Lead sheet is commonly used as both a pitched and a flat roof covering, as well as for gutter linings, flashings, soakers and vertical claddings. Roofing and cladding panels of lead bonded to steel over a plywood, or other, building board, and lead bonded directly to building board are also produced. These may be used for roofing or cladding.
Lead has been used as a roofing material in the British Isles for many hundreds of years. In medieval times, it was used as the roof covering on cathedrals and churches, subsequently being used to roof over many great mansions and country houses. For the last 150 years, it has been greatly used for general domestic construction.
It is a well-proven and long-lasting material, which is extremely malleable and easily dressed into any shape. As it ages, the surface dulls as a coating of lead carbonate is formed due to contact with the atmosphere. This coating protects the lead from further deterioration. The traditional method of manufacture, cast lead sheet, which is formed by pouring molten lead over a bed of sand, has been found to last over 400 years. A small quantity of this type of sheet lead is still produced. It is used for conservation and other high quality work.
For many years milled lead sheet is the method by which the majority of lead sheet has been manufactured. It has a shorter life of 100+ years, as it is thinner than cast lead. It is made by running a slab of lead through a rolling mill several times, until a sheet of the correct thickness is achieved. There is a range of six thicknesses, each being given a British Standard Code - Code 3 (the thinnest and lightest) to Code 8 (the thickest and heaviest).
The type of use will dictate the thickness selected. Large exposed areas of lead (such as are found on flat roofs and in valley/parapet gutters) are subject to considerable thermal expansion, caused by
This asphalt verge has dropped resulting in water penetration. been incorporated.
This large flat roof has been covered in lead divided into a number of bays. Note the stepped detail and the centre gutter.
exposure to the sun. The thicker/heavier sheets (Codes 6-8) are normally selected for such positions, as they are not so easily, and so adversely, affected by extremes of temperature. They can also be installed in larger panels or bays. The thinner grades (Codes 3-5) are used where there is a lower level of exposure involved, eg for flashings and soakers, and for smaller gutters such as those found on pitched roofs. Selection will also be influenced by the quality of the building, the assured lifespan sought and the shape and/or profile of the area to be covered.
Problems associated with lead roofing include:
EXCESSIVE BAY SIZE. Lead is extremely sensitive to changes in temperature. In summer, leadwork that is continuously exposed to the sun, may be subjected to daily changes in temperature of up to 40°C. It is important that the individual pieces of lead sheet or guttering do not exceed maximum recommended sizes, as there is an increased likelihood of failure when they do. This is due to the expansion/contraction cycles involved; the effect will be rippling and cracking of the lead. Overlarge bay size is the most common cause of failure of lead roofing. Guidance on the maximum recommended dimensions for lead covered roofs and gutters is given in the 'Lead Sheet Manuals' published by the Lead Sheet Association.
Maximum size of each bay is dictated by thickness (Code) of the lead. This determines both length and girth.
JUNCTION PROBLEMS - ROLLS. The junction along the length of each bay is normally formed with a roll, more occasionally with a welted upstand (see later). The roll may be hollow or formed over a solid softwood core. A hollow roll is only suitable for use on a pitched roof, as on a flat roof, foot traffic can easily crush it.
The lead from each bay should be freely dressed over the timber roll to form a junction that allows thermal movement. Only the lower sheet should be nailed, if absolutely necessary (and at one side only). Inappropriate fixing will result in buckling and cracking of the sheet, often along the roll.
SOLID ROLL bottom piece may be HOLLOW ROLL
fixed (never the top piece) This |8 |jab|e tQ
top piece ^_^ crushing from foot to move /^f^V over|ap traffic
Rolls may be solid or hollow. Lead is dressed over solid core or metal spring (subsequently withdrawn) to form a detail that will allow thermal movement, prevent capillary action and avoid penetration of any normal level of standing water.
JUNCTION PROBLEMS - DRIPS. A drip is used to form the longitudinal junction between adjoining bays of a flat roof, or of a gutter where the fall is less than 15 degrees. It is a stepped detail that should be both watertight and able to allow thermal movement to properly occur.
Its installation requires care, as any deficiency in design or construction may lead to moisture problems, usually as a result of capillary action. Failure may occur for a number of reasons including those illustrated.
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