ASCE 7 turns the ground snow load into a roof snow load with a short formula and a few factors. The flat-roof value is pf = 0.7 times Ce times Ct times Is times pg, and the sloped-roof value multiplies that by a slope factor. Drift and sliding are separate checks on top. The result is a roof snow load you enter into a takedown as a roof load, and StructLoads carries it down with the dead and live loads.
Start from ground snow
Every snow calculation starts with the ground snow load pg, the snow load on the ground at the site. You take it from the ASCE 7 maps or the ASCE Hazard Tool for the exact location, because it varies strongly by region and elevation. Mountainous and special regions need site-specific values rather than a map reading, so pin pg down first.
The flat-roof formula
The basic conversion of ground to roof snow is one expression:
pf = 0.7 times Ce times Ct times Is times pg
The 0.7 is the basic exposure conversion, and the three factors adjust it for the building.
| Factor | Meaning | Typical range |
|---|---|---|
| Ce | Exposure of the roof to wind | About 0.8 to 1.2 |
| Ct | Thermal, heated or cold roof | 1.0 to 1.2 |
| Is | Importance of the building | 0.8 to 1.2 |
A sheltered, heated, ordinary building sits near the middle; a windswept, unheated, essential facility pushes the factors apart.
The sloped-roof load
A steep roof sheds snow, so the sloped-roof load reduces the flat value with a slope factor Cs that depends on the roof pitch and whether the roof is warm or slippery:
ps = Cs times pf
A flat or low-slope roof has Cs near 1.0, keeping the full flat load, while a steep, slippery roof has a smaller Cs. So the steeper the roof, generally the less balanced snow it holds.
A worked example
Take a heated, ordinary, partially exposed building with a ground snow load pg of 1.5 kPa. With Ce of 1.0, Ct of 1.0 and Is of 1.0, the flat-roof snow load is 0.7 times 1.0 times 1.0 times 1.0 times 1.5, which is about 1.05 kPa. For a low-slope roof with Cs of 1.0, the sloped load is the same 1.05 kPa. That value is then entered as the roof load in the takedown and combined with dead and live load through the ASCE 7 load combinations.
Drift, sliding and unbalanced snow
The flat or sloped value is the balanced case. Real roofs also see snow drifting against parapets and higher adjacent roofs, snow sliding from an upper roof onto a lower one, and unbalanced snow on gable and curved roofs. These are separate checks in ASCE 7 and often govern near steps and parapets. They sit on top of the balanced load, not instead of it, which is why a single number is rarely the whole story. FEMA’s Snow Load Safety Guide, P-957, singles out drifting and sliding as a leading cause of snow-related roof collapse, and notes that drift provisions only entered widely adopted US standards in the late 1980s, so older buildings with roof steps or attached lower wings may never have been designed for them.
How it enters the takedown
The roof snow load behaves like any other area load: multiply it by a support’s tributary area to get its share, then carry it down. It is kept separate from dead and live so the load factors and the roof-or-snow term in the combinations act correctly, the same separation used throughout dead load vs live load and the related typical floor live load values.
Key takeaways: ASCE 7 snow load
Snow load is pf = 0.7 times Ce times Ct times Is times pg for a flat roof, times the slope factor Cs for a sloped roof, starting from the site ground snow pg. Drift, sliding and unbalanced snow are separate checks. The balanced roof snow enters the takedown as a roof load. StructLoads carries the value you enter; a qualified engineer confirms pg, the factors and the governing drift cases.
Quick answers
How do you calculate snow load in ASCE 7?
Start from the ground snow load pg, then compute the flat-roof snow load as pf = 0.7 times Ce times Ct times Is times pg, where Ce is exposure, Ct is thermal and Is is importance. For a sloped roof, multiply by the slope factor Cs to get ps = Cs times pf. Then check drift, sliding and unbalanced snow separately.
What is the ground snow load pg?
Ground snow load pg is the site-specific snow load on the ground, taken from ASCE 7 maps or the ASCE Hazard Tool by location. It is the starting point for the roof snow calculation. Mountainous and case-study regions need site-specific values rather than the map, so confirm pg for the exact location before computing the roof load.
What are the Ce, Ct and Is factors?
Ce is the exposure factor, lower for sheltered roofs and higher for windswept ones. Ct is the thermal factor, near 1.0 for heated buildings and higher for cold roofs. Is is the importance factor, higher for essential facilities. They adjust the basic 0.7 conversion of ground to roof snow up or down for the specific building.
When is the basic snow calculation not enough?
The flat or sloped value is not enough on its own where snow can drift against parapets and higher roofs, slide from an upper roof, or sit unbalanced on a gable or curved roof. Those are separate, often governing, checks in ASCE 7. Rain-on-snow and minimum snow loads can also apply. A qualified engineer confirms which cases govern.