Snow Load Calculator

Decks are flat and don't shed snow. A 4:12 pitched roof loses ~20-30% of snow load through slope-shedding per ASCE 7. A flat roof (or deck) loses zero. Also: open decks have thermal factor Ct = 1.2 (no heated envelope to melt snow from below), while heated home roofs have Ct = 1.0. Combined: flat unheated decks see 30-50% more design snow than the heated pitched roof on the same house. Joist sizing reflects this — a 2×10 at 16″ OC spans 14.8 ft for a heated roof but only 11.7 ft for a deck at the same ground snow.

Only if there's an upwind roof higher than the deck. If you have a 2-story house and the deck is on the back of the 1-story addition (10 ft height difference), wind blows snow off the higher house roof onto the deck and piles it against the house wall. ASCE 7-22 §7.7 estimates drift roughly 1.5 psf per foot of height delta — so 10 ft delta = +15 psf drift on top of the design snow. For a deck that already sees 30 psf flat-roof snow, adding 15 psf drift means structural sizing needs to handle 45 psf, not 30. Most home builders skip this math and end up with deflection issues at the house-side joists in heavy snow years.

No — ASCE 7 ground snow is the 50-year recurrence interval annual maximum snow WEIGHT on the ground (psf), not snowfall depth (inches). Different metrics. Fresh fluffy snow weighs ~5 psf per foot; wet packed snow weighs ~25 psf per foot. So 30 psf ground snow load could mean 6 ft of fluff or 14 inches of wet pack. ASCE 7 captures the worst-case 50-year maximum loading regardless of depth. Always use the published ASCE / IRC ground snow PSF value (or its state default in this calculator), not your snowfall depth.

IRC R301.5 mandates a minimum 40 psf live load for residential decks regardless of snow load — to handle occupant + furniture + party-crowd loading. Even in Florida (0 psf snow), your deck must be designed for 40 psf live. In northern states where snow exceeds 40 psf, snow controls instead. The 'load controller' tag in results shows which one is driving your structural sizing.

Yes, slightly. If the deck is fully covered (roofed pavilion, lanai, screen room with solid roof above), the thermal factor drops from Ct = 1.2 (open structure) to Ct = 1.0 (within heated thermal envelope) — that's 17% less design snow PSF. Partial covers (cantilevered house roof overhang, retractable awnings) don't qualify; they don't shelter the deck thermally. Note that adding a roof structure ALSO adds dead load — usually a wash on net structural impact.

Three sources. (1) Your local building department — they have the ASCE 7 lookup for your jurisdiction. (2) ASCE 7-22 Hazard Tool at https://asce7hazardtool.online — free online lookup with full county precision plus the snow-related parameters. (3) Your state's adopted IRC + amendments document — most states publish a snow load map appendix. The state default in this calculator is the central county; if you're in a mountain or coastal county the actual number can differ by 50-100%.

Rough — within 20%. ASCE commentary mentions roughly 1 psf per 100 ft as a general guideline above the state's typical elevation. For mountain sites above 8,000 ft, the relationship becomes non-linear (some Colorado ski-area sites see 200+ psf). Always confirm with your local building department for elevations above 5,000 ft. The calculator flags this case.

It captures wind's effect on snow distribution. Fully-exposed sites (Ce = 0.9) get 10% LESS design snow because wind blows snow off the deck. Sheltered sites (Ce = 1.2) get 20% MORE because snow piles up without wind disturbance. The default partially-exposed (Ce = 1.0) is the ASCE baseline for typical suburban residential. Get this wrong and a sheltered courtyard deck in PA could be undersized by 20% — the difference between 'fine for 50 years' and 'sagging by year 15'.

Yes — IRC + ASCE allow a 2.5× safety factor over published values. Allowable stress design means your structure could theoretically hold 2.5× the design snow before failure. In practice, this safety factor absorbs the once-in-100-year storms (5+ standard deviations above mean). The 50-year recurrence interval ground snow IS the design value — not a maximum. Decks routinely survive 'storm of the century' events because of this margin. The failure modes you actually see are usually undersized framing + skipped IRC compliance, not unusual storms.