A major renovation of Dereham's main arts venue, the memorial hall, has hit another obstacle after builders discovered the roof needed replacing – at a cost of �32,000.

A detailed inspection of the tiles has revealed they have become porous over time and have let moisture seep through to rust nails and rot underlying battens.

Town clerk Tony Needham said the most cost effective solution would be to replace the whole roof as soon as possible while scaffolding for other parts of the project was still up.

He said: 'If we don't make a decision quickly then the builders cannot take the scaffolding down when they want to. That will delay their work and we will get penalties on the contract.'

A new roof using clay tiles, which was likely to have been the original material used when the hall was first built, should last at least another 50 years.

On Tuesday, January 18, Dereham Town Council will hold an extraordinary meeting to discuss allocating the extra money needed to pay for the work.

Members will be asked to release at least the �32,000 which has been quoted for the new roof from the authority's reserves, while also considering whether to top up the project's contingency fund.

The council had originally allocated �108,428 for unexpected expenses associated with the �2.6m memorial hall project.

In December, just �17,000 was left after a series of obstacles pushed costs up. Among other problems, beams holding up the balcony had to be pulled down and replaced with something stronger, work to strengthen the structure of the foyer was needed and extra water storage was put in.

The council had already recently agreed to increase its reserves to allow for any more obstacles.

As he inspected the ageing roof, Dereham mayor Robert Hambidge said: 'I'm not going to say I'm confident, but I really hope this is the last of the nasties.'

Councillor Linda Monument said she was very much in favour of replacing the roof as soon as possible. 'The sensible approach is to get it done straight away. Once it's done, it will last 50 years or more, but if we don't do it now it could endanger what we are doing elsewhere,' she said.

The renovation of the memorial hall will see new function rooms, dressing rooms and toilets created, along with new flooring, lighting, tiered seating and a glass-roofed atrium.

Mr Needham said he hoped it would still be completed in autumn 2011 despite the many setbacks.