Janitors who clean buildings at Tufts University have overwhelmingly ratified a new four-year contract with the school’s maintenance contractor that will net them a 11.4 percent wage increase and lower health care premiums.

The agreement was reached Monday after a 12-hour marathon bargaining session and just two hours before a midnight deadline for a threatened strike. Tufts was not involved in the negotiations.

Starting in January, the workers will get a 50-cent raise, to $19.85 an hour, with additional increases until their hourly wages reach $21.55 on Jan. 1, 2020, according to 32BJ of the Service Employees International Union, which represents the workers. Night shift workers will get additional hourly differentials starting with 25 cents and increasing to 50 cents over the life of the contract.

Newton-based C&W Services, Tufts’s maintenance contractor, agreed to add three full-time positions starting July 1 at the school’s campuses. Currently, about 60 percent of the approximately 200 janitors who maintain the campus straddling Somerville and Medford work full time.

Under the new contract, the workers will no longer have to pay a $100 monthly premium for family health insurance coverage. C&W, a division of brokerage firm Cushman & Wakefield, also agreed to increases to the workers’ pension and training funds, overtime for janitors who work when the school is closed for a non-state-declared emergency, and three bereavement days for workers who have to travel internationally.

Katheleen Conti can be reached at kconti@globe.com. Follow her on Twitter @GlobeKConti.