# Roof Snow Load Calculator — ASCE 7-16 Balanced Snow Load (psf / kPa) > Free roof snow load calculator: estimate the ASCE 7-16 balanced flat-roof (pf) and sloped-roof (ps) snow load from your ground snow load and editable exposure, thermal, importance and slope factors, with a capacity check and added dead load — metric or imperial, in your browser. Estimate the ASCE 7-16 balanced roof snow load from your ground snow load and fully editable exposure, thermal, importance and slope factors. - **HTML page:** https://siteometry.com/en/tools/roof-snow-load-calculator - **Category:** [Roofing & Siding](https://siteometry.com/en/tools/category/roofing-siding) - **Figures live on the page:** tables, diagrams and worked figures the text refers to ("the table below") are drawn by the page and are not reproduced here. Each section below links to its anchor on the page. - **Runs in the browser:** Siteometry calculators need no account and send no input anywhere — every number below is computed on the visitor's own device. ## Estimate a roof snow load in five steps 1. **Enter the ground snow load** — Type your site's ground snow load pg (or tap a regional hint to start). Confirm the real value with your local building department or the ASCE 7 ground-snow map. 2. **Enter the roof pitch** — Set the pitch as x-in-12 or in degrees. The slope angle drives both the slope factor Cs and the diagram. 3. **Accept or edit the factors** — The exposure (Ce), thermal (Ct), importance (Is) and surface chips pre-fill ASCE 7-16 defaults into editable numbers. Type your own value to override any of them. 4. **Read the loads** — The results rail shows the flat-roof load pf, the sloped-roof load ps, the minimum floor pm, and the governing design load, decomposed in a factor waterfall. 5. **Check capacity (optional)** — Enter your roof's allowable load and any added dead load (solar, re-roofing, tile) for a rough pass / at-risk pre-check — then confirm with a licensed engineer. _On the page: https://siteometry.com/en/tools/roof-snow-load-calculator#howto_ ## What is roof snow load? Roof snow load is the downward pressure that accumulated snow puts on your roof, expressed in pounds per square foot (psf) or kilopascals (kPa). ASCE 7-16 — the standard most US building codes adopt — derives it from the ground snow load pg measured near your site, adjusted for how the roof is exposed, heated and sloped. The balanced flat-roof snow load is pf = 0.7 · Ce · Ct · Is · pg, where Ce is the exposure factor, Ct the thermal factor and Is the risk-category importance factor. The sloped-roof load is ps = Cs · pf, where the slope factor Cs sheds snow off steeper roofs. A minimum-load floor pm (Is · pg, capped at 20 · Is psf) guards against under-counting, so the design load your roof must carry is max(ps, pm). This calculator estimates the balanced snow load — an even blanket of snow across the roof. It does not compute unbalanced, drift or sliding-snow surcharges, which can govern near walls, valleys and roof steps and need a full engineering analysis. _On the page: https://siteometry.com/en/tools/roof-snow-load-calculator#what-is-roof-snow-load_ ## Balanced vs drift load A balanced load is snow spread evenly over the whole roof — the case this tool computes. A drift load is snow blown into a deeper pile against a taller wall, a parapet, or a lower roof next to a higher one. Drifts can be several times heavier than the balanced load over a narrow band, which is why they are a frequent cause of localized roof failures. The illustration below shows both: a uniform balanced band on the left, and an illustrative wind-driven drift piling against an adjacent wall on the right. v1 reports the balanced number only — treat drift-prone geometry as a signal to consult an engineer, not something the estimate covers. _On the page: https://siteometry.com/en/tools/roof-snow-load-calculator#balanced-vs-drift_ ## How pitch reduces the load Steeper roofs shed snow, so the slope factor Cs scales the flat-roof load down as pitch increases. Cs stays at 1.0 up to a breakpoint angle, then falls in a straight line to 0 at 70°. The breakpoint depends on the roof: warmer and more slippery surfaces (metal, for example) start shedding at a shallower angle than cold, non-slippery ones. The curves below are drawn straight from the same breakpoint table the calculator uses. Below the breakpoint a change in pitch does not reduce the load at all — a common surprise for anyone assuming a moderate slope always helps. _On the page: https://siteometry.com/en/tools/roof-snow-load-calculator#slope-and-cs_ ## Exposure & thermal factors The exposure factor Ce accounts for wind: a roof fully exposed to wind loses snow and carries less, while a sheltered roof (tucked among trees or taller buildings) holds more. The thermal factor Ct accounts for heat rising through the roof: a heated building melts some snow (Ct ≈ 1.0), while an unheated or cold-ventilated roof holds it (Ct up to 1.2). The comparison below keeps everything else fixed and varies only Ce for a Terrain B roof — the snow band grows visibly from fully exposed to sheltered. Every factor is a default you can override in the calculator. _On the page: https://siteometry.com/en/tools/roof-snow-load-calculator#exposure-and-thermal_ ## Worked example Take a heated house in a snowy region: ground snow load pg = 40 psf, exposure Ce = 1.0, thermal Ct = 1.1, importance Is = 1.0, and a 6:12 (26.6°) pitch. The flat-roof load is pf = 0.7 · 1.0 · 1.1 · 1.0 · 40 = 30.8 psf. At 26.6° the slope factor Cs is still 1.0, so ps = 30.8 psf. The minimum floor pm = 20 · 1.0 = 20 psf is lower, so the sloped-roof load governs: 30.8 psf. The bars below trace each step of that calculation — the same factor waterfall the calculator shows live in its results rail. _On the page: https://siteometry.com/en/tools/roof-snow-load-calculator#worked-example_ ## How to use this calculator Enter your inputs and the balanced snow load updates instantly — nothing is sent anywhere, everything is computed in your browser. _On the page: https://siteometry.com/en/tools/roof-snow-load-calculator#how-to_ ## Frequently asked questions ### What is ground snow load (pg)? Ground snow load is the reference snow weight on the ground at your site, in psf or kPa, set by building codes from historic snowfall data. Every roof snow load in ASCE 7 starts from pg and adjusts it for exposure, heat and slope. ### How do I find my ground snow load? Your local building department is the authoritative source; many jurisdictions publish a value or adopt the ASCE 7 ground-snow map (ASCE Hazard Tool). The regional hints in this calculator are illustrative ranges only — never use them for a real design. ### Does a steeper pitch reduce roof snow load? Yes, above a breakpoint angle. The slope factor Cs stays at 1.0 up to that angle, then decreases linearly to 0 at 70°. Below the breakpoint, pitch does not reduce the load, and the breakpoint is lower for warm, slippery roofs than for cold, non-slippery ones. ### What is the difference between pf and ps? pf is the balanced flat-roof snow load (0.7 · Ce · Ct · Is · pg). ps is the sloped-roof load, pf multiplied by the slope factor Cs. The design load your roof must carry is the larger of ps and the minimum floor pm. ### Is this a code-compliance or structural check? No. This is an estimate using ASCE 7 formulas with editable factors — not a stamped structural analysis or a code-compliance determination. Before adding solar, re-roofing or building a structure, confirm your design snow load with your building department and have a licensed structural engineer verify your roof's actual capacity. ### Does it calculate snow drift or unbalanced loads? No. v1 computes the balanced snow load only. Drift, unbalanced and sliding-snow surcharges near walls, valleys and roof steps often govern the design and require a full engineering analysis — they are illustrated here but not computed. ### Is my data private? Yes. Every calculation runs in your browser. Nothing you enter is sent to a server, and there is no login. _On the page: https://siteometry.com/en/tools/roof-snow-load-calculator#faq_ ## Related roofing tools Sizing the roof around this snow load? These calculators pick up where the load leaves off. - [Roofing Calculator — Pitch, Area & Materials](https://siteometry.com/en/tools/roofing) - [Roof Truss Calculator — How Many Trusses & Cost](https://siteometry.com/en/tools/roof-truss-calculator) - [Rafter Length Calculator](https://siteometry.com/en/tools/rafter-length-calculator) - [Gutter & Downspout Calculator](https://siteometry.com/en/tools/gutter-downspout-calculator)