Sand Cast Leading
At Dean's Cloister, St George's Chapel, Windsor Castle
Martin Ashley
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| New cast lead roofs to the chapter office, accounts office and Dean’s Cloister |
The Dean' s Cloister was erected about 650 years ago on a grass plot formerly lying within the heart of King Henry III’s Royal Palace at Windsor Castle. It originally linked the Royal Apartments to the Royal Chapel, the latter subsequently becoming the first St George’s Chapel when the Chivalrous Order of Garter Knights and the College of St George were founded in 1348. Later it became the Lady Chapel to the magnificent new building of St George’s Chapel constructed for King Edward IV between 1475 and 1528, and which stands today with the Lady Chapel now as the memorial chapel to Prince Albert. Remarkably, the Dean’s Cloister is still used in just the same way today as it has been, in virtually continuous service, since the foundation of the college in the mid-14th century.
John Sponlee, Master Mason to King Edward III, erected arcading of the highest quality around the cloister in fashionable Perpendicular style. Under substantial lead covered oak roofs, finely carved devotional statue niches were flanked by Purbeck marble shafts at the corners of the four cloister walks. Subsequent and distinctively different stonework details distinguished by the use of Portland stone reflect repairs to the east cloister arcading were carried out under Christopher Wren’s surveyorship in 1661. Pinnacles to the arcade piers were sadly lost at some point, but grotesque beasts still line the corbel tables above the arcade openings onto the garth, the central space of the cloister.
Lead roof coverings from the last comprehensive restoration of the cloister in 1851–2 were largely replaced by copper in the mid 20th century. However, by 2003 when the Dean and Canons of Windsor commenced the first two phases of a further programme of restoration to the Dean’s Cloister, the copper was approaching the end of its design life and there were many other problems. In particular the 1850s leadwork remaining over the south cloister walk was now much repaired and in poor condition, and the lead bays were over-sized by present standards and were failing in places due to thermal fatigue. In addition; outlets from parapet gutter sumps were under-sized and prone to block and overflow, and flashings and pointing were in poor condition at the back of the stone parapets. These problems had all contributed to water damage to the oak roof structures and to the carved stonework arcading and decorative nichework below.
In December 2003, a complicated scaffolding started to rise up from the ancient grass plot within the garth of the Dean’s Cloister. Built rather like a mushroom, the scaffolding poles rose up from the ground to support scaffolding beams cantilevering out over the cloister roofs without actually touching any of the archaeologically significant walls of the surrounding buildings – a deceptively simple design. A high level scaffolding walkway was built over the roof of the Chapter Room together with hoists for manhandling enormously heavy rolls of Code 7, Code 8 and Code 9 sand cast lead into the working area. The scaffolding was left unroofed to allow daylight into the windows of the residences and offices looking out over the cloister until after the Christmas period. In the first week of January 2004 the temporary roof went on, and the real work started in earnest to replace the lead and copper roofs over all four sides of the Dean’s Cloister, St George’s Chapel Ambulatory and the Galilee Porch, the Chapter Room and offices, and St George’s Chapel Vestry, all with superb new sand cast lead supplied by Osiris Lead Limited from their works in Frome, Somerset.
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| Structural beam repairs and re-ordering gutter bays and sumps |
Only the roof of the south cloister walk still retained its Victorian lead roof covering which had been installed by the builder Samuel Cundy in 1851–2. This had probably survived due in part to the shade provided by the Albert Memorial Chapel, but also because ‘Queen Victoria’s Walkway’ stands upon it, a private balustraded walkway across the cloister roof for the sovereign’s discrete access from the Deanery to the Royal Pew overlooking the quire and sanctuary within St George’s Chapel. All of the other roof coverings had been replaced in copper in the early 1950s during the surveyorship of John Seely, 2nd Baron Mottistone and partner of the conservation practice Seely & Paget. At that time copper, being readily available and economical, was used to replace lead roof coverings that had been used here historically. Copper roofing has a design life of 50 to 60 years in comparison with 100 years or sometimes much more for a well-laid lead roof. Following extensive consideration and consultation, the Dean and Canons resolved to reinstate lead roofing upon the Dean’s Cloister so as to revert to a longer and more manageable maintenance cycle appropriate to a community which has so many buildings to maintain in good order.
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| Opening up the archaeologically significant accounts office roof |
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| Accounts office roof with new Code 9 cast lead roof |
Sand casting has been the traditional method of manufacturing lead sheet since Roman times. Melting down pig lead in a large crucible, and running it molten and fizzing down a gently inclined casting table covered with fine, beaten and pressed sand provides a characterful lead sheet to a thickness dictated by the inclination of the table, speed of the pour, and skill of the foundrymen. The sheet is immediately cut with knives to the appropriate width and length, rolled, and removed from the sand bed ready for delivery to site. It has a rough textured face on one side given by the sand bed, and a smooth upper face that tends to be unevenly coloured with blues and browns from oxides and impurities that have floated up and formed on the lead surface. There are differing views on the benefits of laying lead sheet ‘sand side up’ or ‘smooth side up’, but there is no doubt that lead sheet laid sand side up immediately looks mellow and beautiful, and helps to preserve the sense of aged patina that is such an essential characteristic of our ancient and venerable historic buildings. There are also differing views on the use of traditional sand cast lead as opposed to the more commercially available milled lead, whereby thick cast lead sheet is progressively pressed between rollers down to thickness tolerances defined under British Standard 1178: 1982. The Lead Sheet Association advises that where composition of metal is similar, there are no significant differences between the properties or working of either cast and milled lead sheet. However, many years’ experience of seeing both laid confirms the writer’s firm preference for sand cast lead, which although perhaps marginally less precise in terms of dimension, seems more ductile and malleable for bossing and working to neat details. Such qualities might perhaps be due to its relatively benign method of manufacture, rather than undergoing the stresses and work hardening which must occur to some extent through the compaction of the milling process.
P Webb Roofing and Building Services of Datchet in Berkshire, is a Royal Warrant Holder, and a family firm run by Paul Webb. His work at the Dean’s Cloister of repairing and adapting the timber substrates and laying new sand cast lead was carried out with meticulous care and attention. This involved the removal of both old copper and lead roof coverings, lifting sarking boarding and repairing the oak roof-structures below, adapting bay widths, lengths and steps in the light of Lead Sheet Association recommendations, and to ensure future durability, and relaying new sand cast lead to immaculate detail. The Dean’s Cloister roofs carry an astonishing amount of rainwater draining down onto them from the surrounding chapels and residences, and every junction needed consideration in terms of its ability to cope in heavy rain conditions. One particular area of the cloister roof carries water from St George’s Chapel, the Albert Memorial Chapel, and the roofs of the Galilee Porch and the Chapter Room as well as its own rainwater.
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| Dean’s Cloister roof with Albert Memorial Chapel and St George’s Chapel in the background |
Rainwater pipe outlets from the cloister roof tended to block and overflow during rainstorms because they were constricted where they emerged beside stone gargoyles on the parapet walls. An important part of the work was to improve these outlets and to install very carefully detailed emergency overflows (see illustration), so that even if the outlet pipes were to block, storm water could still disperse without damaging the building.
Special care was taken in the east cloister walk where the ceiling had been decorated with heraldry and other images between its oak rafters. The scheme, which dated from 1874, was carried out in ‘sgraffito’, in fashionable revival of a medieval technique of incising plaster to reveal coloured renders beneath.
The project also involved complicated repairs to the accounts office roof structure. Initial inspection suggested this to be a Victorian roof using salvaged timber, but further inspection involving detailed archaeological analysis revealed something much more interesting – possibly the original 15th century oak roof structure. The timbers were subsequently dated by dendrochronology to within the second and third decades of the reign of Henry VIII, and it is thought likely that a date of c1520 will be confirmed by archival research. Early and enormously thick pitch-pine sarking boards to this roof were most unusual, and considerable care was taken to retain them.
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| New lead steps introduced to improve access for roof maintenance |
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| New curved outlet pipe and emergency overflow from gutter sump |
Every detail was worked out between the lead contractor and the professional team. It has to be said that this was not always to current Lead Sheet Association recommendations, as the prime directive was for minimum intervention into the historic building fabric. For example, traditional water-stop laps rather than steps were used on roof slopes, as with sufficient fall, and following the precedent on the roof of St George’s Chapel, they would not alter the essential ‘old roof’ aesthetic of the cloister roofs. Over the north cloister walk, Code 9 lead was used in single sheets to the full length of the slope, as laps were impossible on the very low pitch of the roof. Bossing and working at junctions was perfectly wrought, with no over-working of lead and very aesthetic dressing. Paul Webb successfully achieved a difficult detail for curved, circular-section lead outlet pipes through the parapet walls, having them bent to a curve without distortion, by filling them with sand and gradually working them to a different curve for each one. Paul also chose to employ a Tudor headlap to his lead sheets, which increased the amount of work for his team, but looks splendid, and imparts that sense of quality that a royal building warrants.
The work was completed in autumn 2004, and the project was subsequently runner-up in the prestigious Murdoch award for leadwork in 2004, out of 6,500 entries. The programme of repairs was generously funded by the Friends of St George’s Chapel and also grant-aided under the English Heritage Cathedral Repairs Grant Scheme for 2003/2004 and 2004/2005.
