Repairing the Nave Clerestory at Rochester Cathedral
Ian Stewart
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| The south nave clerestory prior to repairs: distorted lead rainwater pipes and the decaying hood mouldings and mullions are obvious. (Photo: Colin Tolhurst) | Rusting glazing bars were causing the stone of the jambs and mullions to spall as the corroding iron slowly expanded. (Photo: Universal Stone) |
Rochester Cathedral has its origins in the 7th century. In 604 Justus, the first bishop of Rochester, was consecrated by Augustine of Canterbury. However, it was not until 1083 that the present building was begun by the Norman bishop Gundulf, who founded the Benedictine monastic community which survived until the dissolution in 1541. What can be seen today, as so often in such ancient buildings, is an amalgam of different architectural styles surviving from different building and rebuilding programmes over the centuries. These are overlaid by the work of subsequent repairs and restoration. This is particularly true at Rochester, which is an architectural potpourri with examples of excellent work from every era. The overall effect is one of ramshackle charm rather than imposing grandeur. The nave is a typical example of this: the arcades and triforium are fine mid-12th century work, partly rebuilt in the 13th century, and the clerestories and roof were raised in the 15th century and extensively restored in the 19th.
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| Wrongly bedded mullion stones were splitting along the natural grain of the stone. (Photo: Universal Stone) |
The 2003 quinquennial survey of the condition of the cathedral’s fabric identified a number of defects in the nave clerestories that required urgent attention. The hood mouldings of the windows were failing, with many splitting through in the plane of the wall. This was probably exacerbated by the stones being wrongly bedded. Sedimentary rocks, such as sandstone and limestone, have a natural horizontal grain. Experience has shown that these stones are most durable when set in the building in the same orientation, with their beds horizontal. There are exceptions to this rule: in the construction of arches the bedding planes should be at right angles to the curve of the arch, and cornices or copings should be laid with the bedding planes oriented vertically, running back to front, so that they continue to shed water even when eroded.
The window mullions had been fixed in very long lengths with the bedding planes upright and this had resulted in splitting. Many mullions and jambs were also being spalled by the corrosion of the external iron window bars. The masonry and ironwork had all been renewed in the 1870s during Sir George Gilbert Scott’s extensive campaign of restoration.
Repair works were specified as one of three packages for a successful application to English Heritage for grant aid under their cathedrals scheme. The other two packages related to repair and improvement of access arrangements at high level and rainwater disposal. Competitive tenders were obtained and a contract was let for all three packages to Universal Stone of Great Dunmow in Essex. Work commenced in July 2006 and was completed by Christmas 2006.
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| Above: renewed mullions with three stones on their natural bed in place of one long vertically bedded stone. Below: a new cast lead hopper replaces one of the broken cast iron hoppers on the south transept: a repaired lead hopper can be seen in the bottom picture, below left. (Photos: Universal Stone) |
One issue that had to be resolved at the outset was access. Very substantial savings were made by using roped access techniques for the repair of the rainwater goods. The nave clerestories, however, had to be scaffolded. Externally, raising scaffolding from the roofs of the aisles was straightforward. Internal scaffolding was more challenging as it had to be arranged to minimise the impact on the constant use of the cathedral nave not only for services but also for secular functions such as degree ceremonies.
The defects noted above were all visible externally. However, the condition internally was unknown since there is no access at that level. Scaffolding was therefore required not only for inspection and possible repairs but also for health and safety reasons, should the external works dislodge any masonry or glazing inside. The south side was dealt with first and scaffolding was then transferred to the north so that as much as possible of the nave was left unobstructed at any given time.
The clerestory windows had been entirely renewed by Scott in Chilmark stone, a fact confirmed by Bernard Worssam, the consultant geologist. This grey-green stone from the Vale of Wardour in Wiltshire is very similar to Tisbury stone, from which Salisbury Cathedral is largely built. It provides a reasonable match for Reigate stone, which is why Scott favoured it for renewing that material. It cannot quite make up its mind whether it is a sandstone or a limestone, being defined as a slightly glauconitic calcareous sandstone, or a sandy limestone. It was decided to use the same material for repairs since the defects had largely arisen from the workmanship rather than the material itself. The new stone has certainly blended in amongst the old remarkably inconspicuously.
Two minor changes of appearance were accepted, however, on the grounds of practical necessity. First, more joints were introduced in the replacement mullions since the new stones were laid on their natural bed, limiting their available height to about 450mm. Second, lead weatherings were added to the tops of the hood mouldings to improve water shedding and to prolong the life of the stones that were retained and which might also be wrongly bedded, even if they had not yet developed splits. Some of those which had incipient splits were given additional support by drilling and inserting stainless steel dowels bedded in epoxy resin. These changes are not visible from ground level.
It was fortunate that virtually all of the work could be tackled from the outside, leaving the glazing in situ. Mullions and jambs were cut back to the line of the glass and jointed to the internal half which had remained sound. Only one window required complete renewal of the mullions with the consequent resetting of the panels of leaded lights.
It was also possible, with great care, to remove all the iron bars for repair on the bench. The corroded built-in ends were cut off and stainless steel replacement sections welded on. When the bars had been reset in the repaired stonework the glazing then had to be reattached. This also required access from inside. It was not possible to solder on new copper wire ties with the glazing in situ, so holes were very carefully drilled through the hearts of the lead cames on either side of the iron bars to allow copper wire to be threaded through from inside to out and then twisted up to secure the glazing.
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| Hood moulding and stone repairs, before and after: (above) the SPAB-style tile repairs were added in the 1920s; (below) the lead weathering was added to extend the life of new and retained hood mouldings. New Caen stone indents can also be seen here. (Photos: Universal Stone) |
Repairs to the general walling of the clerestories were limited to localised repointing of failed joints and the renewal of a few very badly eroded stones; Kentish Rag in the rubble masonry parapets and Caen in the lower walls. These had been rather beautifully repaired in the 1920s using SPAB-inspired tile, brick and mortar repairs, including repairs to the main stringcourse above the clerestory windows. These were left wherever they were sound but the stringcourse was also given a lead weathering to give it a further lease of life.
The mortars used for all these works were based on a moderately hydraulic lime, rated NHL 3.5, mixed with sharp sand and, in the case of some joints, a little stone dust for colour. No Portland cement was used. Relatively small quantities of mortar were required and the ability to knock up batches of mortar quickly using this dry, bagged lime was much more convenient than using slaked lime in putty form.
At the same time, the early 19th-century lead hopper heads and downpipes on the clerestories were overhauled and splits repaired with discrete welded lead patches. The pipe collars had originally been nailed to wooden blocks set into the masonry. The iron nails had rusted and the blocks had decayed so the pipes were re-attached with stainless steel fixings.
Once the scaffolding was struck and the dust had (literally) settled, the overall result was a very inconspicuous repair, retaining all the patina and irregularity with which people were familiar, but ensuring a robust, weather-tight fabric for the coming decades.
The new leadwork to string course and hood mouldings and the repaired rainwater pipes should ensure the safe shedding of rainfall. The tipped iron window bars will not now damage the stonework into which they are fixed. The inherent problems of the masonry have been reduced by ensuring that the replacement stone is laid on the correct bed to get the best possible weathering properties from the Chilmark stone.
