Assistant Fire Chief Ian Josephson said the grow-op was not yet in operation, but security precautions taken by the owner - like steel rebar in the walls - to prevent grow-rips hampered firefighting efforts.
"It made it very challenging for us to get in there," he said. "We had to wait until it burned through."
"There's not much left of the barn," he said.
Two of the 30 sheep housed in the barn perished in the blaze.
Police and fire officials are still investigating the cause of the fire, but Josephson said it appears to be accidental and not related to the grow-op.
It's also the first grow-op this year discovered during a fire call.
But unless regulations are tightened up around siting and operations of medical marijuana grow-ops, "we're going to end up with more of these," Josephson warned.
Crews from three firehalls responded to the 8 p.m. fire call, and arrived to find the barn located in 6600-block Prest Road engulfed in flames.
It took seven hours to fully extinguish the fire.