Spanning the British Museum’s central Great Court and encircling the grade I-listed Reading Room with a delicate steel and glass roof was an immense challenge for the project’s engineer, Buro Happold. Not only did the weight of the roof have to be carried by the surrounding listed buildings without damaging them or visually intruding on their classical facades, but the solution had to satisfy the planners’ limit on the height of the roof.

The solution is described by Steve Brown, the Buro Happold partner who designed it, as “a bagel cut in half with the edges trimmed flat”. A series of arcs span the courtyard’s facades and the centra Reading Room, overlaid by two interconnecting spirals winding in opposite directions to create a steel lattice.

To explain the engineering, Brown enthusiastically delves into his briefcase and whips out a sheet of cardboard. “The roof is best thought of as a plate of steel with triangular holes cut into it,” he says, bending the cardboard into a shallow arch. “With a lot of breadth you don’t need depth to get the stiffness in the structure.” If you were to cut his sheet of cardboard into thin strips, he points out, each strip on its own would have no stiffness at all.

Stiffness was important: the Georgian buildings lining the Great Court have big, heavy facades that are capable of transferring loads downwards but have no cross walls to give them lateral stability. “If the roof were to push laterally on these facades, they would just fall over,” says Brown, “and I’d have to go and live in Brazil”.

The engineer’s solution was to rest the roof on a series of sliding bearings supported on a reinforced concrete beam mounted behind the parapets of the facades. The sliding bearings stop lateral forces transferring to the facades, so the weight is only transferred vertically.

At its inner edge, the roof meets the drum-shaped Reading Room, located not in the exact centre of the courtyard, but 5 m closer to the north side. The Reading Room is constructed around a series of cast-iron columns tied by wrought iron straps to give it stiffness, much like a barrel. These columns could only take about 10 mm of movement before cracking, and the building itself could take only limited differential movement should the foundations settle. As Brown points out: “There was no capacity for any extra loads either vertically or horizontally.”

The engineer had to encircle the Reading Room with a ring of columns to carry the weight of the new roof and enhance the foundations by underpinning. This complex operation involved cutting a series of holes in the ground around the building’s base using high-pressure water jets and filling the holes with grout – a process known as jet grouting.

The tops of the columns are linked to a compression ring formed from a structural steel beam cast in concrete and mounted on sliding bearings. This collar stops any lateral load transferring to the Reading Room and balances the thrusts from opposite sides of the roof. To install the beam, the arched brick snow gallery around the perimeter of the Reading Room’s domed roof had to be demolished, and to stop differential settlement, the engineers had to ensure that the weight of the new structure matched exactly the weight that had been removed by demolishing the existing gallery.

The huge expanse of roof is restrained at the centre by the compression ring but, with sliding bearings at the perimeter, the roof was unrestrained and could slide out flat. “The roof was so heavy it would collapse,” says Brown. The easiest solution would have been to tie the roof into an arch with cables, but the architect described this as “too messy”. The structure’s stiffness became “a fundamental part of the solution”, says Brown. “All I needed to work out was how big I could make the holes in my steel plate.”

The steel plate Brown describes as forming the roof is in fact a series of fabricated box sections with thickened top and bottom plates to improve their capacity to withstand bending. Brown describes the box section as “an I-beam with the central web cut in half”. I-beams could have been used to construct the roof “but they’d look awful and would have been a pain in the butt to keep clean”.

The next problem was to design the roof to provide an easy transition from the circular Reading Room to the square courtyard. Buro Happold used a customised form-generating software program to resolve both architectural and structural requirements. The easiest way would have been a high arch, but planning restrictions put paid to that. The software generated a series of steels spiralling out from the edge of the Reading Room in two directions, criss-crossing to form thousands of different-sized triangular glazing holes on their way to the courtyard’s perimeter. The transition from the relatively small perimeter of the Reading Room to the lengthy perimeter of the courtyard means the small triangles on the inside become progressively larger towards the outside.

Until then, the high point of the roof had been envisaged as being halfway between the supporting walls, but with the size of the holes progressively increasing towards the outside edge the strength of the structure also had to increase to accommodate the greater spans between the structural nodes – so roof members had to increase in depth as they neared the perimeter. The revised design saw the high point of the roof shifting further to the outside to accommodate the increased weight at the perimeter.

During construction, the weight of the roof was carried by a series of props strategically positioned beneath the entire canopy. However, once these supports were removed, the entire 800 tonnes would be carried at the inner and outer edges. Because this would cause the roof to deflect, the design had to allow for this deflection and the roof had to be deliberately constructed out of shape.

When it came to assembly, the designers were keen to find a system to ensure that the team on site could not force a piece of steel into the wrong place. Thus each of the 6000 steel sections forming the roof is unique: “The last thing we wanted on a Friday night when it was almost beer o’clock was a guy hitting a piece with a hammer to get it to fit – it could have thrown the whole structure out,” says Brown. Steel fabricator Waagner-Biró came up with a way of manufacturing each section slightly differently so that each section could be connected only to one particular node.

Waagner-Biró also brought to the design a practical knowledge of assembling large steel structures. The strength of the welded joints between sections was critical because the roof will move constantly under the heat of the sun. Waagner-Biró simplified the weld design to make it easier to assemble the steel sections on site. The roof was expected to be the most difficult part of the scheme to construct. In the event, “the roof went in without a problem”, says Carl Wright, project director of construction manager Mace.