Roof Snow Load Calculator (ASCE 7-22)

Ground snow load (pg) is the historical weight of snow measured on flat, open ground by weather stations. Roof snow load (pf for flat, ps for sloped) is the actual engineering weight that the roof structure experiences. Roof snow load is typically 20% to 50% lower than ground snow load because wind blows snow off the roof, heat escaping from the building melts snow, and gravity causes snow to slide off pitched slopes.

A rain-on-snow event is by far the most dangerous weather hazard for roof structures. When heavy liquid rain falls onto an existing layer of snow, the porous snow pack absorbs the water like a giant sponge rather than shedding it into the gutters. A 12-inch snow layer that initially weighed a safe 12 psf can double or triple in weight to 30+ psf in a matter of hours as rainwater saturates the pack.

Key warning signs of imminent roof failure include: 1) Interior doors that suddenly stick or will not latch; 2) New drywall cracks spreading across ceilings or above door frames; 3) Visual sagging in ceiling drywall or visible deflection in exposed attic rafters/trusses; 4) Creaking, groaning, or sharp popping sounds from the attic framing; and 5) Severe water leaking through light fixtures or ceilings.

Metal roofs feature smooth, low-friction painted or bare metal surfaces. In ASCE 7-22 Section 7.4, unobstructed metal roofs qualify as 'slippery surfaces.' As soon as interior attic heat or outdoor solar radiation warms the metal panels, a thin boundary layer of water forms between the metal and snow, causing the entire snow pack to slide off the roof in sheets, preventing heavy long-term accumulation.

Snow guards (cleats, brackets, or continuous pipe rails) are mechanical devices installed along the lower eaves of a metal or slate roof. Because metal roofs shed snow rapidly, sudden snow avalanches can crush gutters, destroy ground-level HVAC units, break landscaping, and severely injure pedestrians standing below. Snow guards hold the snow pack in place, forcing it to melt gradually in controlled volume.

The safest method is using an extendable roof rake from the ground, pulling snow downward in gentle 1-foot layers starting from the eaves and working upward. Never stand on a snow-covered roof, and never use metal shovels, picks, or salt pellets that puncture shingles and corrode aluminum flashing. Leave a 2-to-3 inch buffer of snow on the shingles to avoid damaging granules.

When winter wind blows across a pitched gable or hip roof, it strips snow from the windward (upwind) roof plane and deposits it in concentrated drifts on the leeward (downwind) slope. Under ASCE 7-22 Section 7.6, the leeward slope must be engineered to carry an unbalanced drift load that can be up to 150% to 250% heavier than the uniform design snow load.

Proper attic insulation (R-49 to R-60 in northern climates) and balanced 1:300 soffit-to-ridge ventilation prevent heat from escaping into the attic. If heat escapes, it creates uneven melting where snow melts on upper slopes and refreezes into massive, heavy ice dams at the unheated eaves. A cold, well-ventilated roof deck maintains uniform snow distribution and prevents localized ice dam weight spikes.

You can find your official ground snow load by calling your local city or county municipal building inspection department, checking local residential code amendments (IRC Table R301.2(1)), or using the official online ASCE 7 Hazard Tool (asce7hazardtool.online), which provides geo-located ground snow data based on USGS and NOAA historical snowfall mapping.