Ground Snow Load: Site Lookup, Roof Design, and Code Rules

The ground snow load for a site is the weight, in pounds per square foot, of snow that accumulates on a flat, unobstructed patch of ground at that location, and in the United States it is read from the maps published in ASCE 7-22. Engineers write it as p_g. It is the starting number for every structural snow calculation that follows, and it is a weight, not a depth: a foot of dry January powder and a foot of wet March slush measure the same on a yardstick but can differ by a factor of five or more in what they do to a roof.

What the Number Actually Measures

p_g is the weight of naturally accumulated snow on level ground with no wind drifting, no melting, and no sheltering effects. That baseline matters because every later adjustment in the design process is a multiplier on this one value. If p_g is wrong, everything downstream is wrong.

The reason depth alone is unreliable comes down to density. Fresh, dry snow falling in still, cold air weighs roughly 3 to 4 pounds per cubic foot. Snow that has sat a day or two and taken some wind compresses to 12 to 19 pounds per cubic foot. Wet, sticky snow runs 17 to 25 pounds per cubic foot, and old compacted snow (firn) can exceed 35. Slush approaches the density of water itself, and one inch of water across a square foot of roof weighs about 5.2 pounds. Engineers convert snow to its water equivalent to find the real load, which is why two visually identical snowfalls can produce very different structural demands.

Finding the Ground Snow Load for Your Site

The authoritative source is ASCE 7-22, “Minimum Design Loads and Associated Criteria for Buildings and Other Structures,” which contains ground snow load maps for the contiguous United States and a separate table for Alaska.¹ Hawaii has zero ground snow load except in mountainous regions approved by local building officials.²

The practical way to pull a value is the Applied Technology Council’s online tool, ATC Hazards by Location, which accepts a street address or GPS coordinates and returns the ASCE 7-22 ground snow load for that exact point.³ A point lookup is more accurate than reading between contour lines on a printed map, and it matters most where a site sits near the boundary between two load zones.

Why Older Snow Load Numbers Shouldn’t Be Recycled

ASCE 7-22 changed how the maps are built. Earlier editions used a 50-year mean recurrence interval and then applied a 1.6 load factor to reach a design strength value. ASCE 7-22 moved to reliability-targeted loads: the mapped values are calibrated so that structures across the country achieve the same probability of failure regardless of local snow climate, and the load factor applied to the mapped number is 1.0 instead of 1.6.⁴

For most locations, the factored flat roof load under ASCE 7-22 lands between 0.95 and 1.15 times the old ASCE 7-16 factored load, averaging about 1.05. A handful of spots moved more than that.⁴ Reusing a p_g from older plans is therefore not safe practice; the number needs to be looked up fresh against the current edition in force locally.

ASCE 7-22 also moved the risk category adjustment out of the formula and into the maps. The old snow importance factor (I_s) is gone. A hospital (Risk Category IV) now looks up p_g on a different map than a standard office building (Risk Category II), with the higher reliability target baked into the mapped value.⁵ So the right map depends on what the building is for, not just where it sits.

Case Study Regions

Some areas have snow climates too variable for a nationwide map to capture, and ASCE 7 flags these as Case Study regions requiring site-specific analysis instead of a map lookup. High-altitude mountain zones and certain microclimates fall here. ASCE 7-22 shrank the extent of these regions by more than 90 percent compared with the ASCE 7-10 and 7-16 editions, drawing on more than 40 additional years of snow data and state-specific studies.⁴ If a site still falls inside one, the building official has to approve the snow load used in design.

Turning Ground Snow Load Into a Roof Design Load

The ground value is not the load the roof is designed for. The flat roof snow load p_f comes from ASCE 7-22 Equation 7.3-1:

p_f = 0.7 × C_e × C_t × p_g

The 0.7 multiplier reflects an empirical finding: roofs typically catch about 70 percent of the snow that lands on open ground nearby.⁶ The other two factors tune the result to local conditions.

The exposure factor C_e accounts for how much wind reaches the roof. A roof fully exposed in open terrain loses snow to wind and gets a lower C_e. A roof sheltered by dense trees or taller buildings holds more snow and gets a higher C_e. Values run from below 1.0 at windy, exposed sites to 1.2 or more at heavily sheltered ones.

The thermal factor C_t reflects how much heat escapes through the roof. A well-heated building with a poorly insulated roof melts snow faster and sits near 1.0. Unheated structures run 1.2, and freezer buildings 1.3, because snow persists longer on a cold surface.

ASCE 7-22 also sets a minimum flat roof snow load p_m that applies when the calculated p_f is too low. In places with light but persistent snowfall, this minimum keeps roofs from being designed for an unrealistically small number just because the formula produces one. Both values must be checked and the greater one used.⁶

Pitched Roofs

Steeper roofs shed snow more readily, and ASCE 7-22 applies a slope factor C_s that reduces the flat roof snow load for pitched roofs:

p_s = C_s × p_f

The steeper the pitch, the lower C_s. The reduction also depends on roof surface (slippery metal sheds snow faster than rough asphalt shingles) and whether the roof is heated from below. Unheated roofs with rough surfaces get the smallest reduction because snow clings longest.

There is a catch. Snow that slides off a steep roof lands somewhere. If it lands on a lower adjacent roof, that lower roof picks up a sliding surcharge. Steepening an upper roof to reduce its own load can push the problem downward instead of eliminating it.

Loads the Flat Formula Doesn’t Cover

Real roofs rarely see uniform snow. Three additional conditions often control the design of specific areas.

Snow drifting. Wind pushes snow across a roof and piles it against obstructions and changes in roof height. A lower roof wing beside a taller block, a parapet, or a rooftop unit all create drift zones. ASCE 7-22 requires engineers to calculate both leeward drifts (snow blown off a higher surface onto a lower one) and windward drifts (snow scooped up and piled against an obstruction from the upwind side).⁷ Where both types overlap, as at reentrant corners or roof steps, designers take the larger of the two rather than adding them; adding would overestimate the load. Drift loads routinely exceed the uniform flat roof load by a factor of two or more on lower roofs and near parapets.

Sliding snow. Snow on a sloped upper roof can release in a sheet and land on a lower roof. ASCE 7-22 requires a sliding snow surcharge on the lower roof, applied in addition to the balanced snow load, concentrated near the base of the upper slope.

Rain-on-snow surcharge. Rain falling on an existing snow pack saturates it and spikes the load. ASCE 7-22 raised the rain-on-snow surcharge to 8 psf, up from 5 psf in earlier editions. It applies to roofs with low slopes where water cannot drain quickly, and it can occur during a warming event that a building owner would reasonably assume is reducing the snow load.

Code Enforcement and Local Amendments

Snow load design is not optional. The International Building Code requires it through Section 1608, and the 2024 IBC pulls its design snow loads from Chapter 7 of ASCE 7, using the reliability-targeted ground snow load values from the ASCE 7-22 maps or the IBC’s reproduced figures. The design roof load must also meet or exceed the minimum live load from IBC Section 1607, which sets a floor regardless of what the snow calculation produces.² Residential construction falls under the International Residential Code, which references the same ASCE 7 provisions for structural adequacy.

Local building departments enforce both codes at plan review before issuing permits. A design submitted with outdated snow load data or the wrong ASCE 7 edition can stall the permit, and a building that fails to meet code faces denied occupancy or professional liability claims against the engineer of record.

Local jurisdictions can also adopt amendments that raise snow load requirements above the national standard. A mountain county may mandate a higher p_g than the ASCE map shows for that location. The obligation to check for and apply any local overlay sits with the design professional before the structural design is finalized.

When Existing Buildings Get Pulled In

Existing buildings are not exempt from current snow load requirements simply because they were built under an older code. The International Existing Building Code requires a structural upgrade whenever an alteration increases the design dead, live, or snow load by more than 5 percent, including snow drift effects. If that threshold is crossed, existing gravity load-carrying elements such as columns and foundations must be brought into compliance with the current IBC requirements for new construction.⁸

Several common renovation activities can cross the 5 percent line:

Adding roof insulation. Changing the thermal properties of the roof assembly can raise the thermal factor C_t, so the roof retains snow longer and the design snow load rises.
Installing snow guards. Snow retention systems prevent sliding, which increases the total load the roof must carry.
Replacing roofing material. Swapping a lightweight membrane for heavier tiles or adding ballast layers increases the dead load.
Raising parapet height. Taller parapets create new drift conditions that may not have been in the original design.

One exception: adding a second layer of roof covering weighing 3 psf or less over a single existing layer does not trigger the upgrade requirement.⁸ For anything beyond that, a reroofing project is worth running past a structural engineer to see whether the new assembly changes the load profile enough to require reinforcement before work begins.

1
ASCE Library. Snow Loads: Guide to the Snow Load Provisions of ASCE 7-22
2
International Code Council. 2024 International Building Code Chapter 16 Structural Design
3
Applied Technology Council. ATC Hazards by Location – Website and API
4
STRUCTURE Magazine. Ground Snow Loads for ASCE 7-22
5
STRUCTURE Magazine. Snow and Rain Loads in ASCE 7-22
6
STRUCTURE Magazine. ASCE 7-22 Flat Roof Snow Load Versus Minimum Snow Load
7
Federal Emergency Management Agency. FEMA Design Guide: Three-Dimensional Roof Snowdrifts
8
UpCodes. IEBC Section 806 Structural