Roof Snow Load: Warning Signs and Safe Removal in Colorado

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A deep spring storm on the Front Range can drop heavy, wet snow on a roof in a single night, and homeowners understandably start wondering how much weight the house is carrying. Learning to calculate snow load in rough terms, recognize the warning signs of an overstressed roof and remove snow without climbing a ladder can prevent both structural damage and injuries. This guide explains how snow load is described, why the same depth of snow can weigh very different amounts, how design snow loads are set by local building departments, which parts of a roof collect the most snow, what warning signs call for immediate action and how to clear snow safely from the ground. It draws on FEMA’s Snow Load Safety Guide and OSHA’s hazard alert on rooftop snow removal. This is general information, not an engineering assessment; a licensed structural engineer is the right resource for questions about a specific roof’s capacity.

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How to Calculate Snow Load in Plain Terms

Snow load is simply the weight of snow resting on a surface, usually expressed in pounds per square foot (psf). OSHA’s rooftop snow removal hazard alert describes it the same way and notes that the weight varies with the snow’s water content.

A rough estimate comes down to two numbers: how deep the snow is and how heavy that type of snow is per foot of depth. Multiply the depth in feet by the weight per foot of depth, and the result is an approximate load in pounds per square foot.

FEMA’s Snow Load Safety Guide (FEMA P-957) gives the key ranges:

• Fresh snow: the weight of 1 foot of fresh snow ranges from about 3 psf for light, dry snow to about 21 psf for wet, heavy snow.
• Ice: 1 inch of ice weighs a little less than 5 psf, and 1 foot of ice weighs approximately 57 psf.

Using those figures, a simple illustration shows why snow type matters so much. Two feet of light, dry powder might add only about 6 psf. Two feet of wet, heavy snow could add roughly 42 psf, about seven times as much. Add a couple of inches of ice at the eaves and that edge of the roof picks up close to another 10 psf on top of the snow.

Snow that has settled, been rained on, or partly melted and refrozen generally becomes denser than fresh snow, so an older snowpack can weigh more than its depth suggests. FEMA does not give a single figure for packed snow, and densities vary widely, which is why a depth-only estimate should be treated as a rough guide rather than a precise number. Some people estimate density by cutting a sample of known area from the snowpack, melting or weighing it and scaling up, but even that only describes one spot on the roof.

A Worked Example

Imagine a garage roof measuring 24 feet by 24 feet, or 576 square feet, after a late-March storm. A yardstick pushed through the snow at a few spots on the lower roof edge reads about 18 inches, or 1.5 feet. The storm was warm and the snow is wet and sticky, so the heavy end of FEMA’s range, about 21 psf per foot, is the cautious assumption.

1. Depth times weight per foot: 1.5 feet multiplied by 21 psf is roughly 31.5 psf.
2. Add ice: if a 2-inch band of ice has formed along the eaves, that edge picks up a little under 10 psf more.
3. Total weight on the roof: 31.5 psf multiplied by 576 square feet is a little over 18,000 pounds of snow, before counting any drift or ice.

If the same 18 inches had been light, dry powder at about 3 psf per foot, the load would be closer to 4.5 psf. The math is simple; the uncertainty lies in the density assumption and in how unevenly the snow is spread. That is why the estimate is best used to decide whether to watch the roof closely or call a professional, not to decide whether the roof is safe.

Why Ground Snow Is Not the Same as Roof Snow

It is tempting to measure the snow in the yard and assume the roof holds the same amount. FEMA cautions that roof snow loads vary from ground snow based on multiple factors, and that measuring ground snow weight and assuming it equals the roof snow load is incorrect. OSHA similarly describes ground snow as only a rough indication. Wind can scour snow off one part of a roof and pile it onto another, sun can melt one slope faster than the other, and heat escaping from the house can change what stays on the roof.

Design Snow Loads and Colorado Elevation

Every roof is designed to carry a certain snow load, and that number comes from the building code. FEMA explains that the code identifies a ground snow load for a location, which designers use as the starting point to calculate the roof’s design snow load. The roof value is then adjusted for factors such as wind exposure, roof slope, roof shape, obstructions and the building’s thermal condition.

In Colorado, those values vary enormously with elevation and terrain. A house on the plains east of Denver, a home in the foothills above Boulder and a cabin near a mountain pass can have very different design snow loads, even within the same county. Local building departments set or adopt those values, and some mountain jurisdictions publish their own snow load requirements. The best source for a specific address is the city or county building department. FEMA also notes that when drawings are not available, the local building department should be able to tell an owner which code edition was in place when the building was constructed.

Age matters too. FEMA points out that building codes began to address drifting and unbalanced snow loads only in the 1970s and 1980s, so a building constructed decades ago may not have been designed for snow loads as they are understood today. Structural deterioration, such as rot or damaged framing, can also reduce what a roof can safely carry, a point OSHA makes as well.

A homeowner cannot calculate a roof’s capacity with a tape measure. The useful comparison is between a rough estimate of the snow load and the roof’s design capacity from the plans, the building department or an engineer. FEMA recommends getting that design capacity before the snow season, not during a storm.

Where Snow Piles Up: Drifts, Valleys and Low Roofs

Snow rarely sits evenly on a roof. FEMA’s guide describes drifting and sliding snow as major contributors to unbalanced loads, and it lists several spots on houses where snow tends to accumulate:

• The downwind side of the ridge. Wind carries snow over the peak and drops it on the leeward slope.
• Valleys. FEMA notes that valleys at the intersection of sloping residential roofs accumulate greater snow depth than elsewhere. Our guide to roof valleys explains why these areas also tend to leak.
• Lower roofs next to taller ones. A porch, garage or addition roof below a taller wall catches both drifting snow and snow sliding off the upper roof.
• Around chimneys, dormers and skylights. FEMA lists chimneys, dormer windows, porch roofs and skylights as residential features that collect drifts.

Those concentrated loads are why a roof can be stressed in one area even when the average snow depth looks modest.

Metal Roofs and Snow Shedding

Roofing material affects how snow behaves. FEMA explains that textured materials such as asphalt shingles do not shed snow as easily, while slippery materials such as metal panels shed it more readily, and that snow guards or cleats inhibit sliding. A metal roof can reduce how much snow stays on the roof, but it can also release large slabs suddenly onto walkways, decks, lower roofs and vehicles. Our guide to metal roof cost covers snow guards and other details worth budgeting for in snowy areas.

Ice Dams, Attic Ventilation and Gutters

Heat matters as much as snowfall. FEMA describes how heat from an uninsulated or poorly ventilated attic can melt snow from beneath; the meltwater runs down to the colder eave and refreezes, forming an ice dam. According to FEMA, ice dams create two problems: they prevent snow from sliding off, which adds to an unbalanced load at the eave, and they can push water into the building.

Ice is heavy, as the FEMA figures show, and an ice dam several inches thick concentrates that weight along the roof edge and gutters. Our guide to ice dams explains how they form and how to prevent them. The long-term fix is usually a combination of air sealing, insulation and ventilation so the roof deck stays cold; FEMA notes that a well-ventilated attic has a temperature similar to outdoor air, so heat from the house does not melt roof snow. Our guide to attic ventilation covers intake and exhaust vents and common mistakes.

Gutters take a beating as well. Ice-filled gutters can pull away from the fascia, and heavy snow sliding off the roof can bend or tear them loose. Some gutter guards can help keep debris out, but they can also hold snow and ice in certain designs. Our guide to gutter guards compares the options.

Warning Signs of Too Much Snow on the Roof

FEMA’s guide lists warning signs that an overstressed roof deck or framing may display before failure. Adapted for houses, the signs worth watching for include:

• Sagging ceilings or a visibly sagging roofline.
• Popping, cracking or creaking noises from the roof or attic.
• Sagging roof sheathing or bowing truss or rafter members visible in the attic.
• Interior doors or windows that suddenly stick or will not open or close.
• Cracked or split wood framing members.
• New cracks in walls, drywall or masonry.
• Severe roof leaks.

FEMA’s guidance is blunt: if any of these warning signs are observed, the building should be promptly evacuated and a qualified design professional, such as a professional engineer, should be contacted to perform a detailed structural inspection. FEMA also warns that if there is any concern snow loads may cause a collapse, all removal activity should stop and the building should be evacuated.

Not every drip during a thaw is structural, of course. Many leaks during snowy weather come from ice dams, flashing problems or worn roofing. Our guide to roof leaks helps sort out common causes. When leaks appear together with sagging, cracking sounds or sticking doors, treat it as a possible structural issue, not a routine repair.

How to Remove Snow From a Roof Safely

The safest place to remove roof snow from is the ground. OSHA’s hazard alert on snow removal reports that falls cause the most worker fatalities and injuries during rooftop snow removal, with people falling off roof edges, through snow-covered skylights and from ladders. OSHA recommends methods that avoid going on the roof whenever possible, such as using snow rakes or drag lines from the ground.

Use a Roof Rake From the Ground

A roof rake is a wide, shallow blade on a long telescoping handle. Working from the ground, pull snow down from the lower few feet of the roof, especially at the eaves where ice dams form. OSHA advises removing small amounts at a time to avoid strain and to avoid being hit or buried by falling snow. FEMA adds that roof snow can slide at any moment, so stay back from the eave and outside the sliding range.

FEMA’s guide recommends a non-metallic snow rake where possible, because metal rakes can damage roofing material. It also advises leaving at least 2 inches of snow on the roof rather than scraping to the surface, since complete removal can damage the roof covering, and never using sharp tools such as picks to chip ice.

Watch for Power Lines and Falling Ice

Long-handled rakes can reach overhead power lines, and OSHA lists shock and electrocution among the hazards of snow removal. Look up before extending any pole near the service drop. FEMA also warns that dislodged icicles can cause injuries even from a short height, so keep people and pets clear of the area below.

Do Not Climb Onto a Snowy Roof

Snow hides skylights, vents and roof edges, and a snow-covered roof is slippery and unpredictable. OSHA notes that a surface weighed down by snow should be evaluated to determine whether it can be safely accessed because it may be at risk of collapse. For anything beyond what can be reached from the ground, FEMA recommends a licensed, insured roofing contractor experienced in roof snow removal. Professionals working at height are expected to use fall protection, and a homeowner on a ladder with a shovel has none.

Remove Snow Evenly

FEMA cautions against creating new unbalanced loads during removal. Clearing one side of a roof completely while the other stays loaded, or piling raked snow onto a lower roof or deck, can make things worse. Remove drifted areas first, work evenly, and dispose of snow on the ground away from exits, downspouts and vents.

Planning Ahead Before the Next Big Storm

The best time to think about snow load is in the fall. A few steps help:

• Find out the roof’s design snow load from building plans or the local building department, or ask an engineer if the house has had additions or alterations.
• Look in the attic for water stains, sagging sheathing, cracked rafters or truss damage, and correct ventilation and insulation problems that lead to ice dams.
• Clean gutters and downspouts so meltwater can drain.
• Buy a roof rake before the first storm, when stores still have them in stock.
• Line up a roofing contractor who does snow removal, especially for homes in the foothills and mountains.

For buyers, a home inspection can reveal attic ventilation problems, past leak stains, damaged framing and roof conditions that affect how a house handles snow. Our guide to hiring a home inspector explains what to expect, and the structure and exterior hub collects related roof and attic guides.

References

• Snow Load Safety Guide (FEMA P-957) — Federal Emergency Management Agency
• Falls and Other Hazards to Workers Removing Snow from Rooftops and Other Elevated Surfaces — Occupational Safety and Health Administration
• Winter Weather: Hazards and Precautions — Occupational Safety and Health Administration

Buying a home in the foothills or anywhere snow piles up on the Front Range? Contact us and we can help you find an inspector who checks the attic, roof framing and ventilation before winter arrives.