With snow piling up, some roofs are coming down.

The Amwell Township Volunteer Fire Department on Monday had to remove its trucks and equipment from its firehouse because the roof was sagging. Construction crews were hoping to save the roof while two other departments stored the equipment.

In nearby Fallowfield, firefighters were able to save all but one of 11 sheep in a one-story barn that collapsed Monday afternoon. Rescue trucks with hydraulic equipment lifted the sheet metal roof off the wood-frame building to save the sheep, but some horses were reportedly trapped.

Two other barn collapses were reported Monday in Mount Pleasant and Elizabeth Township.

All of that followed the collapse of the roof of a closed ice arena on the South Side on Monday and the roof failure at the Rostraver Ice Garden on Sunday.

So now it's only natural to ask: Are we all in peril of being crushed under deadly snow and debris on our next trip to Giant Eagle or Home Depot?

Probably not.

But there are some commercial buildings, mostly with flat roofs, that could be at risk.

Contractors say building owners should pay attention to their roofs and check their design blueprints to evaluate the danger.

Depth of snow is less a factor than its wetness and the damage caused by the freeze-thaw cycle, especially if a roof is already compromised by the addition over the years of heavy equipment, such as air conditioners, that were not accounted for in the original design.

"It's not how deep the snow is, it's the weight of the snow," said Jack Scalo of Burns and Scalo Roofing Co.

A foot of dry snow weighs about three pounds per cubic foot. But a foot of wet snow can weigh up to 21 pounds. Roofs in the northeast are supposed to be designed to support 30 pounds per cubic foot.

So it's obvious that a If a heavy rain which saturates the snow on the roofs now, it could bring more collapses.

"This problem is going to magnify," said Mr. Scalo.

Ice is the other issue. As heat rises inside buildings, especially in big commercial structures without a lot of insulation in the ceiling, the ice under the snow starts to melt. But with the gutters frozen solid, the water has nowhere to go. Then the water re-freezes at night. That cycle, day after day, can weaken a roof.

"These are expansion and contraction cycles with enormous extremes," said Gary Kassem, president of SingleSource Roofing.

He said building owners should be particularly concerned if they've added equipment or altered the roof over the years.

If their building is at risk, building owners should hire a contractor to remove the snow. That job isn't as easy as it sounds. It generally has to be done by hand, it can take days, it costs thousands of dollars and it's easy to damage the roof.

"You have to be very careful of what's underneath the snow," said Mr. Kassem.

But he said crews don't have to remove all the snow.

"Don't even touch the last six inches," he said. "Just get the top few feet off of it and it's like a normal winter."

There's a science to removal. Most crews will remove snow from the weakest part of the roof first, meaning the point farthest from any supports, and usually shovel it away in strips using plastic shovels. They never use snowblowers or metal shovels for fear of damaging the roof.

It's labor-intensive.

"I can tell you that we put 8 to 10 men on a roof and it usually takes multiple days," said Mr. Scalo. "On a Home Depot-size building, it will take four days."

Building owners who think their roof is in danger shouldn't try to remove the snow themselves, said Allegheny County Public Works Director Joe Olczak.

"I wouldn't recommend getting up on the roof unless you're a professional," he said. "I'd recommend they hire somebody."

Here are some signs that a roof is under stress:

1. Sagging steel.

2. Splitting wood supports.

3. Sprinkler heads pushed down below ceiling tiles.

4. Doors popping open or doors and windows that are hard to open.

5. Bowing utility pipes or electrical conduit near the ceiling.

6. Creaking sounds.

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