PETERBOROUGH — Selectmen approved a 15-ton weight limit on the Main Street Bridge Tuesday night, in a recommendation that could for up to two years impact downtown deliveries and the Fire Department, but is not expected to disrupt ConVal School District bus schedules.

The weight limit, recommended by engineers and Public Works Director Rodney Bartlett, goes into effect Friday.

Due to the bridge’s poor condition, engineers are also recommending complete replacement of the Main Street bridge, built in 1940, rather than rehabilitation, as had been planned.

The engineers — Hoyle, Tanner & Associates Inc., Manchester — issued their report on Dec. 22 and Bartlett made his recommendation to the Select Board Tuesday night to post the weight limit, saying it would be in effect until bridge replacement is complete, possibly for as long as two years.

In their report, the engineers wrote, “Overall, the bridge appears to be in poor condition.” This is due, they wrote, to freeze-thaw activity, and de-icing chemicals that have permeated the concrete.

Bartlett told the Select Board Tuesday night he will go door-to-door to notify merchants, trucking companies, and the school bus company about the weight limit in addition to using e-mail, the newspaper and the town Web site.

He said that, initially, police will be stationed at the bridge during times of heavy truck traffic (mornings) and will give warnings and written information to drivers of large trucks that cross the bridge.

In his formal recommendation to the board, Bartlett wrote, “Grove St. will become the main access to the downtown for large delivery trucks.”

He wrote that tractor-trailer trucks that travel without special permits can weigh as much as 40 tons.

Tuesday night, he told the Select Board fire trucks are too heavy to use the bridge.

Buses, however, are a different story.

Dave Carter, terminal manager for Goffstown Truck, which runs the buses for the ConVal School District, told the Monadnock Ledger-Transcript on Wednesday that, after speaking with Bartlett, he plans no changes in bus routes as a result of the new bridge weight limit.

He said a loaded bus weighs 29,800 pounds, which is less than the 30,000-pound limit.

On Tuesday, Carter had said in a telephone interview that most of the school buses are not fully loaded.