The union representing Harvard’s janitors reached a tentative agreement with the University last night, drawing up a six-year contract that will gradually raise the starting wage for custodial workers to $18.50 per hour.

The proposed settlement, which Harvard’s janitors will vote on tomorrow, also institutes a system providing higher wages to janitors working later hours or multiple consecutive days and increases vacation time, sick days and disability benefits.

Under the previous contract between the University and the union, which expired last night at midnight, custodial employees in their first three years at Harvard are paid $13.50 per hour.

If Harvard’s janitors ratify last night’s agreement, Service Employees International Union (SEIU) Local 615, which represents most of Harvard’s janitors, will have come close to achieving several of its initial goals, including an hourly wage of $20.

Representatives from both sides expressed satisfaction with the deal struck last night.

“We’re very pleased to have come to terms on a tentative agreement involving contract provisions that we believe are fair to both the University and the custodians,” said Harvard spokesman Joe Wrinn.

SEIU Local 615 spokeswoman Courtney Snegroff said that the union is pleased with the proposed terms.

“We feel very good about it,” she said. “Victory was made on a lot of fronts.”

Last night’s agreement came after a period of intense negotiation in advance of the contract’s expiration.

Brian S. Rider, the chief negotiator on behalf of the janitors, had left the negotiations on Monday demanding that the starting wage increase to $19.50 per hour over the next five years. Harvard had countered with a six-year agreement that would have gradually increased janitors’ starting hourly wage to $17.

“The discussions we have had with union representatives have been productive and mutually respectful,” Wrinn said. “We had been making steady progress toward a new agreement for several weeks, and both sides worked very hard in the past few days to arrive at an acceptable middle ground.”

Rider was not available for comment last night.

All the previous contract terms between the janitors and the University will remain in place until the janitors ratify the proposed contract, according to Wrinn.

Under the tentative agreement, all janitors who work the late-night shift are set to receive an extra 50 cents per hour, Snegroff said. While janitors currently receive the same pay regardless of shift, the union had originally aimed for 7 percent higher wages for those working the late-night shift, which would have amounted to just under an extra dollar per hour.

Janitors who work for more than six consecutive days will receive extra pay beginning on the sixth day, Snegroff explained.

Harvard’s janitors will also receive several days of vacation during the winter recess or an equal amount of time off at another point during the year and six to twelve sick days, depending on seniority.

Janitors who have worked at Harvard for at least seven years will receive 100 percent short-term disability coverage, up from 75 percent.

Wrinn added that the University will provide an additional contribution to “a joint-education fund for opportunity and advancement for SEIU workers.”

SEIU officials will hold meetings today and tomorrow to lay out the proposed contract to the janitors, and the contract will be put to a majority vote at several meetings across the campus tomorrow, Snegroff said.

If ratified, the new contract will expire on Nov. 15, 2011.

—Staff writer Daniel J.T. Schuker can be reached at dschuker@fas.harvard.edu.