The addition is expected to be ready in October, after which the church will be demolished to make way for a new church, Jackson said.
The congregation of about 200 must raise money for the new $1 million church and the $70,000 cost of demolishing the old one, church officials said.
"We hated to move out, but we had no alternative for fear (the roof) would come down," said Bill Mason, 69, vice chairman of the church's trustee board.
"It really hasn't been easy, but this is, I think, one of those times when your faith is truly called upon," Jackson said. "We are so appreciative that God has brought us this far. At first we didn't know what we would do, but we knew we had to stay together."
On the Sunday after church officials learned the church was unsafe and would have to be torn down, the service was held at Northern Middle School.
That service was held without Jackson, whose father died the night he learned he would lose his church. His mother had died shortly before.
"I didn't have time to grieve because I had a church that was grieving," said Jackson, 56, who has been the church's pastor for 17 years.
"It was really difficult to come back in here and tell these people they don't have a church, that they wouldn't be able to worship here anymore," he said.
The church was condemned because the walls were leaning too far out to support the roof, said Mike Heyser, the city's building inspector. Condemnation notices weren't posted because the congregation agreed voluntarily to leave, he said.
The danger was discovered when the church hired architect Kurt Cushwa to design an addition to the fellowship hall, Cushwa said.
A water leak had seeped through two layers of shingles, rotting the truss that supports the roof, officials said.
The truss was all that was holding up the roof, which is so unstable a heavy snow could bring it down, Cushwa said.
"It could look just like it does now one minute and the roof is laying on the floor another minute and you have no warning," Cushwa said.
Since May 1997, the congregation has been holding services at noon on Sundays at Christ's Reformed Church at 130 W. Franklin St.
The Rev. Don Stevenson said he invited the Ebenezer congregation to use the church.
"The thing that I have sensed is how they have needed to and wanted to and have stayed together. Being away from their own home has caused some homesickness I'm sure," Stevenson said.
Ebenezer is called "Big Bethel" by the older parishioners, who remember the Bethel Street Meeting House that the church's first parishioners built across Bethel Street in 1838.
The church was founded by Thomas Henry, a freed slave, according to a book on the church's history by Marguerite Doleman.
The church's Moller organ, stained-glass windows and balcony railings will be moved to the new church, Jackson said.
The new church will seat 350 people compared with the old church, which can accommodate 300 people, he said.
Hagerstown's City Council agreed to waive the $607 in permit fees for the fellowship hall because of the church's predicament.
Councilman J. Wallace McClure said the city needs to help the older churches remain in the downtown area.
"The churches are vital," contributing to the character of downtown neighborhoods, he said.