Shank said work could be completed as early as tonight.
Money for materials came from Project Open Space, he said.
The barn is being built with "$32,000 in materials and that much if not more in (volunteer) labor," Shank said.
Cost of the work had the job been contracted out would have been $49,000, Shank said.
"Farmers are kind of letting their work go so we can get this barn done," said Betsy Herbst, 42. Her husband, Dave Herbst, 43, owns Misty Meadow Dairy Farm in Smithsburg.
"He's got wheat at home waiting for him."
Jeff Poffenberger, 42, of Powell Road in Sharpsburg, who is on the board of directors of Washington County Agriculture Center, was on hand with his son, Jesse Rohrer, 14, and his father, William Poffenberger, 63.
"I feel this is one way our county can demonstrate support for agriculture," said Jeff Poffenberger, who directed the roof work. "It will be the first time my son will have his animals under a roof instead of under a tent."
He said this area's Ag Expo is one of the few fairs he's seen that show animals in tents.
"In other years, we rented as many as seven tents," Shank said. "This will eliminate renting four tents."
He said that tents have poor air circulation that can affect the animals.
Two pole barns already stand in an L-shape. The new barn, like at least one of the existing barns, will be used to house animals for show.
"We'll have more facilities for anybody who wants to use them," Dave Herbst said. "It's needed to attract regional and national shows here to help pay the bills."
Sue M. Lowery, 36, a member of the Ag Expo board, drove the blue front-end loader and helped stack 20- and 18-foot metal roof sheets before the refreshments arrived.
"They go through five gallons a night," Betsy Herbst said, as she poured lemonade mix into the barrel-shaped red and white thermos.
"We're relying on people's dedication to the project to get it done," Poffenberger said. "We're saving the Ag center money that can be used for other projects."
Plans call for the new barn to be initiated July 12-13 for a pygmy goat show.