A series of setbacks has hit refurbishment plans at St Margaret’s Hall in Bradford on Avon, after problems with the roof and floor were discovered when work began.

The project budget stood at £790,000 when builders started in June and the revamp – which includes retractable seating, a new roof, front access for disabled visitors and removal of the stage to provide a mezzanine level and office space – was due to be finished by December.

Now the work is facing delays and extra costs.

Bradford on Avon Town Council clerk Diane Holmes said: “It is disappointing that these hidden defects have been uncovered, but when dealing with an old building like this there are bound to be unknowns.”

When the roof was strip-ped, metal angle battens, due to be reused, were seen to be corroded. They broke when metal clips were fixed in.

Project manager Ian Muir said: “The option we came up with was to re-pitch the roof, using timber.”

An estimated cost of £78,000 will come from a contingency fund and general reserves, but work could be delayed by eight weeks.

A survey by structural engineers in 2012 showed battens covered in insulation foam, but no significant defects were identified in the area they exposed.

Dismantling of the stage and floor strengthening work uncovered large voids filled with rubble, thought to be pits from when the hall was a dye house. More pits, which could drop under the weight of the seating, may be found.

An agreement was made at last Tuesday’s town council meeting to use ground penetrating radar, costing £1,400, to check it out.

If that is inconclusive, the floor must be taken up and rescanned. Any pits found will need to be filled in and topped with reinforced concrete.