Most Capital Region homes are built for roughly 40 to 50 pounds per square foot of roof snow load. That is about four feet of light fresh powder, but as little as one to two feet of dense settled or refrozen snow. Clear the roof when packed depth passes about a foot, or before rain falls on an existing pack.
Not all snow weighs the same
This is the part most homeowners get wrong, and it is the whole ballgame. Depth tells you very little on its own. What matters is density, and density changes dramatically with temperature, age and moisture.
Fresh, cold, dry powder is mostly air. Settled snow that has been sitting for a week has compacted under its own weight. Snow that has been through a thaw and refreeze cycle, or that has absorbed rain, can be several times heavier than the same depth of powder. A roof that comfortably carried three feet of January powder can be in genuine trouble under fourteen inches of February ice pack.
| Snow type | Rough weight per foot of depth | Depth to reach 40 lb/sq ft |
|---|---|---|
| Fresh light powder | About 3 lb per sq ft | Around 4 feet |
| Settled snow | About 8 to 12 lb per sq ft | Around 3 to 4 feet |
| Packed or old snow | About 15 to 20 lb per sq ft | Around 2 feet |
| Ice, or snow soaked with rain | About 40 to 57 lb per sq ft | Around 1 foot |
Warning signs your roof is overloaded
- Doors or windows that suddenly stick or will not close properly
- New cracks in drywall or plaster, particularly near the ceiling or above door frames
- Visible sagging in the roofline or ceiling
- Creaking, popping or cracking noises from the structure
- Sprinkler heads in commercial buildings dropping below the ceiling line
- Doors at the top of an interior stairwell binding against the frame
- Water staining on ceilings, which usually indicates an ice dam rather than load
Why rain on snow is the dangerous combination
The single most hazardous weather sequence for a roof in this region is a significant snowfall followed by rain or a warm wet event. Snow absorbs the rain like a sponge rather than shedding it, and the weight can double or worse in a matter of hours without the depth changing much at all.
If there is a substantial snow pack on your roof and rain is in the forecast, that is the moment to act, not after. It is also the sequence that produces most winter roof failures in the Northeast, and it is entirely predictable from the forecast.
Flat and low slope roofs are a different problem
Pitched residential roofs shed some of their load naturally. Flat and low slope commercial roofs do not, they collect. Worse, wind pushes snow into drifts against parapet walls, mechanical screens and rooftop units, which concentrates load in exactly the places the structure is least able to take it.
Drift loading, not uniform depth, is what causes most commercial roof failures. If you manage a warehouse, retail or multi-family building with a flat roof, the depth in the middle is not the number that matters. The depth against the parapets is.
When to call someone instead of doing it yourself
Raking a single-story roof from the ground with a proper roof rake is a reasonable homeowner job. Standing on a ladder in the winter to reach a second story is not, and neither is walking on a snow covered pitched roof. Every winter the region produces avoidable serious injuries from exactly those two decisions.
Call a contractor if the roof cannot be reached safely from the ground, if there is already an ice dam formed, if the roof is slate, tile or metal, if it is a low slope commercial roof, or if you are seeing any of the structural warning signs above.
