Commercial roofs in central Wisconsin are designed to carry a lot of snow. Most winters, most roofs never come close to their limit. The trouble comes from the exceptions: drifts piled against a wall, snow sliding off a higher roof onto a lower one, a heavy rain on top of a deep snowpack, or an older roof that is carrying more equipment than it was designed for.
What snow actually weighs
Depth alone tells you very little. Fresh powder is mostly air; old, settled snow and snow soaked with rain are dense. A widely used rule of thumb puts roughly 5 pounds per square foot in any of these:
- 10 to 12 inches of fresh snow
- 3 to 5 inches of packed or old snow
- About 1 inch of ice
Water itself weighs about 5.2 pounds per square foot for every inch of depth, which is why rain falling on snow is such a concern: the snowpack soaks it up like a sponge and holds it on the roof.
Drifts are the real danger
Snow loads are rarely even. Wind strips snow off the open middle of the roof and piles it against parapets, higher walls, penthouses and rooftop units. A building next to a taller one gets snow drifting and sliding down onto its roof. Those drifts can be many times the load on the rest of the roof, concentrated in exactly the places where the structure may not expect it. When clearing a roof, drifts come first.
Roof leaking right now?
Wausau Superior Flat Roofing repairs, coats and replaces commercial and flat roofs across Wausau and central Wisconsin, with a written price before any work starts. Request a quote online and we’ll call you back.
Warning signs of overload
- Visible sagging in the roof or ceiling
- New cracks in drywall, plaster or masonry
- Doors and windows that suddenly stick
- Creaking, popping or cracking sounds
- Sprinkler heads that have dropped below the ceiling line
If you see these, get people out of the area first, then call a structural engineer. Snow removal comes after the building is safe to work on.
How to clear a flat roof without damaging it
- Plastic tools only. Metal shovels and snow blowers cut membranes and tear flashings.
- Leave a layer. A few inches of snow protects the membrane.
- Clear in a pattern. Remove snow evenly and don't pile it in one spot on the roof.
- Open the drains. So the next thaw can drain instead of ponding. See ice dams on flat roofs.
- Fall protection. Open edges and skylights are hidden under snow.
- Check afterward. Look for cuts and damage once the roof is visible.
On homes, flat porch and garage roofs collect snow from the sloped roof above. A roof rake from the ground handles most of it; leave the rest to someone with the right equipment. See commercial roof snow removal.