The fire started in an outhouse, and police believed the roof may have contained asbes-tos.
They issued a warning to all householders in Armagh, instructing them to keep all doors and windows closed.
Fire crews from Armagh, Keady and Dungannon and a water tanker from Pomeroy were deployed.
Fire and Rescue Service group commander Walter Johnston said the uncertainty about what was inside the building forced firefighters to take major precautions.
He said: "The fire started about 4.20pm in an outhouse at a farm and because the owner no longer lives on the premises there was no- one around to tell us what was inside.
"There was an asbestos corrugated roof on the premises, but you get that all around the country. There was no risk provided you dampen it down.
"We suspected there may have been gas cylinders, but we couldn't find any. Because we didn't know what was inside we had to take every precaution.'
Written by Belfast News Letter
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