Darren Tutt of the Lead Sheet Association looks at the dos and don’ts of working with rolled lead sheet that will enable architects to get the best out of it.
Rolled lead sheet is a beautiful material to work with, and the Lead .Sheet Association (LSA) spends a lot of time supporting and advising the architectural community to ensure they get the best out of this product.
As well as the obvious aesthetic merits of lead, you can also expect tremendous longevity from a well-installed lead roof. Its life cycle costs and green credentials – it is fully recycled in the UK – are further benefits.
However, to make sure you get the longest life out of a lead roof there are a few steps that need to be followed. Here are our key tips to get you started.
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 is to get the ventilation detail right.
The LSA recommends the use of a ventilated warm roof for most new roof details as it 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 per cent effective vapour control layer and without ventilation, any moisture penetration could be trapped in the insulation area.
When looking to renovate 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 the use or heating levels of the building in the future for the life of the lead sheet.
Definition – 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 75 mm 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 75 mm 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.
The table shows the maximum length and girth for the five codes of lead sheet that are used for linings of both box and tapered gutters.
By following these guidelines, you and your clients should have peace of mind and a roof that stands the test of time. All the technical details referred to in this article can be found in ‘Rolled Lead Sheet – The Complete Manual’ which refers to the use of Rolled Lead Sheet BS EN 12588. The LSA has also recently developed an app that brings together some key details that can be easily accessed.
Darren Tutt is technical officer at the Lead Sheet Association