When to use: Determine design snow loads on a roof per ASCE 7-16 Chapter 7. Start from the mapped ground snow load pg, apply the exposure (Ce), thermal (Ct), and importance (Is) factors to get the flat-roof load pf, then reduce by the slope factor Cs to get the sloped-roof load ps. A minimum snow load pmgoverns for low-slope roofs (θ < 15°).
Calculate design roof snow loads per ASCE 7-16 Chapter 7. Convert the mapped ground snow load Pg into flat-roof snow load Pf using exposure, thermal, and importance factors, then reduce to the sloped-roof design load Ps using the roof slope factor Cs. Includes the minimum snow load check for low-slope roofs.
The flat-roof snow load is Pf = 0.7·Ce·Ct·Is·Pg where Ce is the exposure factor (0.9 fully exposed to 1.1 sheltered), Ct is the thermal factor (1.0 heated, up to 1.3 unheated freezer), and Is is the importance factor from Risk Category. The sloped-roof load Ps = Cs·Pf uses the slope reduction factor Cs that decreases from 1.0 to 0 as slope increases from 30° to 70° (warm roofs) or 37.5° to 70° (cold roofs). A minimum load Pm governs for low-slope roofs (θ < 15°).
Flat-roof snow: Pf = 0.7·Ce·Ct·Is·Pg. Slope factor (warm roof, non-slippery): Cs = 1.0 for θ ≤ 30°; Cs = (70 − θ)/40 for 30° < θ < 70°; Cs = 0 for θ ≥ 70°. Sloped-roof snow: Ps = Cs·Pf. Minimum (θ < 15°): Pm = Is·Pg for Pg ≤ 20 psf; Pm = 20·Is for Pg > 20 psf.
Use for any roof in a snow-prone region to establish gravity loads for structural design of rafters, purlins, beams, and columns. Drift loads on lower roofs adjacent to higher structures (ASCE 7 §7.7–7.8), sliding snow, and rain-on-snow surcharges require separate calculations not included in this flat/sloped roof tool.
Pg is obtained from ASCE 7 Figure 7.2-1 (ground snow load map) for most US locations, or from the local Authority Having Jurisdiction (AHJ). Site-specific case studies apply in complex terrain. ASCE 7 also tabulates Pg for many cities.
Ce accounts for how wind sweeps snow off the roof. Fully exposed roofs in open terrain (Ce = 0.9) receive less accumulated snow than sheltered roofs surrounded by trees or buildings (Ce = 1.1). Exposure B is partially exposed (Ce = 1.0), which is the default.
For low-slope roofs (θ < 15°) and Pg ≤ 20 psf, the minimum load Pm = Is·Pg can govern when Pf is reduced by favorable Ce and Ct values. This minimum prevents under-design of nearly-flat roofs where snow has little opportunity to slide off.
Yes, but the slope reduction begins at a lower angle for slippery surfaces (metal, slate, glass). ASCE 7 §7.4 provides separate Cs curves for slippery roofs: Cs begins declining above 15° (warm) or 10° (cold) and reaches zero sooner, giving larger load reductions for steep metal roofs.