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Snow load in northern New England
Roofs here are designed for loads most of the country never sees. Understanding what yours carries is the difference between a nervous February and an informed one.
Depth is the wrong measurement
People judge snow by how deep it is. What a roof feels is weight, and the same depth can weigh wildly different amounts depending on what it is made of.
Fresh dry powder is light. The same snow after a thaw and refreeze, compacted, holding meltwater and topped with a rain event, can weigh several times more without looking much different from the ground. Rain falling onto an existing snowpack is the condition to pay attention to, because the pack absorbs it and the load climbs fast with no visible change.
What roofs here are built for
Ground snow loads in this part of New Hampshire are among the highest in the country, and building codes here set design loads accordingly. A house built to current code, with its structure intact, is engineered for a serious winter.
The buildings that get into trouble are usually older, altered, or not houses: additions framed lighter than the original, porch and deck roofs, carports, sheds and barns. Low slope roofs hold more than steep ones, and a roof that has had a wall removed underneath it at some point is worth a second thought.
Signs a load is worth acting on
- Doors or windows that suddenly stick, or that have gone out of square
- New cracks in ceiling or wall finishes, especially near the middle of a span
- A visible sag or dip along the ridge or in a roof plane
- Creaking or popping noises that are new
- Drifted snow piled deep on one slope after wind, or where a lower roof meets a wall
Drifting is the one people miss. Wind moves the load onto one side, and where a lower roof meets a taller wall it can pile far deeper than anywhere else on the building.
Clearing it, and doing it safely
A roof rake from the ground, taking the lower few feet off, does most of the useful work. It reduces the load at the eaves and takes away the material an ice dam would otherwise be built from.
Nobody should be on a snow covered roof. It is the single most dangerous thing a homeowner decides to do in winter, and falls and roof collapses both happen during clearing. If it needs more than a rake from the ground, that is a phone call rather than a Saturday.
Where this meets ice dams
They are related but not the same problem. Load is about weight on the structure. Dams are about heat escaping into the attic, melting the pack from below and refreezing at a cold eave. Clearing snow helps both, but only one of them has a permanent fix, and it is in the attic. More on ice dams.
Worried about a load right now?
If you are seeing sticking doors, new cracks or a visible sag, call rather than send a message, and stay out from under it.