With a client like no other, the stipulations put on WellMax Scaffolding’s team to get the job right and keep the priceless features at the British Museum safe were tough. Tim Clark reports on how the team rose to the challenge

Project British Museum refurbishment Client The British Museum Main contractor Curo Construction Scaffolding subcontractor WellMax Scaffolding Contract value (scaffolding subcontract) £500,000 Start date April 2020 Completion April 2021

They arrive in the dead of winter. Their mission is to assemble a pinpoint-accurate scaffold, hidden behind a world-famous facade, to enable the safe replacement of an ageing roof on one of the world’s most famous museums.

WellMax Scaffolding estimating director Chris Maxwell-Smith says the unnamed crew work like ninjas. So skilled are they at their trade and so sought after are their skills that he vows to keep their identities a secret. “If I tell you who they are or give their names they’ll get poached by a rival,” he jokes, as he reflects on how his firm took on the job of erecting a safe, stable platform for working at height on the Grade-I listed British Museum’s south colonnade.

Completed in 1847, the neo-classical colonnade is one of the most prominent Victorian-era edifices in Europe. The famous fluted columns of the museum’s grand portico support a copper roof, which shelters the main entrance, but by 2020 the roof was in urgent need of repair or replacement.

Main contractor Curo Construction was tasked with replacing the old copper panels and undertaking structural reinforcement work to strengthen the roof and prevent water ingress. The work included painstaking restoration of lime-plaster soffits, featuring 42 oblong coffers, which form the ceiling of the portico along the colonnade. This sensitive work was tackled using traditional plastering techniques.

“We had the structural element of where we could place scaffolding and the visual brief, so [visible] scaffolding had to be minimised”

Gaining access to the upper levels of the huge museum was an engineering feat in itself. Specialist scaffolder WellMax was contracted to design and assemble a safe working space for the main contractor, which would not “affect or disturb the building”, while appearing as discreet as possible.

The thorough brief from the British Museum provided a roster of constraints, including structural and aesthetic requirements, dealing with the public and – given the timing of the job, which started just over a week after the first lockdown in England – COVID-safe working practices.

Among those constraints was the need to ensure the scaffolding made minimal contact with the fabric of the historic building, and that the scaffold poles should not pass in front of the 13.7-metre-high Portland-stone columns, to maintain the museum’s iconic facade during the works. The structure had to be designed to withstand wind loading without putting any strain on the museum itself.

Maxwell-Smith says: “Even now, what strikes me is the actual scale of the building. You don’t grasp just how big the British Museum is until you walk right up to it. The columns are 1.5 metres wide, and the volume of stone used in the building and the ground-bearing weight has meant that no extra load can really be applied – or only very carefully.”

The British Museum was built before many building regulations, such as those overseeing ground loads or weight distribution, were codified. The first issue to overcome involved finding a way to assemble the scaffolding behind the columns without placing excessive load on either the building or the ground immediately around it.

Maxwell-Smith says: “Most buildings aren’t designed to have temporary structures placed on them, so they aren’t [necessarily able] to bear the extra weight.

“The museum building pre-dates general-access loading conventions, so you had two facets to the build: we had the structural element of where we could place scaffolding and the visual brief, so [visible] scaffolding had to be minimised. The museum didn’t want to see any uprights in front of the building – everything had to be hidden behind the columns.”

The scaffold’s hefty roof was designed to move with the wind – cleverly cutting peak load

Structural engineers working for The British Museum stipulated where each of the scaffold bases could be placed, accurate to the nearest millimetre, with limited areas of 450mm by 2,000mm behind each column. One key decision was that the scaffold bases could only be placed where it was known that masonry walls and piers existed underneath. The scaffolding design also ran counter to the usual norms by taking up as little space as possible, in contrast with the typical practice of spreading load as widely as possible at ground level.

The answer was deceptively simple, Maxwell-Smith recalls: “We said, ‘why don’t we use modular beams?’” – which are 450mm wide. “We decided to stand those up and minimise the work [to install the scaffold structure] for the guys working at height. The modular beams also came with pre-built bracing. That was the first hurdle to overcome.”

Pre-constructed and widely available, scaffold beams form a core part of a toolkit for large projects, where they are typically installed horizontally. Using the 450mm beams as a pair of uprights allowed the team to focus the scaffold loadings on the particular areas in a uniform manner, while also reducing equipment, labour time and risk. The same alloy beams were used to form bridging sections between the support towers.

Scaffold props were allowed to bear on non-decorative flat surfaces of the facade. No restraints were permitted on the stone columns, however, and no fixings could be attached to the building fabric. Where the scaffold met the columns, guttering was used to form a protective case over the column itself.

The scaffold also included a cantilever section, jutting out at the top of the structure to provide access to the pediment – the triangular section of the facade directly above the columns – where further restoration was required. Knee bracing was installed to transfer the cantilevered loads back to the structural wall.

The working platform, with knee bracing and alloy beams

The triangular pediment features a recessed space called a tympanum, which is elaborately decorated with an array of carved stone figures. Created by celebrated sculptor Sir Richard Westmacott, the artwork ranks alongside many of the artefacts inside the building in terms of historical significance.

“The statues within the portico itself are priceless items,” Maxwell-Smith says. “It’s interesting as the British Museum doesn’t have an insurance policy. You can’t replace the statues as they are literally irreplaceable.” He adds that his team were, unsurprisingly, “very, very careful” when constructing the scaffold elements in the vicinity of the carvings.

“We chose not to restrain the roof fully, and actually allow it to move subtly up and down. That meant that the loads we put into the building were much smaller”

The biggest challenge, however, was not the art but the elements. The scaffold and its large temporary roof had to counteract wind loading safely, and do so without placing any significant additional load on the museum building.

Maxwell-Smith says: “To restrain a roof of that size [over the museum] would take an immense weight [of ballast] and we simply didn’t have that capacity in the building. It was something the museum simply wouldn’t allow us to do.”

The team came up with a surprising solution to this conundrum. “We chose not to restrain the roof fully, and actually allow it to move subtly up and down,” Maxwell-Smith explains. “That meant that the loads we put into the building were much smaller.” To control this movement, stop tubes were placed on the scaffold. With a minimum spacing of 79mm between the top chord of the beam and the stop tube, the roof could rise in the wind by 65-85mm, which meant the weight of the roof itself would help to counteract the applied wind load.

This novel design reduced the need for kentledge concrete blocks to stabilise the structure, which would have added to the overall weight placed on the building. Where static loading was needed to minimise wind movement, water butts were often hung from the scaffold in preference to concrete blocks.

Maxwell-Smith recalls: “It was quite scary at times and [the structure] moved quite a lot, but we had confidence in our designers and we did structural modelling. We went in there as a team, with senior management, and health and safety and contract managers, to ensure it was both designed and installed correctly.”

The “mesmerising” new copper roof

With the complex task mapped out, it took six weeks to place the scaffolding for the soffits on the east and west wings of the museum, with the overall project taking 18 weeks.

Once all of the scaffolding had been painstakingly put in place, the structure above the columns was wrapped, not just to mask the construction work but to meet the client’s desire to maintain the look of the building as far as possible. WellMax sourced and installed a mesh printed with high-resolution digital images of the famous portico and its sculpture, designed to replicate what was covered up as closely as possible.

One year after the scaffolding went up, the restoration works were finished. “The completed works are absolutely fantastic to see,” Maxwell-Smith says. “The copper, which has replaced the ageing roof, is mesmerising. It’s a shame it can only be seen with a helicopter.”

The successful work to restore this landmark building went on to be named Project of the Year by a Specialist Contractor at the 2021 CN Specialist Awards. Entries for the 2022 Specialist Awards are open until 15 April.