What is Snow Load?
Snow load is the downward force exerted on a structure by the weight of accumulated snow. It is measured in pounds per square foot (psf) and is a critical factor in structural engineering, particularly for roof design. Excessive snow load can cause roof collapse, structural damage, or building failure if the structure was not designed to handle the weight.
Snow load depends on the depth of snow, its density (which varies based on whether it is fresh, settled, or ice-crusted), and the roof geometry. Building codes specify minimum design snow loads based on geographic location, elevation, and building importance.
Snow Load Formulas
Snow Density Reference
| Snow Type | Density (lb/cu ft) | Weight per inch depth (psf) |
|---|---|---|
| Fresh, light snow | 5–10 | 0.4–0.8 |
| Average settled snow | 15–20 | 1.3–1.7 |
| Packed / old snow | 20–40 | 1.7–3.3 |
| Ice | 57 | 4.7 |
Worked Example
A 1,200 sq ft roof with 12 inches of settled snow (15 lb/cu ft density):
Roof load = 0.7 × 15 = 10.5 psf
Total weight = 10.5 × 1,200 = 12,600 lbs
Frequently Asked Questions
What is the typical design snow load?
Design snow loads vary by location, from 0 psf in southern regions to over 100 psf in heavy snowfall mountain areas. Check your local building code or ASCE 7 for the ground snow load (Pg) for your location.
When should I be concerned about snow load?
Be concerned when accumulation exceeds your roof's design load, typically when heavy wet snow or ice builds up beyond normal levels. Warning signs include creaking sounds, visible sagging, doors sticking, and cracking in walls or ceilings.
Does roof pitch affect snow load?
Yes, steeper roofs shed snow more easily and retain less accumulation. Flat roofs collect the most snow. Building codes apply a slope factor that reduces the design snow load for pitched roofs.