How we restored a historic tower to make it watertight

Beckford’s Tower was constructed between 1826 and 1827 for novelist and MP William Beckford, designed by architect Henry Goodridge. The tower functioned as a study retreat and housed Beckford’s collection of art, objects and rare books, made possible through wealth from the transatlantic slave trade. Following his death in 1844, the building was used as a mortuary chapel and a private residence. The building was Grade I listed in 1972 and subsequently became the responsibility of the Bath Preservation Trust (BPT). Between 1997 and 2000, the trust carried out a comprehensive restoration, including dismantling and reconstructing the lantern at the top of the tower.

By 2019 the condition of the lantern was again causing concern and the building was placed on Historic England’s At Risk register. In 2021 the trust appointed my practice, Thomas Ford & Partners, as conservation architect and lead consultant for a project, supported by the National Lottery Heritage Fund, to repair and redisplay the tower.

Our approach was the same as it would be for any historic building. It was based on developing a clear understanding of the building’s history: its construction, materials, maintenance, previous repairs, and current defects. We were fortunate in that the BPT’s senior curator, Amy Frost, had an exceptional knowledge of the building and of the surviving archive information.

The building’s problems were specifically related to water penetration, which had damaged the joinery at lantern level as well as the elaborate plaster ceiling of the belvedere. Water penetration and consequential damage in the same area had also been a significant issue before the restoration some 20 years earlier.

The construction of the tower is significant. Up to and including the belvedere, it is a square shaft solidly constructed in Bath stone. Above the belvedere, the plan form becomes octagonal and continues in stone for a further storey. At lantern level, and above, the structure is entirely timber, ultimately capped with an elaborate cast-iron roof. The lantern structure consists of a series of eight substantial timber posts with the spaces between them fitted with timber panels, identical to panelled doors in construction. There is an inset gutter around the base of the lantern and a triangular sump at each corner above the belvedere.

At the start of the project, two of the lantern panels had been significantly affected by rot, as had the three associated structural posts, the sole plate beneath, and the bed of the inset gutter. Leaks from the triangular lead-lined sumps had damaged two areas of the ornate plaster belvedere ceiling. The timber panels had been clad with plywood sheets to mitigate the water penetration but these sheets were themselves failing.

To put these defects into context, it is worth noting that the tower is 47m high and 220m above sea level, on the top of a hill high above the city of Bath. It is, to say the least, a highly exposed location and the lantern appears to have suffered problems with water penetration throughout the tower’s history.

Information from the previous restoration proved very informative. The lead work of the gutter around the base of the lantern was traditionally detailed to a high standard but it had nevertheless started to leak within two years of completion, leading to the gutter being lined with a fibreglass-reinforced resin. A letter subsequently written by the architect responsible for those repairs made it clear he had advised against using lead detailing and was not surprised it had failed to prevent water entry.

Also of note was that the panels and posts on the south-west side of the lantern that were found to be rotten in 2021 had also been replaced in 2000.

Taking all of this evidence into account, we concluded that the original materials and detailing were unsuited to the exposed location and orientation. In high winds, and with the eddies that form around towers, it seemed very probable that rainwater could be driven at the leadwork from almost any angle, which would explain why the traditional detailing had failed. The increased frequency of severe weather events resulting from climate change would only exacerbate these historical problems.

Although the repair or replication of the original detailing is correct in conservation terms, where this approach has failed it raises broader practical and philosophical questions. Ultimately, it was agreed that the preservation of the tower’s appearance and structural integrity was more important than using a traditional approach. Following lengthy discussions with Historic England, who were part-funding the repair, it was agreed that the gutter leadwork would be replaced with a specialist, liquid-applied, fibre-reinforced, material to create a jointless lining. The damaged panels and posts were to be replaced in Accoya – a modified timber that is both rot-resistant and highly stable. We also re-detailed the junction between the gutter upstand and the lantern joinery. Here the cover flashing had previously been let into a groove in the timber, which had proved a weak point. The triangular ‘sumps’ had simply failed because the lead had been laid in single sheets with no scope for thermal movement. These have now been relined in lead but with improved detailing.

Climate change and extreme weather events

Although undoubtedly exacerbated by increasing levels of rainfall and extreme weather events, the tower’s underlying problems appear to be a result of its historic detailing. The use of a structure composed entirely of painted timber at this sort of elevation and exposure is unusual. Had the lantern structure been of stone, it seems unlikely that these problems would have arisen.

The catchment area for rainwater is quite small and there is no evidence that a lack of capacity has been at the root of any of these problems. There are no gutters other than the inset gutter around the lantern and the only rainwater pipes are short ones that connect this gutter with the sumps below. From the sumps, the rainwater is simply discharged away from the face of the building.

Lessons for making historic buildings watertight

All historic buildings are unique. The more we see and understand, the more we learn. But this information should only ever inform our approach to other buildings rather than dictate it. The process of understanding each building – its history, significance, construction, quirks, and problems – should be gone through on every occasion.

As architects, we must always do our best to provide the optimum overall solutions within the many constraints and conflicting demands of historic buildings, with particular regard to future maintenance and how this can be carried out safely and cost-effectively. This is critically important in relation to rainwater collection and disposal systems.

Always provide the building owner with a clear maintenance plan and a recommendation for regular periodic inspections. It is also worth emphasising the likely physical and financial consequences of failing to carry out this maintenance.

Consider the effects of increased rainfall on the capacity of rainwater systems, such as gutters and downpipes, and improve resilience wherever possible. In addition to increasing capacity and improving water flow, consider failsafe solutions to deal with unexpected blockages – for example, providing overflows to parapet gutters.

Consider the design of leaf traps, particularly in sumps. Some leaf traps seem to actively encourage blockages and can be sealed entirely by a single plastic bag or a small number of leaves.

Clive England is a director at Thomas Ford & Partners

See other ideas to adapt our buildings to our changing climate: Powerhouse Company’s floating office, tips on shading design, retrofitting houses to reduce overheating, the role of facades in avoiding overheating