When roof snow needs removing

Does roof snow usually need removing at all?

Usually not. Roofs are designed for a snow load, and a well-built roof in good condition is meant to carry the snow its location normally gets. FEMA's Snow Load Safety Guide (FEMA P-957) says that knowing the design snow load of the structure is key to deciding when to begin removal, that it can be found on construction drawings, and that if there are no drawings, an owner should have a licensed design professional determine the structure's capacity before the snow season.

The same guide is blunt that you cannot judge roof load by measuring snow on the ground: roof snow loads differ from ground snow for several reasons, and assuming they are the same is incorrect. So the decision is less about how much snow you see and more about the roof, where the snow has gathered, and how heavy it is.

How heavy is snow on a roof?

It varies far more than depth suggests. FEMA's guide puts the weight of a foot of fresh snow at about 3 pounds per square foot for light, dry snow and about 21 pounds per square foot for wet, heavy snow. Ice is heavier still: one inch weighs a little less than 5 pounds per square foot, and a foot of ice roughly 57. FEMA notes that this is part of why preventing ice buildup on a roof matters.

FEMA also notes that western snow is typically lighter and less dense than East Coast snow, with exceptions, and that local authorities and structural engineers know regional snow best. In the Missoula and Bitterroot valleys, the loads to watch are a dry pack topped by a wet storm, rain falling onto existing snow, and ice that builds at the eaves.

Where on a roof does snow build up dangerously?

Where the load is uneven. FEMA describes unbalanced snow loading, where snow collects in some areas more than others, as a greater risk to a roof than a uniform load. Drifts form on lower roofs in the wind shadow of higher parts of the building and against obstructions such as a higher roof wall, a chimney, rooftop equipment, a parapet or dormer windows.

Sliding snow is the other cause. FEMA explains that snow from a higher roof can slide onto a lower roof below it, and names porch and sunroom roofs and entrance canopies beneath gabled roofs as common examples. A small porch roof under a large main roof can end up carrying far more snow than the roof above it. For the hazard that sliding snow creates for people below, see our guide to icicles and roof snow slides near entrances.

What are the warning signs a roof is overloaded?

FEMA P-957 lists warning signs common in wood, metal and steel buildings: sagging ceiling tiles or boards and ceiling boards falling out of the grid; sagging sprinkler lines or heads, or sprinkler heads deflecting below suspended ceilings; popping, cracking and creaking noises; sagging roof members, including metal decking or plywood sheathing; bowing truss bottom chords or web members; doors and windows that can no longer be opened or closed; cracked or split wood members; cracks in walls or masonry; severe roof leaks; and water collecting where it should not on low-slope roofs.

FEMA's instruction if you see any of them is to evacuate the building promptly and contact a qualified design professional, such as a professional engineer, for a detailed structural inspection. Its warning box adds that if there is any concern snow loads may cause a collapse, all removal activity should stop and the building should be evacuated. This is not a situation to fix with a shovel.

Which buildings are most at risk?

Older buildings, buildings with additions, and outbuildings. FEMA notes that drifting loads and unbalanced snow loads were added to building codes relatively recently compared with the age of many buildings, that older pre-engineered metal buildings designed before drifting loads were addressed can be vulnerable, and that many older agricultural structures were not required to be designed to a specific snow load. It also says secondary structures such as entrance canopies and porch roofs do not typically perform as well as primary buildings, partly because they are often additions to the main structure.

For a western Montana property, that list often includes the barn, the shop, the carport, the porch roof and the newer lower wing of the house. Those are the places to look first after a big storm or a rain-on-snow event.

Who should remove snow from a roof?

FEMA's guidance is that roof snow removal should be done by a licensed, insured professional roofing contractor with experience removing snow from roofs, because of their familiarity with safety protocols. OSHA's winter guidance warns that snow-covered roofs can hide hazards such as skylights that people fall through, and tells employers to consider removal methods that keep workers off roofs entirely.

We do not do roof work or roof snow removal. If your roof needs it, call a roofing contractor who does, and if you have any of the warning signs above, call a structural engineer first. Your insurer may also have guidance for policyholders; ask them.

What does safe roof snow removal involve?

It helps to know what a professional should be doing, so you can recognize good practice. FEMA lists leaving at least 2 inches of snow on the roof to protect the roof covering, not using mechanical snow removal equipment, using plastic rather than metal shovels and no sharp tools, removing drifted snow first at elevation changes and around equipment, removing snow evenly so the remaining load stays balanced, not stockpiling snow on the roof, and keeping removed snow away from exits, downspouts, vents and equipment below.

For people on the ground, FEMA's safety measures include keeping foot traffic away from areas where snow and ice will fall, watching for dislodged icicles, and staying outside the sliding range of the eave when using a non-metallic roof rake from the ground. Metal rakes can damage roofing and should be avoided.

What can you do before the next big storm?

FEMA recommends that preventive steps happen before the snow season, not just before a storm. For a homeowner that means knowing roughly what the roof was designed for, getting a professional opinion on any older building, addition or outbuilding you are unsure about, and keeping an eye on drift and slide zones through the winter.

It also means dealing with what sits above the roof. Branches overhanging a roof add their own load when they carry snow and ice, and a limb that fails under wet snow can land on the roof with far more force than the snow itself. Our guide to winter tree care and snow load explains which trees are most at risk, and our tree services team prunes limbs back from rooflines before winter. If ice is forming at the eaves, read our guide to ice dams in Montana homes.

How can you keep an eye on a roof without climbing on it?

From the ground and from inside. After a heavy storm, a windy spell or rain on snow, walk around the building and look at the places FEMA describes as drift and slide zones: lower roofs beside taller ones, the areas around chimneys, dormers and skylights, and porch or entrance roofs below a larger gable. Compare the snow depth there with the rest of the roof. A drift that is much deeper than the general cover is the load to ask a professional about.

Then look inside, at the warning signs listed above: ceilings, doors and windows that start sticking, new cracks, new noises. In outbuildings, look up at the framing. Note dates and take photos, because a roofing contractor or engineer will want to know how quickly things changed. None of this requires a ladder, and FEMA's and OSHA's guidance both point toward keeping people off snow-covered roofs.

Where does HJ fit in?

On the ground. Snow coming off a roof, by sliding or by a roofer clearing it, lands on walks, steps, decks and driveways, and FEMA's guidance is to keep it clear of exits and downspouts. Our residential snow removal arrangements cover driveways, walks, steps, landings and decks where access requires it, and our emergency tree and storm response handles trees and limbs that come down on roofs and structures. Our full snow and 24-hour storm services are available across Missoula and Ravalli counties. This guide is part of our Montana Winter Property Guide.

If a limb is threatening your roof or the ground below your eaves needs clearing, call (406) 215-7319 or contact us.

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