Period character guide: Lead work

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Watching the roofer gently work the lead around my new skylights I once again marvelled at how this soft, supple, silver grey material is so resilient, useful and attractive. Lead has been part of each of the houses I have worked on; criss-crossing leadedlight windows; snaking its way between joists in the form of water pipes; adorning the exterior walls in the shape of rainwater hopper heads; serving as the outer sheath of ancient electric cables; and helping weatherproof the roofs.

Lead’s waterproofing and aesthetic properties, coupled with its flexibility and durability, mean that it has been widely used both practically and decoratively in buildings for centuries. The Romans were quick to exploit Britain’s extensive lead deposits, along with the silver held within them, and Britain remained a major supplier of the world’s lead until the mid-19th century.

While the Roman craftsmen used lead to construct water ducts and storage tanks, the Normans employed it for roofing. In medieval times it was used extensively on church roofs and, as time went on, lead was not only laid to form large flat areas but to create more complex roofs involving curved shapes such as with the dome of St Paul’s Cathedral in London. By the 17th and 18th centuries it was widely employed on all types of buildings.

Traditionally lead sheet was cast on beds of sand, giving it a distinctive grained texture. Smoother, milled lead sheet was produced by a rolling process from the mid 1700s and was extensively used from around 1900; today this is the most commonly employed form of lead.

Machine-cast or direct method lead sheet was developed in Australia in the mid 1950s and introduced into the UK in the 1980s but, like sandcast lead, cannot be produced to the consistent thickness tolerances achieved by milled lead.

Founded in 1825, Norman & Underwood still manufactures sand-cast lead at its factory in Leicester. Jon Castleman, the managing director, explained to me that the process starts in the ‘pigging shop’, where scrap lead is collected and sorted. The lead is then put into gas fired melting pots where it is heated to 621°F (327°C).

Melting pots
Impurities in the molten lead float to the surface and this ‘dross’ is skimmed off before the lead is transferred into ‘pig’ moulds by hand ladling. From here the ‘pigs‘, each weighing around 88lbs, are transferred to melting pots in the casting shop.

The casting process at Norman & Underwood consists of ‘throwing’ molten lead down a bed of prepared damp sand and, according to Jon, has changed little since Roman times. A predetermined amount of sand, which is watered and thoroughly mixed using wooden paddles, is spread across the casting table. It is levelled off by two men using a bar of wood called a ‘strickle’ which is supported by the side beams of the table. The strickle is then used to ram the sand and solidify it and again level it off. Once level, the sand is smoothed by copper ‘planes’.

At the head of the table melted lead is ladled into the ‘head pan’. This is then lifted by means of a pulley wheel and handle and the molten lead is thrown down the surface of the table. While still molten, the lead is planed off to give its required thickness using the strickle. Jon explained that the thickness is determined by a number of factors: the speed at which the lead is thrown down the table, the heat of the metal, the speed with which it is planed off and the skill of the men.

By the time the men reach the bottom of the table, the lead behind them has set so they can mark and cut it to size virtually immediately. It is then rolled and weighed. Any lead left on the table is recycled.

In addition to the lead used on roofs, Norman & Underwood manufactures lead rainwater goods, including hopper heads and downpipes. If these are required with mouldings and patterns, they can be pressed into the sand on the table before the sheet is thrown. These are then cut out and formed by hand into the various items.

Living with lead
Although lead should be virtually maintenancefree, it has a low mechanical strength so is liable to crack or split. For this reason it is advisable to check its condition periodically.

As well as being a roof covering, lead is used in conjunction with tiles and other roofing materials to form waterproof junctions around chimneys and at parapet walls. It is also employed to provide weathering to ledges and cornices.

If lead rainwater downpipes, hopper heads and gutters become damaged they are likely to leak into the building which may cause long-lasting harm if left unchecked. Where lead water supply pipes still exist it is best to replace them with modern plumbing because we now know of the dangers of lead poisoning but do try, as I have done, to leave the old pipes in place beside the new ones as they are part of the building’s history. Lead waste pipes from sinks and baths can continue to be used.

Making repairs
The normal life expectancy of a roof covered with cast lead is at least 200 years and for milled lead between 100 and 200 years. Even so, the efficiency and longevity of lead roofing is largely dependent upon the skill of those who laid it. Repairs need to be undertaken only by expert craftsmen as mistakes are easy to make.

Lead is shaped by beating or by lead burning. Beating can result in the lead being stretched and thinned, making it vulnerable to cracks appearing. Burning achieves a long-lasting seam but can scorch the timber beneath and has been known to cause serious fires so ensure that great ‘The normal life expectancy of a roof covered with cast-lead is at least 200 years’ care is taken. Working with lead is a specialised job and incorrect detailing or sizing of individual sheets can result in expensive remedial works.

It is worth ensuring that the contractor you employ has the necessary training and is familiar with the Lead Sheet Association’s publication The Complete Manual which outlines all the specification and design considerations.

Word for word: A lead work glossary:

• CODE Refers to the thickness of lead sheet with a range from code three to code eight. When specifying lead, it is important to use the right code for the purpose.
• PATINATION OIL Forms a semi-permeable protective membrane and is applied to new lead to minimise the possibility of staining.
• ROLL JOINTS Allow for expansion in lead roofing while forming a weatherproof joint. Traditionally, lead roofs were laid with joints formed of hollow rolls.
• WIPED JOINTS Made between lead pipes. The pipes and solder are heated with a blowtorch and then the lead is wiped smooth, historically with a moleskin cloth. Wiping a joint is a skilled job best carried out by an experienced plumber.

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WORDS ROGER HUNT PHOTOGRAPHS ROGER HUNT; THE LEAD CONTRACTORS ASSOCIATION; GSL; NORMAN & UNDERWOOD
Featured in the April 2010 issue of Period Living

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