The condition of the 64-metre tower at the Grade I listed building has deteriorated in recent years due to its exposed position.
Restoration is now underway as part of the much-maligned £2.2 million scheme to repair the building, which dates back to 1897.
Coun Bryan Lodge, Sheffield Council’s cabinet member for finance, said: “Restoration of stonework on the tower is costing £70,000 alone.
“The tower is one of the city’s tallest structures and exposed to the elements and, over the years, the condition of stonework has deteriorated.
“Pieces have been falling off while other sections need to be repaired.”
Every inch of the gritstone tower is being examined for decay and some sections of masonry are being chipped out and replaced.
Coun Lodge said: “This is necessary work which is needed to preserve a Grade I listed building and also protect the health and safety of people in the streets below.
“We don’t want pieces of the Town Hall dropping off and hitting people on the head.”
Bird netting will also be replaced to protect the outside of the building and inside rooms will be modernised so the council can reduce spending on rented buildings elsewhere.
Lib Dems say the cost of the project should be reduced so there is more money for services.
But Coun Lodge said: “We have an accommodation strategy the Liberal Democrats supported when they were in power, which will save us £30 million.
“This is not about posh offices - some rooms in the Town Hall have broken windows, while others are quite run-down inside.”