The Building Conservation Directory 2020

81 C AT H E D R A L COMMU N I C AT I O N S T H E B U I L D I N G CON S E R VAT I ON D I R E C TO R Y 2 0 2 0 ROOFING 3.1 SAND CAST LEAD JONATHAN CASTLEMAN L EAD IS one of the earliest metals known to man and has been in use for some 7,000 years. The English lead mines worked by the Romans are well known, and sheet lead has been used in this country for roofing since Saxon times. Christopher Wren wrote to a friend when choosing lead to cover the dome of St Paul’s, ‘Lead is certainly the best and lightest covering and being of our own growth and manufacture, and lasting, if properly laid for many hundreds of years.’ Lead was used extensively for roofing buildings in the medieval period, especially in the late 14th and in the 15th century when the open timber roofs of flat pitch were developed. Many examples of lead roofs from this period exist today, although the lead covering is likely to have been recast since it was originally laid. The oldest surviving lead sheet on a roof is likely to date from the 17th and 18th centuries, although it is rare to find examples from before the middle of the 18th century. Among examples of original work of this period still in excellent condition, are the roofs of St Paul’s Cathedral, Westminster Abbey and York Minster. Until the invention of the milling process at the end of the 17th century, all lead used on buildings was sand cast. This cast lead was of considerable thickness, usually far thicker than that used on buildings today. It was not until the 19th century that milled lead (or rolled lead as it is now known) came into common use. Lead is one of the most corrosion- resistant metals and is therefore at little risk of being attacked by polluted atmospheres. Although lead has a tendency for expansion and contraction on roof slopes, it is an almost ideal material for roofing and gutters. Lead can be easily worked into shape, easily jointed and fixed, and is readily available. Flatter pitched roofs are better suited to lead as there is reduced stress on the material when expansion and contraction takes place. If properly laid however, lead can also be used on steeply pitched roofs. Cast lead is available in the thicker codes only, codes 6 (2.65mm), 7 (3.15mm) and 8 (3.55mm). At these thicknesses lead is commonly considered a heavy roofing material, however a cast lead roof is lighter than most slate or tiled roofs and is of a similar weight to thatch or asphalt. Experienced architects and plumbers are of the general opinion that cast lead lasts longer than milled lead, although there appears to be no scientific explanation for this. One reason given is that having been poured over open sand beds, it sets in its natural form without any built-in stresses. REPAIRS It is a key principle of conservation work that as little should be changed as possible, so any new materials required should match the original. Where lead is concerned, this is best achieved by using same production methods, so where original old cast lead roofs must be renewed, the new material should be cast rather than milled, and alternative materials should only be used where essential. This is particularly important where roofs of steep pitch are visible from the ground. One of the most common causes of defects in lead sheet is wear and tear caused by thermal movement. Lead has a low mechanical strength and is liable to crack or split when subjected to continuous expansion and contraction. Other common causes of failure include structural issues where the timber substrate under the lead has failed, poorly designed or inadequate fixings, and underside corrosion caused by a reaction between the timber substrate or by poor ventilation under the lead. When lead sheeting becomes worn or split it can be repaired by lead welding, and it is only when it is beyond repair that the lead needs to be removed, recast and relaid, making it one of the most recyclable materials around. This can give rise to considerable savings over completely renewing a roof. THE ART OF SAND CASTING LEAD Today, cast lead is manufactured following the same basic methods as that which the Romans used to cast their lead. The process of casting lead starts with scrap, collected from various sources. The collected scrap lead is then inspected for signs of solder because the material has a lower melting point than lead, creating problems in A church roof of sand-cast lead dating from the 1800s which is still performing well. (Photo: Jonathan Castleman)