The collapse occurred around 3 p.m., the same time school was letting out, said Principal Lisa Kane.
"I was inside the building speaking with some children and we heard this noise and we ran outside," said Kane, who described the collapse sound as "like thunder."
No children were hurt or nearby during the collapse.
The roof of Lewis Hall was being redone as part of a large-scale $700,000 restoration and expansion, according to Archdiocese of Washington, D.C. Spokeswoman Susan Gibbs.
George Tanner, the site superintendent for Kane Construction, said the two men were installing support trusses that would hold up the roof when they collapsed "like dominoes." He said the collapse may have occurred due to a wind gust.
"We just talked to the guy that was on top of the trusses and he said it was a big gust of wind," Tanner said. He said the construction workers, both subcontractors with JB Drywall, suffered only minor injuries, but were transported to a hospital.
"They're both OK," Tanner said. "One guy that was on top actually broke his nose, but that's all."
Tanner said it would probably take about a week to get new trusses in to restart building the roof.
Kane said after the collapse she and a group of faculty members stood around the two men, whose names Tanner did not know, and prayed. Monsignor Cary Hill blessed the men before they got in the ambulance.
"We believe in the power of prayer," Kane said. "My thoughts and prayers are with the workers who were injured."
Kane said she was "extremely grateful that at no time were children in danger."
A group of children reportedly saw a third man jump off a roof, according to Gibbs and Kane, but Tanner and construction worker Adolfo Batres of JB Drywall said that did not happen, and may have just been somebody getting off the adjacent roof of the elementary school.
Regardless, a counselor will be on hand at the school Thursday to talk with any upset children, and letters explaining what happened will be sent home to parents, Gibbs said.
Batres said he was on a lift below the trusses when they collapsed.
"We were almost done, we only had like eight more to go," Batres said. "I was lucky, you know, that's all I can say. God saved me."