Bringing a derelict, roofless church back to life and converting it into a state-of-the-art recording studio for one of London's great orchestras was always going to take more than one weekend. The dead cows and eleven hundred corpses didn't help, either.
"The bodies were taken out before we started on site," says Stuart Benham, site manager for Wates Construction. "I think there were about 1100 down there." He is talking about the contents of the crypt and graveyard of St Luke's, a church beside London's Old Street that structural engineer Arup and contractor Wates is converting into a £16m educational centre for the London Symphony Orchestra.

Dealing with the bodies was only one of many challenges faced by the project team. The roofless and derelict church was suffering from bad subsidence, yet had to be turned into a state-of-the-art concert venue and satellite broadcasting and recording studio. It also had to be impervious to external noise – not easy, given that it is underneath the flight path into Heathrow and next to the main road.

The grade I-listed church was designed by Nicholas Hawksmoor and completed in 1733. It was abandoned in 1959 because the subsidence affecting the structure was so bad; it was stabilised by the crude resort of taking off the roof to reduce the weight on the foundations.

The only way to deal with the aircraft noise was to install an extremely heavy, dense roof structure, something that directly conflicted with the fragile state of the church structure. Extensive investigation was carried out to see how much weight the walls could take.

Luckily, parish records were available. "We could understand the timescale in which the settlement took place," says Michael Courtney, Arup's associate director responsible for the structural engineering. Foundation movement became a problem soon after the church was built and culminated in underpinning of the south and east walls in the 19th century. However, the north wall and the adjoining section of the east wall continued to move and so the decision was taken to deconsecrate the church in 1959.

Records showed a 17th-century pond to the north-east of the site, and Courtney conjectured that wooden piles had been used to support the walls against a high water table. In the 19th-century, water extraction for a burgeoning industry would have lowered the water table, thus exposing the piles to air and causing them to rot.

This hypothesis was dismissed, however, when ground investigations revealed the unexpected cause of the subsidence. "We discovered the depth of fill varied from 2.5 m at the east end and 1 m at the west. The differential compaction of this caused the cracking," explains Courtney. Fill is a mixture of earth and clay, plus detritus left over by human activity, and this steadily rises over the centuries. Under this is a stable layer of undisturbed gravel. "We noticed the gravel suddenly stepped down by two metres. This strongly suggests someone excavated it for building purposes a very long time ago," Courtney goes on. "When they built the church, they wouldn't have known that."

Because the walls were cracked and clearly weakened, a decision was taken to support the roof independently of the walls and stabilise the north wall. The walls have simply been stitched together and repointed. Luckily the heavy tower on the west side was on stable ground and could be left undisturbed, apart from some localised stone repair.

Courtney counsels caution with this type of restoration: "Buildings tell you things that calculations don't; traditional materials and modern building techniques work in different ways."

Before work could start on the roof, new basements had to be created and the crypt prepared. The crypt underneath the church was full of bodies and was exhumed by the appropriately named specialist Necropolis before the contract commenced in January 2001. The crypt was demolished and the graveyard dug out to form new basements on the south and north side of the church. The basements were taken down to the level of the base of the foundations of the walls, and the north wall was underpinned. "We had to pour a huge amount of concrete underneath the foundations to support the wall," says Wates' Benham. A newly constructed reinforced concrete slab forms the roof of the basements and the floor of the church. "The critical thing was not to undermine the foundations of the tower – it is very heavy," says Courtney.

The structural engineer's solution for the new roof is independent steel columns to provide support. Originally there were going to be 12 columns sitting on the concrete columns supporting the floor slab, but this would have affected the auditorium space. Instead just four large-diameter columns have been used.

The tops of these branch out so each column supports the end of three roof trusses. Courtney is delighted with these "trees", which he describes as very efficient. "The shape reflects and reacts to the forces and gives you a lovely open space in the middle."

Precast concrete slabs, 150 mm thick, block out the noise and are supported by six roof trusses resting on the columns. An insitu slab was considered too difficult to install because of the roof slope. Underneath the slab are two separate layers of plywood to further reduce noise transmission. A neat touch is roof-level lighting rigs that double up as tie-rods. Outside, the roof is finished with Westmoreland slate.

Inevitably there have been a few minor hiccups. "There were still 30 bodies and two cows down here when we started doing the footings and the south basement," recalls Benham. "We had to bring back Necropolis and the archaeologists." One particularly difficult job was installing the temporary supports while new openings were formed in the old walls to link the new basements. "The temporary supports were quite tricky. Even though you had the drawings, you never knew how it would work out," he remembers.

Work is now well advanced on the church. The roof is nearly finished and clad in its new slate. The forest of scaffolding in and outside the building is due to be struck any day now, once again revealing the architectural glory of Hawksmoor's work to the passers-by in Old Street. People all over the world will also be able to share the benefits of the finished restoration in December when the first satellite broadcasts of the LSO's music begin.