Snow Load Calculation — ASCE 7 Ground and Roof Snow Loads
Snow load calculation per ASCE 7-22 Chapter 7 converts ground snow load (pg) into the design roof snow load (ps) by applying terrain, exposure, thermal, and importance factors. This reference covers the full calculation procedure, flat and sloped roof loads, drift calculations, unbalanced loads, and typical pg values by US region.
Snow Load Calculation Procedure Overview
1. Determine ground snow load: pg (from ASCE 7 Figure 7.2-1 or state maps)
2. Compute flat roof snow load: pf = 0.7 × Ce × Ct × Is × pg
3. Check minimum pf:
- pf ≥ Is × 20 psf (pg > 20 psf areas)
- pf ≥ Is × pg (pg ≤ 20 psf areas)
4. Adjust for roof slope: ps = Cs × pf
5. Add drift loads on lower roofs
6. Check unbalanced loads for pitched and curved roofs
Key Snow Load Factors
Exposure Factor Ce (ASCE 7 Table 7.3-1)
| Roof Exposure | Terrain Category B | Terrain C | Terrain D |
|---|---|---|---|
| Fully exposed | 0.9 | 0.9 | 0.8 |
| Partially exposed | 1.0 | 1.0 | 0.9 |
| Sheltered | 1.2 | 1.1 | 1.0 |
- Fully exposed: No shelter from trees, terrain features, or adjacent structures; windy site
- Partially exposed: Most roofs in suburban/industrial areas
- Sheltered: Dense forest or surrounded by tall buildings that block wind
Thermal Factor Ct (ASCE 7 Table 7.3-2)
| Structure Use | Ct |
|---|---|
| Heated structure (most buildings) | 1.0 |
| Structures kept just above freezing (cold storage, greenhouses) | 1.1 |
| Unheated structures | 1.2 |
| Continuously heated freezers | 1.3 |
| Structures with R ≥ 30 insulation (energy-efficient) | 1.1 |
Importance Factor Is (ASCE 7 Table 1.5-2)
| Risk Category | Is |
|---|---|
| I (low hazard — agricultural) | 0.80 |
| II (standard occupancy) | 1.00 |
| III (substantial hazard — schools, hospitals) | 1.10 |
| IV (essential facilities — emergency response) | 1.20 |
Flat Roof Snow Load
pf = 0.7 × Ce × Ct × Is × pg
Minimum pf:
If pg > 20 psf: pf ≥ Is × 20 psf
If pg ≤ 20 psf: pf ≥ Is × pg
Example: Office building (Risk II), suburban location (Terrain C, partially exposed), heated
- pg = 30 psf (location-specific)
- Ce = 1.0, Ct = 1.0, Is = 1.0
- pf = 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21.0 psf
- Minimum = 1.0 × 20 = 20 psf → pf = 21 psf governs
Sloped Roof Snow Load
ps = Cs × pf
Cs = roof slope factor from ASCE 7 Figure 7.4-1
Cs values for warm roofs (Ct ≤ 1.0), unobstructed slippery surface:
| Roof Slope (degrees) | Roof Slope (rise:run) | Cs |
|---|---|---|
| 0–5° | 0–1:12 | 1.0 |
| 10° | ~2:12 | 1.0 |
| 15° | ~3:12 | 0.93 |
| 20° | ~4:12 | 0.87 |
| 25° | ~5:12 | 0.80 |
| 30° | ~6:12 | 0.73 |
| 35° | ~7:12 | 0.67 |
| 40° | ~8:12 | 0.60 |
| 50° | ~12:12 | 0.40 |
| 60° | ~21:12 | 0.20 |
| 70° | ~33:12 | 0.0 |
For rough surfaces (metal roofing, shingles): Cs = 1.0 up to 15°, then linear to 0 at 70°.
Roof Snow Drift Calculations
Drifts form when wind-transported snow accumulates against parapets, at lower roofs, or in roof valleys.
Leeward (Downwind) Drift — ASCE 7 Section 7.7.1
hb = pf / γ (balanced snow depth, ft)
hd = 0.43 × (lu)^(1/3) × (pg + 10)^(1/4) − 0.5 (ft)
Where:
lu = length of upper roof (ft)
γ = snow density = 0.13pg + 14 ≤ 30 pcf
Drift width: w = 4hd (if hd/hb < 0.2: treat as no drift)
Maximum drift surcharge: pd = γ × hd (psf)
Windward (Upwind) Drift
Windward drift = 0.75 × leeward drift height (triangular distribution)
Parapet Drift
Parapet acts as a windward obstruction. Use upper roof length = distance from parapet to ridge or far end.
Unbalanced Snow Loads
For gable roofs (ASCE 7 Section 7.6.1) when W ≥ 20 ft and pf ≥ 20 psf or pg ≥ 20 psf:
Windward slope: 0.3 × ps (reduce snow) Leeward slope: ps + hd × γ/2 (add drift on leeward)
For curved roofs and multi-span roofs, additional provisions apply per Section 7.6.2–7.6.3.
Ground Snow Load Map — Selected US Locations
| Region | pg (psf) | Notes |
|---|---|---|
| Florida, Gulf Coast, S. California | 0–5 | Essentially no snow design required |
| Mid-Atlantic (DC, Richmond) | 15–25 | Moderate; drift check needed |
| Northeast coast (NYC, Boston) | 25–40 | Significant; full drift analysis |
| Great Lakes (Cleveland, Chicago) | 25–40 | Lake-effect areas higher |
| Midwest (Kansas City, St. Louis) | 15–25 | Variable by year |
| Mountain West (Denver foothills) | 30–50 | Altitude dependent |
| Colorado mountains (Vail, Aspen) | 100–250+ | Extreme; site-specific required |
| Pacific Northwest (Portland, Seattle) | 10–20 | Mild coast; mountains higher |
| Minnesota, Wisconsin | 40–60 | High ground snow loads |
| New England mountains (VT, NH, ME) | 50–100+ | Case study or site-specific |
| Alaska (Anchorage) | 50–100 | Site-specific by municipality |
Note: ASCE 7 Figure 7.2-1 provides the authoritative ground snow load map. CS areas require site-specific studies. Always verify against the local building authority's adopted map.
Rain-on-Snow Surcharge
For pg between 0 and 20 psf: add rain-on-snow surcharge = 5 psf to the design roof snow load (Section 7.10).
This accounts for rain that falls on an existing snow pack, increasing the total load beyond the dry snow weight. Not required for slopes > 1/2:12 (slope shedding assumed).
Frequently Asked Questions
What is the flat roof snow load for a 30 psf ground snow load? For a standard office (Is = 1.0, Ce = 1.0, Ct = 1.0): pf = 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21 psf. Minimum = 20 psf, so 21 psf governs. For a sheltered site (Ce = 1.2): pf = 0.7 × 1.2 × 1.0 × 1.0 × 30 = 25.2 psf.
Do I need to check snow drift on a single-storey building? Yes, whenever there are parapets or roof features that create wind shadow. A 24 in parapet on a 50 ft wide roof with pg = 30 psf can generate drift surcharges of 30–50 psf on the parapet-adjacent zone. Drift loads are often the controlling design case for roof structure adjacent to walls.
When is the Cs factor less than 1.0? For sloped roofs with slope > 5° (warm roof) or > 15° (cold roof), snow slides off and Cs < 1.0. Slippery metal roofs shed snow faster; rough asphalt shingles retain more. Cs = 0 at 70° (essentially vertical) — no design snow load for steep slopes.
Is snow load additive with wind? In LRFD: LC3 = 1.2D + 1.6S + 0.5W (or L). Snow and wind are partially combined. Full simultaneous application of both at maximum values has low probability. ASCE 7 Section 2.3 combinations reflect this through the 0.5 companion load factor.
What is the difference between balanced and unbalanced snow load? Balanced snow load is the uniform flat or sloped roof snow load ps applied symmetrically across the entire roof, assuming wind has not redistributed the snow. Unbalanced snow load accounts for wind blowing snow off the windward slope and depositing it on the leeward slope, creating an asymmetric loading condition. For gable roofs wider than 20 ft with pf ≥ 20 psf or pg ≥ 20 psf, ASCE 7-22 Section 7.6.1 requires checking unbalanced loads: 0.3ps on the windward slope and ps plus a drift surcharge on the leeward slope. Unbalanced loads typically govern the design of rafters and ridge connections because they create significant bending and thrust that symmetric loading does not produce.
How does the thermal factor Ct affect roof snow load? The thermal factor Ct increases the design snow load for structures where less heat escapes through the roof to melt accumulated snow. For a standard heated building, Ct = 1.0. For unheated structures Ct = 1.2, meaning the flat roof snow load is 20% higher than for a heated structure with the same pg, Ce, and Is values. Cold-storage buildings kept just above freezing use Ct = 1.1, and continuously heated freezers use Ct = 1.3 because the interior is intentionally kept cold and the roof surface remains at near-ambient temperature. Selecting the correct Ct is particularly important for industrial and agricultural structures that may be unheated or minimally conditioned.
When does snow drift govern the structural design? Snow drift governs when the drift surcharge load on the lower roof or at a parapet significantly exceeds the balanced snow load. This commonly occurs on buildings with large upper roof areas (long lu), high ground snow loads (pg ≥ 30 psf), and significant step height between roof levels. The leeward drift height hd from ASCE 7-22 Section 7.7.1 is proportional to lu^(1/3) × (pg + 10)^(1/4); when hd exceeds the balanced snow depth hb = pf/γ, the drift load can be 2–4 times the uniform balanced load. Drift loads typically govern the design of roof framing adjacent to penthouse walls, taller building sections, and rooftop equipment screens.
Run This Calculation
→ Snow Load Calculator — ASCE 7 roof snow load (ps) from ground snow load (pg), exposure category, thermal factor, and importance factor.
→ Beam Capacity Calculator — verify roof beam capacity under factored snow + dead load per AISC 360.
Related pages
- Load Combinations ASCE 7 — how snow load fits into LRFD/ASD combinations
- Deflection Limits Reference — serviceability check under snow + dead load
- Steel Beam Span Guide — roof beam span capacity under snow loading
- Steel Roof Framing Reference — purlin and joist sizing for roof snow loads
- Live Load Reference — ASCE 7 — minimum roof live loads to combine with snow
- Reference tables directory
- How to verify calculator results
- Wind Load Calculation
- structural wind load calculator
Snow loads per ASCE 7-22 Chapter 7. Ground snow loads must be determined from ASCE 7 Figure 7.2-1 or local authority's adopted map. CS zones require site-specific ground snow load studies. Confirm with the authority having jurisdiction before finalizing design loads.
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