The union representing 340 Harvard janitors ratified a new six-year contract with the University on Friday that will gradually raise the starting wage for custodial workers to $18.50 per hour.

After the two parties reached a tentative agreement late last Tuesday, the Service Employees International Union (SEIU) Local 615 put the proposed deal to a vote among the janitors throughout the day Thursday.

The janitors overwhelmingly approved the contract, with fewer than 10 workers voting against it, according to SEIU Local 615 spokeswoman Courtney Snegroff.

“People are very satisfied,” she said.

Harvard spokesman Joe Wrinn said last week that Harvard considered the contract’s terms “fair to both the University and the custodians,” calling it “an acceptable middle ground.”

Negotiators from Harvard and the SEIU Local 615 first sat down at the table on Sept. 22 and vetted 60 different proposals during 14 negotiating sessions, according to Wrinn. The two sides finally struck the tentative agreement after a 12-hour session the day the old contract was set to expire.

Starting janitors at Harvard are currently paid $13.50 per hour, a wage set by the previous contract. The first increase will come in July 2006, when the starting wage will rise to $14.50 per hour. Crew chiefs with at least three years of experience, the highest-paid worker category in the contract, will receive $20.50 per hour in 2011.

“The wages are definitely one of the strong points of the contract,” Snegroff said, calling the wage hikes “unprecedented.”

But she also said that Harvard’s wages still will not be on par with those offered at its local peer institutions, such as Boston College, Boston University, and MIT.

“It brings [Harvard’s janitors] closer, but, in a few years, those schools will be making a lot more,” she said.

Under the contract, custodians working the late-night shift will receive an additional 50 cents per hour, and janitors who work for more than six consecutive days will receive time-and-a-half rates beginning on the sixth day.

Harvard’s janitors will also receive three to four days of vacation during the winter recess or an equal amount of time off at another point during the year, as well as six to 12 sick days, depending on seniority.

The contract also provides for 100 percent short-term disability coverage to custodians who have worked at Harvard for at least seven years, up from 75 percent.

The University will also make a $300,000 contribution to an education and training fund for SEIU members.

The contract stipulates that Harvard will also continue to work, during the next two years, toward the goal, set out in the old contract, of increasing the proportion of full-time workers at the University to 60 percent.

Snegroff noted that the janitors on the bargaining committee were disappointed that the University had not agreed to provide higher wages for workers who do heavy lifting. But she reiterated that the reaction to the new terms was “very positive” on the whole.

The contract will expire on Nov. 15, 2011.