The former Oamaru Chief Post Office building of 1884 has been home to the council for the past 30 years and has a category 1 Heritage NZ classification.
From this Monday an Oamaru contractor, Breen Construction, will begin the first major reroof in decades.
"The replacement of the roof is believed to be the first for several decades, and comes at a point where it is more cost-effective than continuing to maintain a deteriorating roof," Waitaki District Council project manager Grant Rhodes said.
Notably, the existing guttering had not been able to handle heavy rain, causing leaks into the heritage building.
"Certain parts of the roofing are original, including the zinc tiles and lead on the tower," Mr Rhodes said.
"Some original clock-face glazing, which has cracked over the last 121 years, will be replaced with heritage glass with Heritage NZ approval.
"We’ll be taking excellent care of one of Oamaru’s iconic heritage buildings with this work"
The work, budgeted at $1million, included new Colorsteel and butanol roofing, guttering renewal and installing stormwater overflows within the main roof.
"There will also be upgrades to the lift in the Thames St building, the servicing and relocation of heat pumps and air conditioning to a central location and a refurbishment of the flagpole."
Mr Rhodes said the clock tower would also be washed and repaired, roof railings and zinc tiles repainted, copper guttering and downpipes repaired and the masonry given a chemical treatment, steam-clean and repointing.
"The clocktower windows will also be repaired and repainted, and the fluorescent lighting behind the clock face will be replaced with LED lighting."
The building has been listed since 1990 and the council took over the building in September 1994 after a $2.5m refurbishment.
Mr Rhodes said the roof replacement was approved in the 2024-25 enhanced annual plan, and the work was planned to be finished by February.
