Snow Load Roof Calculator (ASCE 7)
ASCE 7 Chapter 7 provides the standard procedure for determining design snow loads on roofs. The flat roof snow load pf = 0.7 * Ce * Ct * Is * pg converts the ground snow load pg into a roof design load, accounting for exposure, thermal conditions, and occupancy importance. The factor 0.7 reflects that roofs generally accumulate less snow than the ground due to wind and heat loss. For sloped roofs, apply the slope factor Cs to reduce pf. Ground snow load pg must be taken from ASCE 7 Figure 7.2-1 or site-specific studies; it ranges from 0 psf in Florida to over 100 psf in parts of Alaska.
ASCE 7 snow load formulas
pf = 0.7 × Ce × Ct × Is × pg
ps = Cs × pf
Minimum roof snow load: pf must not be less than Is * 20 psf where pg greater than or equal to 20 psf, or Is * pg where pg less than 20 psf (ASCE 7 Section 7.3.4).
Special snow load cases
- Unbalanced snow loads on gable roofs can be critical and must be checked per ASCE 7 Section 7.6.
- Drift loads at roof steps, parapets, and adjacent structures are often significantly higher than the flat roof load. See ASCE 7 Sections 7.7 and 7.8.
- Ponding (progressive roof deflection from rain on snow) can cause collapse and must be checked per ASCE 7 Section 7.11.
- Rain-on-snow surcharge of 5 psf applies when pg is less than 20 psf (ASCE 7 Section 7.10).
Frequently asked questions
What is the flat roof snow load formula in ASCE 7?
pf = 0.7 * Ce * Ct * Is * pg, where pf is the flat roof snow load (psf), pg is the ground snow load from ASCE 7 Figure 7.2-1, Ce is the exposure factor, Ct is the thermal factor, and Is is the importance factor for snow.
What is the exposure factor Ce?
Ce accounts for wind exposure that sweeps snow off the roof. For fully exposed roofs in open terrain (Exposure C/D), Ce = 0.7. For sheltered roofs (Exposure B), Ce = 1.1. Partially exposed roofs use Ce = 1.0. Values are from ASCE 7 Table 7.3-1.
What is the thermal factor Ct?
Ct accounts for heat loss through the roof, which melts snow. Heated buildings (Ct = 1.0), slightly ventilated cold roofs (Ct = 1.1), unheated open structures (Ct = 1.2), and freezer buildings (Ct = 1.3). From ASCE 7 Table 7.3-2.
What is the importance factor Is for snow?
Is increases design loads for essential or high-occupancy facilities. Risk Category I = 0.80, Risk Category II = 1.00, Risk Category III = 1.10, Risk Category IV = 1.20. From ASCE 7 Table 1.5-2.
How does roof slope affect snow load?
For sloped roofs, the design snow load ps = Cs * pf, where Cs is the roof slope factor from ASCE 7 Figure 7.4-1. Steeper warm roofs shed snow more readily (lower Cs). For slopes greater than 70 degrees, ps = 0 (no snow accumulation assumed).
Official sources
- American Society of Civil Engineers: ASCE 7-22 Minimum Design Loads, Chapter 7 Snow Loads.
- International Building Code (ICC): IBC 2021 Section 1608 Snow Loads.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.