Repairing the deteriorated concrete at 39-year-old Ralph Wilson Stadium will cost $4 million to $5 million in order to keep the stadium in shape for the next 10 to 15 years, an engineering study has concluded.

The damage is “not severe,” the county’s public works commissioner said Thursday, but close to a third of the concrete frames that hold up the upper deck have “minor to medium” deterioration from water and salt that have seeped in over time.

“The concrete’s condition is appropriate for a stadium that old,” said Erie County Public Works Commissioner John C. Loffredo. “It’s not extreme in either way, good or bad. It’s appropriate.”

The $4 million estimate surprised County Legislator Timothy R. Hogues, D-Buffalo, who said he had anticipated a higher estimate.

“Some of the calls I’ve gotten from constituents were concerned about the actual safety of the structure and the age of it,” said Hogues, who is chairman of the Legislature’s Finance and Management Committee.

Erie County hired the engineering firm DiDonato Associates to assess the condition of the concrete as county officials negotiate a new lease with the Buffalo Bills that could call for renovating the county-owned stadium in Orchard Park.

The study showed that about 70 percent of the concrete frames that hold up the stadium’s upper deck are in good shape but that the remaining 30 percent will need repairs, Loffredo told legislators Thursday. Those repairs, he said, would put the structure “in reasonable shape for another 10 to 15 years. Then you have to look at it again.”

The county is exploring options for the concrete work, Loffredo said, but that decision likely would be made in conjunction with lease negotiations. The county is in three-way talks with the Bills and state officials over renewing the team’s lease after it expires in July 2013. “We’re looking at the options,” Loffredo said of the concrete work.

“It’s kind of tied in to the lease talks and putting improvements in the stadium. It would have to dovetail with that.”

The Buffalo News has reported that the Bills are seeking stadium renovations estimated at $200 million to $220 million and that state and county officials have pressed for a commitment to keep the team in Western New York.

Those in the community who have been pushing for a new stadium in downtown Buffalo were eagerly awaiting completion of the study. Major players in the lease negotiations, however, have called the proposed outer harbor stadium a nonstarter. Others, Hogues said, have questioned whether it’s worth investing public money in the NFL’s sixth-oldest stadium.

The DiDonato study has not yet been released, but Loffredo gave legislators an update in a budget hearing Thursday.

Deterioration of the stadium’s concrete is caused when salt used to de-ice the upper deck in the winter and water seep into expansion joints and into the concrete below, Loffredo said. Some of the recommended repair work would include sealing and waterproofing those areas to prevent more erosion in the future.

The county has periodically hired engineering firms to assess the condition of the concrete and has done work to repair the structure in the past. The latest study, which was estimated to cost the county up to $210,000, updated the previous assessments.

The work recommended by DiDonato was in line with what county officials expected, Loffredo said. He compared the needed repair work with that done on a bridge to keep it structurally sound over time.

“It’s nothing that you would say, ‘That’s it, end of life, we’ve got to walk away, we’ve got to look at something else,’ ” Loffredo said. “No.”