At least two roofs collapsed in York Thursday as a storm dropped as much as 20 inches of snow, but a York-based structural engineer said property owners shouldn't worry about all the snow on their roofs unless they know the roof is compromised.

Investigations will probably show the two roof collapses -- a home in York Township and a fire hall in Lewisberry Township -- were the result of existing weaknesses or other structural issues, said Josh Carney; president of Carney Engineering Group in York.

Homeowners can just allow the snow to melt from their sloped roofs, he said, unless they're aware of an issued that has deteriorated or weakened the roof.

Flat roofs can pose problems only if the system engineered for drainage isn't working properly, he said.

Roofs in the York area are rated to withstand about 30 pounds of square feet, and there's a buffer built-in so they won't fail until they reach a weight of about 45 or 50 pounds per square feet, he said. All the snow that fell yesterday amounts to only about 20 pounds per square feet, he said.

The snow on roofs doesn't get heavier just because it's melting and converting to liquid, he said.

"This was not what I would consider to be a historic storm," he said. "It will just drain off the roof."

Dover had the most snow in York County Thursday, with 20 inches.