Formula used
Flat roof snow load (informational, simplified): pf = 0.7 × Ce × Ct × Is × pg
- pg: Ground snow load (psf or kPa)
- Ce: Exposure factor
- Ct: Thermal factor
- Is: Importance factor
Sloped roof load (simplified): ps = Cs × pf, where Cs accounts for slope and roof surface slipperiness. This tool linearly reduces Cs from 30° to 70° for slippery roofs, and from ~15° to 70° for non‑slippery roofs. For low slopes (<5°), Cs ≈ 1.
Optional surcharges are added: ptotal = proof + pr + pd.
How to use this calculator
- Select the unit system: US (psf) or Metric (kPa).
- Enter the ground snow load pg for your site.
- Enter factors: exposure Ce, thermal Ct, and importance Is.
- Choose roof type and slope; enable slippery if applicable.
- Include optional surcharges for rain‑on‑snow and drifting when required.
- Click Calculate to view pf, ps, and total design load.
- Use Download CSV or Download PDF to export results.
Example data table
| Units | pg | Ce | Ct | Is | Roof | Slope° | Slippery | pr | pd | Use |
|---|---|---|---|---|---|---|---|---|---|---|
| US | 30 | 1.0 | 1.0 | 1.0 | Flat | 0 | No | 0 | 0 | |
| US | 40 | 0.9 | 1.1 | 1.0 | Sloped | 25 | No | 0 | 2 | |
| US | 60 | 1.2 | 1.0 | 1.1 | Sloped | 45 | Yes | 5 | 4 | |
| Metric | 1.5 | 1.0 | 1.0 | 1.0 | Flat | 2 | No | 0 | 0 | |
| Metric | 2.2 | 1.0 | 0.9 | 1.0 | Sloped | 35 | Yes | 0.2 | 0.1 |
Click Apply to load the example into the form, then calculate.
Ground snow load and factors overview
These ranges are informational. Always verify with current standards and local amendments.
| Factor | Typical range | What it represents | Notes |
|---|---|---|---|
| Ce (Exposure) | ~0.9 – 1.2 | Wind exposure and terrain effects | Higher values for exposed sites; lower for sheltered conditions. |
| Ct (Thermal) | ~0.8 – 1.2 | Heat loss characteristics of the building | Warm roofs may reduce load; cold roofs may increase load. |
| Is (Importance) | ~0.8 – 1.2 | Risk/occupancy importance | Essential facilities often use higher values; storage may be lower. |
| pg | Varies by location | Ground snow load baseline | Obtain from official maps, local code, or a licensed engineer. |
Slope reduction (Cs) quick reference
Computed with this tool’s simplified linear model for demonstration.
| Slope (°) | Cs — non‑slippery | Cs — slippery |
|---|
Worked examples (pf, ps, total) in psf and kPa
Examples match the same equations used above: pf = 0.7·Ce·Ct·Is·pg, ps = Cs·pf, total includes pr and pd.
| # | Inputs | pf (psf) | ps (psf) | Total (psf) | pf (kPa) | ps (kPa) | Total (kPa) |
|---|
Parapet and snow drift notes
Drifts can form at parapets, around higher-to-lower roof steps, penthouses, and equipment. Their magnitude depends on wind exposure, upwind fetch, roof height differences, and parapet height.
- Identify potential drift locations: steps, parapets, ridges, valleys, and obstructions.
- Obtain drift surcharge from authoritative methods or a licensed engineer.
- Enter that surcharge as pd to include it in Total design snow load.
- Consider local drifting patterns; wind directionality can cause asymmetric loading.
Unbalanced loading and sliding snow
Gable and shed roofs may experience unbalanced snow loading due to wind scour or sliding from the upper surface to eaves or lower roofs, especially on slippery materials.
- Evaluate unbalanced cases on opposing roof surfaces when appropriate.
- On slippery roofs, check eave accumulation and sliding forces on guards.
- Verify bearing and connections at lower roofs receiving sliding snow.
- Use the slippery roof toggle to reflect reduced retained snow on steep surfaces.
Rain-on-snow surcharge guidance
Rain-on-snow may add significant weight if drains are impaired or melting is slow. It is sensitive to climate, roof thermal behavior, drainage design, and maintenance.
- Confirm whether the jurisdiction specifies a minimum surcharge.
- Assess ponding risk and ensure clear, redundant drainage paths.
- Account for heated vs. unheated roofs and insulation levels.
- Enter the surcharge as pr to include it in the total.
| Scenario | Drainage condition | Thermal behavior | Design note |
|---|---|---|---|
| Cold roof, good drainage | Primary drains clear | Minimal melting | Lower likelihood, still verify surcharge requirements |
| Warm roof, marginal drainage | Partially obstructed | Intermittent melting | Consider meaningful surcharge; review ponding stability |
| Warm roof, blocked drains | Blocked / frozen | Persistent meltwater | High surcharge risk; prioritize drainage remediation |
Roof‑zone placement guidance
Divide the roof into zones to reflect where snow tends to accumulate or scour. Zoning helps apply different loads to critical areas instead of using a single uniform value everywhere.
- Corners and edges: Often wind‑scoured; check for local reductions and unbalanced cases.
- Field: Representative interior area; use computed flat or sloped roof load.
- Ridge: May scour on windward; consider unbalanced loading on opposing slopes.
- Eaves / lower roofs: Sliding/accumulation from upper slopes; verify bearing and connections.
- Valleys: Flow convergence; potential accumulation and drift from adjacent roof surfaces.
- Obstructions/parapets/steps: Drift formation downwind of height changes and equipment.
| Zone | Typical condition | Snow behavior | Design note |
|---|---|---|---|
| Corner/Edge | High wind exposure | Scour; possible unbalanced patterns | Consider asymmetric loading; guard/railing sliding checks |
| Field | Interior roof area | Near uniform retained snow | Use pf or ps from calculator |
| Ridge | Peak of sloped roofs | Windward scour; leeward retention | Evaluate unbalanced case across ridgeline |
| Eave/Lower roof | Downslope termination or adjacent lower roof | Sliding snow accumulation | Check sliding loads, ponding, and local reinforcement |
| Valley | Intersecting roof planes | Accumulation due to flow convergence | Account for increased load line along valley |
| Parapet/Obstruction | Height change or equipment | Leeward drift formation | Use drift surcharge (pd) for downwind zone |
Quick workflow
- Compute base load: pf (flat) or ps (sloped) using inputs.
- Define zones: corners/edges, field, ridge, eaves/lower roofs, valleys, obstructions.
- Apply modifiers per zone: unbalanced patterns, sliding, and drift surcharge pd.
- Include rain‑on‑snow pr if specified by climate or code.
- Verify members, connections, and drainage specific to high‑risk zones.