Roof Snow Load Calculator — ASCE 7 Flat Roof Formula
Calculate flat and sloped roof snow loads using the ASCE 7 formula Pf = 0.7 × Ce × Ct × I × Pg. Enter your ground snow load, exposure factor, thermal factor, and importance factor to get code-compliant design loads for structural engineering.
The ASCE 7 Flat Roof Snow Load Formula Explained
The ASCE 7 standard defines flat roof snow load as Pf = 0.7 × Ce × Ct × I × Pg. This formula is the starting point for any structural snow load calculation in the United States. The 0.7 coefficient accounts for the fact that roofs accumulate less snow than the ground due to heat loss, wind, and geometry.
Example: a partially exposed heated residential building in a 40-psf ground snow zone calculates as Pf = 0.7 × 1.0 × 1.0 × 1.0 × 40 = 28 psf flat roof snow load. A cold storage building in the same zone with full shelter: Pf = 0.7 × 1.2 × 1.3 × 1.0 × 40 = 43.7 psf — 56% more load from the same ground snow.
The minimum flat roof design load per ASCE 7 is 20 psf for Pg ≥ 20 psf regions, and Pg for regions where Pg < 20 psf. This floor prevents under-engineering in moderate snow regions where the formula might yield an unrealistically low number.
Sloped Roof Snow Load: Applying the Cs Factor
For sloped roofs, ASCE 7 multiplies the flat roof snow load by a slope reduction factor Cs: Ps = Cs × Pf. For warm roofs (Ct ≤ 1.0) with slippery surfaces, Cs drops to 0 at 70°. For warm roofs with non-slippery surfaces, Cs = 1.0 for pitches below 15° and decreases linearly to 0 at 70°.
Cold roofs (Ct = 1.1) retain full snow load to steeper angles before the reduction kicks in. Unobstructed slippery surfaces (metal, slate, glass) get the maximum slope credit. If your roof has skylights, parapets, or equipment that could obstruct sliding, use the non-slippery reduction curve regardless of surface material.
Note: this calculator computes balanced snow load only. Unbalanced loads, drift loads at parapets and step changes, and sliding snow loads at lower roofs require separate calculations per ASCE 7 Sections 7.6–7.9. For complex roofs, consult a licensed structural engineer.
Roof Snow Load by Region: Quick Reference
Ground snow load varies enormously by location. Coastal Southeast US: 0–10 psf. Midwest plains: 15–30 psf. Mid-Atlantic and New England: 25–50 psf. Upper Midwest and Mountain West: 40–100 psf. Sierra Nevada and Cascade peaks: 200–300 psf at elevation.
Using the ASCE 7 formula for a standard residential building (Ce=1.0, Ct=1.0, I=1.0), flat roof design loads come to: 10 psf ground → 7 psf roof; 25 psf ground → 17.5 psf roof; 50 psf ground → 35 psf roof; 100 psf ground → 70 psf roof. Always verify your specific location's Pg from the adopted building code or a geotechnical engineer's site assessment for mountain or special study areas.
Frequently Asked Questions
What is the ASCE 7 flat roof snow load formula?
The ASCE 7 flat roof snow load formula is Pf = 0.7 × Ce × Ct × I × Pg. Pf is the design flat roof snow load (psf), Ce is the exposure factor (0.7–1.3), Ct is the thermal factor (1.0–1.3), I is the importance factor (0.8–1.2), and Pg is the ground snow load from ASCE 7 Figure 7.2-1 for your location.
How do I find the ground snow load (Pg) for my location?
Ground snow load (Pg) values are provided in ASCE 7 Figure 7.2-1 and local building codes. Values range from 0 psf in the deep south to 100+ psf in mountain regions. Many states adopt modified maps in their building codes — always check your local jurisdiction's adopted code, not just the national ASCE 7 map.
What exposure factor (Ce) should I use for a flat roof?
Ce = 0.9 for fully exposed roofs in open terrain with no obstructions. Ce = 1.0 for partially exposed roofs in suburban or light wooded terrain. Ce = 1.2 for sheltered roofs surrounded by trees or taller structures. Lower exposure factors mean less snow accumulates due to wind scour.
What thermal factor (Ct) applies to a flat roof?
Ct = 1.0 for heated structures with standard insulation. Ct = 1.1 for structures kept just above freezing (like unheated garages). Ct = 1.2 for unheated open-air structures. Ct = 1.3 for cold storage or refrigerator buildings where roof temperature is below freezing.
Does a flat roof carry more snow load than a sloped roof?
Yes. A flat roof retains the full calculated Pf value because snow doesn't slide off. Sloped roofs apply a slope reduction factor (Cs) that reduces load for pitches above 15° for cold roofs and above 30° for warm roofs. Roofs steeper than 70° receive zero design snow load per ASCE 7.