RCI May 2017

PITCHED ROOFING Rolled lead sheeting: ensuring the roof stands the test of time Sean Benwell, technical officer at the Lead Sheet Association, looks at the best ways to ensure you get the longest life out of a lead roof The longevity and aesthetics that you can expect from a well installed lead roof are far superior to many other products available on the market, particularly in relation to man-made products. However, to make sure you get the longest life out of a lead roof there are a few steps that need to be followed. Ventilation: the key to a long-lasting roof Lead roof and cladding ventilation is one of the most common design flaws dealt with in the LSA’s technical department, so we can’t stress enough how important it is to get the ventilation detail right. Substrate (18mm min) 50mm battens fixed with direction of fall Vapour check layer Roof deck The LSA recommends the use of a ventilated warm roof for most new roof details. A ventilated warm roof design allows through ventilation above the insulation to disperse any moisture that may percolate through the vapour control layer. Although recommended, it is usually difficult to achieve a 100% effective vapour control layer and without ventilation, any moisture penetration could be trapped in the insulation area. When looking at renovating existing leadwork, the underside of the old lead will give an indication as to whether there has been a condensation problem. If corrosion is minimal, the existing roof construction will usually be satisfactory for new lead sheet, provided that there is to be no change in 074 MAY 2017 RCIMAG.COM the use or heating levels of the building in the future for the life of the lead sheet. Pitch – flat to pitched A lead roof changes from flat to pitched at 11 degrees; joints across the fall become laps instead of drips and the lead is nailed directly to the substrate. The length of lap required is related to the roof pitch, however, a 75mm vertical lap must be maintained at all times. The LSA would recommend that a minimum fall of 1 in 80 be used for any flat roof design. The joints used in the direction of the fall may be wood cored rolls, hollow rolls, or welts, and the size of each bay is limited in relation to the thickness of the lead sheet used. Bays are secured in position with two rows of copper or stainless steel nails at the top of each sheet and in the top third of an undercloak if wood cored rolls are used. The lap length is related to the roof pitch and always measured from the lowest fixing. However, a 75mm vertical lap must be maintained at all times. The substrate must always be a fully supported roof construction which conforms to the current Building Regulations and the recommendations of British Standards 5250, 6229 and 6915. The importance of good guttering A well-designed lead gutter will help to minimise the chance of water ingress, gutters should be designed with a minimum fall ratio of 1 in 80. Lead-lined gutters are used where a flat or pitched roof abuts a vertical wall, where two pitched roofs are joined by a horizontal valley gutter and between adjacent areas of lead roofing. To allow for thermal movement, lead gutter linings must always be divided into separate pieces (bays). As with all lead sheet installations, thermal movement must be accounted for when specifying the dimensions of the gutter. To allow for thermal movement the LSA recommends that gutters are divided into bays with the size of each bay dependant on the code of lead sheet used. Table 8 shows the maximum length and girth for the five codes of lead sheet that are used for linings of both box and tapered gutters. The gutters used on lead roofs can be divided into two main groups, box gutters and tapered gutters. Box gutters usually have parallel sides with a minimum base width of 225mm. When using a box gutter with a lead covered roof, the upstand of a gutter should not be less than 100mm. A tapered gutter occurs when a pitched roof abuts a vertical wall or where a horizontal valley gutter joins two pitched roofs. With a tapered gutter the pitched roof merges into the sole of the gutter without upstands. According to the fall of the gutter and the pitch of the roof, the gutter sole will increase in width at each drip. Therefore, the gutter is wider at the highest point with at least 150mm width at the lowest point of the gutter. By following the guidelines above, you and your client should have peace of mind in addition to a roof that stands the test of time. If you need help with a technical enquiry you can visit the Lead Sheet Association’s website at www.leadsheet.co.uk where there is a wealth of technical information. Alternatively, you can call the technical team on 01622 872 432 or email technical@leadsheet.co.uk Diagram shows a ventilated warm roof Above: Lead pitched roof; Right: tapered lead gutter Lead sheet on underlay 50 mm MIN Insulation Ceiling Unventilated air space Breather membrane Ventilated air space


RCI May 2017
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