ROOF SNOW LOAD

Enter your local ground snow load (pg) in psf — pull this from your local building department or snowloadinfo.com. Common values: Boston 35, Chicago 25, Denver 25, Buffalo 50, mountain regions 70–100+. Pick the roof type (heated for houses with conditioned space below, unheated for barns and detached garages) and exposure (most suburban homes are "partially exposed").

Enter the roof pitch in rise per 12 inches. The calculator returns the flat-roof load (pf) and the sloped-roof load (ps), which accounts for snow sliding off steeper roofs above ~30°. This is the design load that rafters, trusses, and sheathing must support per ASCE 7-22.

For a heated suburban house with pg = 30 psf, partially exposed, 6/12 pitch roof:

pf = 0.7 × Ce × Ct × I × pg = 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21 psf flat load.

Roof angle: arctan(6/12) = 26.6° — under the 30° threshold for slope reduction, so Cs = 1.0.

Sloped load (ps): 21 psf — same as flat in this case.

For a steep 12/12 roof (45°) with same pg = 30 psf: Cs = (70 − 45) ÷ 40 = 0.625. ps = 0.625 × 21 = 13.1 psf — significant load reduction because snow slides off.

For an unheated barn (Ct = 1.2) at 6/12 pitch with pg = 50 psf (heavy snow region): pf = 0.7 × 1.0 × 1.2 × 1.0 × 50 = 42 psf. That's the design load every rafter and truss must carry.

Using pg as design load directly. Ground snow ≠ roof snow. Roofs typically see 60–80% of ground load due to wind effects, thermal melting, and slope. The 0.7 base factor handles this.

Picking "fully exposed" for a typical suburban site. Most homes are "partially exposed" — surrounded by some trees or buildings that reduce wind drift effects. Fully exposed is open prairie or coastal sites.

Forgetting the importance factor. I = 1.0 for residential (Risk Category II). Schools, hospitals, and emergency facilities use I = 1.1 or 1.2. The calculator assumes I = 1.0.

Underestimating drift loads. The calculated ps is the BALANCED snow load — uniform across the roof. Drift loads against walls, parapets, or roof level changes can be 2–3× the balanced load. Engineered analysis required for those locations.

ASCE 7 simplified: pf = 0.7 × Ce × Ct × I × pg. I = 1.0 residential.

Typical Ce: 0.9 fully exposed, 1.0 partially exposed, 1.2 sheltered.

Ct: 1.0 heated, 1.2 unheated.

Slope reduction: kicks in above 30°. ps = ((70 − angle) ÷ 40) × pf, with Cs capped at 1.0.

Drift loads against walls and roof level changes: not handled by this formula, requires engineered analysis.

Verify pg with your local building department — ASCE map values can be wrong by 5–10 psf in mountainous or microclimate areas.