Just before 11 p.m. on December 20, 2023, most of the roof on a builders’ supply dispatch center in Anchorage, Alaska gave way under heavy snow. The structure was deemed destroyed. Nobody was inside. The failure came one month after local officials warned that snow loads could become disastrous, especially on flat roofs, during what had already become a record snow season.
The Anchorage collapse followed a familiar chain: a flat roof, a heavy season, and a building that had drained and shed snow well enough in past winters. Roofs fail because of decisions made years earlier, and the time to fix those decisions is before the flakes fly. Setting a roof to drain properly starts with accurate layout, and the comparison of digital levels vs bubble levels matters more on a low-slope roof than almost anywhere else.
Why Snow Loads Bring Down Roofs
Snow is heavy, and its weight depends on how it falls and how long it sits. A foot of fresh powder weighs little; the same foot after a thaw and refreeze can weigh five times as much. The table below gives the densities contractors work with.
| Snow type | Density (lb per cubic foot) | Load per foot of depth |
|---|---|---|
| Fresh powder | 5 to 12 | 5 to 12 psf |
| Packed snow | 15 to 25 | 15 to 25 psf |
| Wet or slushy snow | 25 to 40 | 25 to 40 psf |
| Ice | 57 | 57 psf |
Snow density and load numbers
Designers read the ground snow load from maps in ASCE 7, which gives the load a roof must resist at its specific site. Anchorage carries a design ground snow load around 40 pounds per square foot, and parts of Alaska exceed 100 psf. A single wet storm can drop 2 to 3 feet of dense snow, which on its own approaches the design load before any drifting is added.
How accumulation differs by roof type
Sloped roofs shed snow as it slides, which is why pitched roofs rarely collapse. Flat and low-slope roofs hold everything that falls, and wind piles snow into drifts against parapets, rooftop units, and the walls of taller neighbors. Drift loads can run two to three times the uniform ground load, which is how a roof that passed inspection in November fails in January. Snow that slides off an upper roof lands on the lower roof with extra force, and roof steps and valleys collect it; designers add a surcharge for that sliding snow, and some owners install snow fences to hold it in place on steep slopes. Measuring and setting the drainage slope is where electronic spirit levels earn their keep, because a roof that looks flat to the eye can still hold several inches of water and snow.
Designing Roofs for Snow: Loads, Members, and Connections
Roof framing is sized for the worst realistic combination of weights, not the average winter. Dead load covers the roof itself, live load covers people and equipment during construction and maintenance, and snow load covers the seasonal accumulation. In most snow country, the governing combination is 1.2 times dead load plus 1.6 times snow load.
Load combinations that govern
- 1.2D + 1.6S: the basic snow check for most roofs.
- 0.9D + 1.6S: the uplift and overturning check for sloped roofs.
- Drift and sliding loads added at parapets, valleys, and roof steps.
- Rain-on-snow surcharge where drains can block.
Members and connections
Joists, rafters, and trusses are sized from span tables that keep deflection under about L/240, and they rarely fail in bending alone. Collapses usually start at connections and splices. Hangers, straps, and hold-downs transfer the load into the walls, and every fastener is a potential failure point. Span tables assume the members are braced against rotation; unbraced compression edges buckle early, so blocking and bridging are not optional extras on a snow roof. A member that lifts under sliding snow or wind puts its fasteners in tension, and engineers size those connections for it, because tension anchorage length is generally longer than compression anchorage length for the same capacity.
Flat Roof Vulnerabilities: Slope, Drainage, and Ponding
A flat roof is not supposed to be flat. Codes and membrane manufacturers want a minimum slope of 1/4 inch per foot so water runs to the drains, and that slope has to exist in the structure, not just in the cover board. When water stands, it adds weight, freezes, and repeats the cycle with every storm.
Ponding and the 5.2 psf rule
One inch of water weighs 5.2 pounds per square foot. Six inches of trapped water adds 31 psf on top of whatever snow has already fallen. Ponding also hides early deflection: the roof sags, collects more water, and sags further. That is why interior drains need strainers and why secondary overflow scuppers, required on many buildings, give water a way out when the primary path blocks. Scuppers through the parapet act as overflow drains, and the code sizes them so water never climbs the curb.
Measuring slope before winter
Contractors verify drainage with instruments they already own. The types of levels used in leveling run from simple spirit vials to self-leveling rotary lasers, and the right tool depends on roof size and crew. On a small roof, a long straightedge and a level prove the slope in minutes; on a big deck, a laser saves a full day.
Insulation, Heat Loss, and Snow Behavior
Insulation changes how snow behaves on a roof. A well-insulated roof stays cold, so snow accumulates evenly and melts slowly. A leaky roof loses heat, melts snow in patches, and turns the meltwater into ice at the cold eaves. Codes recognize that roofs need more thermal protection than walls, because heat rises and the top of the building loses it fastest.
Ice dam mechanics
Ice dams form when heat escapes through the ceiling, melts snow on the warm part of the roof, and the water refreezes at the cold overhang. The dam backs water up under the shingles and into the walls. The fix is a combination of air sealing, ventilation, and enough insulation to keep the roof deck cold in the first place.
R-value targets by climate zone
| Climate zone | Attic R-value | Wall R-value |
|---|---|---|
| Zone 5 | R-49 | R-20 to R-30 |
| Zone 6 | R-49 to R-60 | R-20 to R-30 |
| Zone 7 | R-60 | R-25 to R-30 |
| Zone 8 | R-60 | R-25 to R-30 |
Anchorage sits in zone 8, which is one reason the 2023-24 season hit so hard: the record loads exceeded what older buildings were designed to carry. Attic ventilation, roughly 1 square foot of vent for every 150 square feet of ceiling, keeps the deck cold and flushes the moisture that drives rot.
Inspecting and Maintaining Roofs Before Snow Season
The inspection season is autumn, not the middle of January. A half-day walk on a dry roof in October prevents a collapse in February. The checklist below covers the items that matter most.
Pre-winter checklist
- Clear leaves and debris from drains, scuppers, and gutters.
- Verify slope and look for ponding with a level or straightedge.
- Inspect flashing at parapets, penetrations, and rooftop units.
- Check insulation and ventilation in the attic or roof cavity.
- Repair blisters, cracks, and low spots while the membrane is workable.
- Plan snow removal and confirm who is responsible for it.
Snow removal guidelines
Remove snow when the accumulation approaches the design load, not when it looks deep. As a rule of thumb, for a 40 psf design load that is roughly 2 feet of packed snow. Use a roof rake from the ground on residential buildings, and hire crews with plastic shovels for commercial decks. Never chip ice, never pile snow in one spot, and keep workers off the roof while it is loaded.
Simple tools still earn their place in the inspection. The accuracy of water levels is underrated for checking drain slope on a small roof, and a water level costs a fraction of a laser while giving the same answer on a 30-foot run. Document the inspection with photos and dates, and keep the record with the building file; it becomes the evidence of due diligence if a claim follows a collapse.
Connections and Anchors That Keep Roofs in Place
The roof-to-wall connection is the last line of defense. Anchor bolts, hold-downs, and straps tie the framing to the structure below, and they have to be designed for gravity and uplift at the same time.
Lessons from the Anchorage collapse
- Official warnings about snow loads deserve immediate action, not a review meeting.
- Flat and low-slope roofs need extra scrutiny in record seasons.
- Check roofs before winter, when repairs are still possible.
- An empty building can still fail, which makes inspection scheduling a safety decision.
On masonry buildings the connection depends on embedment. Anchoring in masonry structures requires the depth and edge distance shown on the structural drawings, and skipping that step turns a snow load problem into a wall failure. Anchor bolt spacing and embedment follow the drawings, and a quick field check with a torque wrench is worth the hour. A roof that survived twenty winters can fail in the twenty-first when a record season arrives. Design to the code for the site, keep the drains clear, and measure the slope before the first storm.