Snow Load Calculator

What This Snow Load Calculator Does

Snow sitting on a roof is heavier than most people expect, and how much of that weight the roof actually has to carry depends on more than just how much snow falls. This snow load calculator takes the ground snow load for your area, along with your roof's pitch, exposure to wind, how the building is heated, and its occupancy risk category, and works out two numbers that matter for framing and engineering: the flat-roof snow load and the final sloped-roof design snow load, given in pounds per square foot (psf), kilopascals (kPa), and kilograms per square meter (kg/m²).

Change any input and the results, the live roof diagram, and the step-by-step math all update together. Enter a steeper pitch and watch the design load drop as the slope factor kicks in. Switch the roof surface from standard shingles to a slippery, unobstructed metal roof and see how much sooner that reduction starts. Add your roof's rated design capacity in the advanced options and the calculator will tell you, plainly, whether that roof has enough margin for the snow load you just calculated.

Important: This Is a Planning Tool, Not a Stamped Engineering Calculation

It's worth being upfront about what this tool is for. Snow load design is safety-critical structural engineering, and the actual ground snow load for a specific address, along with the exposure and thermal conditions a licensed engineer would use, comes from official building code snow maps, local amendments, and site-specific judgment — not a general online calculator.

What this tool gives you is a realistic, planning-level estimate using the same general method most building codes are built around, so you can budget a project, sanity-check a truss or rafter quote, or simply understand where the numbers on an engineered drawing come from. Before any roof, carport, pergola, or outbuilding is built or modified, always confirm the actual required snow load with your local building department and have the structure engineered and stamped by a licensed professional.

The Snow Load Formula, Explained in Plain Terms

The calculation behind this tool follows the same basic shape used across most modern building codes, based on the ASCE 7 structural standard. It starts with the ground snow load, Pg, which is the weight of snow measured on open, level ground in your area, published on local snow load maps by state, county, or elevation.

That ground value gets scaled down to a flat-roof snow load, pf, using the formula pf = 0.7 × Ce × Ct × Is × Pg. The 0.7 factor already accounts for the fact that wind generally clears some snow off a roof compared to open ground. Ce, Ct, and Is are three multipliers — covered in the sections below — that adjust for how exposed the roof is to wind, how much heat escapes through the roof from below, and how important the building is to keep standing during a severe storm.

From there, the flat-roof value is adjusted one more time by the roof slope factor, Cs, to get the final sloped-roof snow load, ps = Cs × pf. This is the number that actually goes into sizing rafters, trusses, and structural beams, since it reflects both how much snow accumulates and how well the roof's shape and surface let that snow slide off on its own.

Exposure Factor (Ce): How Wind Changes the Load

Wind is one of the biggest variables in how much snow actually stays on a roof. A roof out in open, flat country with nothing to block the wind will have snow scoured off it constantly, while a roof tucked tightly among dense trees or taller buildings barely feels the wind at all, letting snow pile up closer to what fell.

This calculator groups that effect into four exposure options, each with a typical Ce value: fully exposed open terrain (Ce 0.8), fully exposed but with more rolling or wooded surroundings (Ce 0.9), the partially exposed default most buildings fall into (Ce 1.0), and sheltered sites surrounded by dense trees or structures (Ce 1.2). A lower Ce means wind is doing more of the work of clearing snow off the roof; a higher Ce means the roof has to plan on holding more of it.

Thermal Factor (Ct): Heat Loss Melts Snow From Below

A heated building constantly loses a small amount of warmth through the roof, and that warmth melts the bottom layer of snow sitting on top, which is one reason a heated house typically carries less accumulated snow than an unheated shed sitting right next to it under the same storm.

This calculator covers four common cases: a normally heated structure (Ct 1.0), a building kept just above freezing like an unheated but insulated garage (Ct 1.1), a fully unheated structure like a pole barn or detached shed (Ct 1.2), and a continuously heated greenhouse (Ct 0.85), which melts snow fastest of all. Picking the right thermal condition matters — an unheated outbuilding can end up with a meaningfully higher design snow load than a heated home right next to it, even under identical weather.

Risk / Importance Category (Is): Why the Building's Purpose Matters

Not every building carries the same consequence if it were ever overloaded. Building codes account for this with a risk (or importance) category that nudges the design snow load up or down based on how critical the structure is to keep standing.

Category I covers low-hazard structures like minor storage buildings, with a reduced factor of 0.8. Category II is the standard default most homes, offices, and typical buildings use, at 1.0. Category III covers buildings where failure poses a substantial risk to human life, like schools, at 1.1. Category IV covers essential facilities that need to remain functional even after a severe storm, like hospitals and fire stations, at 1.2 — the highest multiplier of the four.

Roof Slope Factor (Cs): Why Steep Roofs Carry Less Snow

A flat roof holds onto essentially all the snow that lands on it, but as a roof gets steeper, gravity starts doing more of the work of sliding that snow off before it ever builds up. The slope factor, Cs, captures this by starting at 1.0 for low-slope roofs and gradually stepping down toward zero as the roof gets steeper, up to a 70° slope where the code assumes essentially no snow stays put at all.

How quickly Cs starts dropping depends on two things this calculator lets you set: how warm the roof stays (through the thermal option above) and how slippery the roofing surface is. A warm, slippery metal roof starts losing load at a fairly shallow slope, since melted water underneath helps snow slide right off. A cold, standard shingle roof holds onto snow up to a much steeper angle before the same effect kicks in, since there's more friction and no melt layer helping things along.

Why Low-Slope Roofs Have a Minimum Snow Load

For roofs at 15° or flatter, building codes set a floor under the flat-roof snow load, no matter how favorable the exposure, heating, and importance multipliers work out. That minimum equals the importance factor times the ground snow load itself (capped at a Pg of 20 psf for this purpose), which keeps a low-slope roof from ever being designed for an unrealistically small snow load just because the math on paper came out low.

This calculator checks for that condition automatically whenever the roof angle comes out to 15° or less, and if the code-minimum value is higher than the calculated flat-roof load, it uses the minimum instead — flagged clearly in both the results panel and the step-by-step solution so it's never a hidden adjustment.

What This Calculator Doesn't Cover: Drift, Sliding, and Unbalanced Loads

Real-world snow load design goes further than the uniform load this calculator estimates. Snow drifting against a taller adjacent roof, a parapet wall, or a rooftop unit can pile up dramatically deeper in one spot than the uniform load alone would suggest. Sloped roofs can also see unbalanced loads, where wind deposits more snow on the leeward side than the windward side. And snow sliding off a steep upper roof onto a lower one can add a sudden, concentrated load exactly where it lands.

These situations require the specific geometry of a building and are exactly the kind of detail a structural engineer accounts for in a stamped design. This calculator's uniform load estimate is the right starting point for budgeting and planning, but any roof with adjacent taller structures, valleys, or drift-prone geometry should always get a full engineered review.

Worked Example: A Heated Home in a Moderate Snow Zone

Picture a heated single-family home with a ground snow load of 30 psf, a 6/12 roof pitch, partially exposed siting, standard asphalt shingles, and a Category II risk classification. The flat-roof snow load works out to 0.7 × 1.0 (Ce) × 1.0 (Ct) × 1.0 (Is) × 30 psf = 21 psf. At a 6/12 pitch, the roof angle comes out to about 26.6°, which sits below the 30° threshold for a standard, warm roof, so the slope factor stays at Cs = 1.0 and the final design snow load stays at 21 psf.

Now compare that to an unheated detached garage on the same lot, same ground snow load and pitch, but with Ct bumped up to 1.2 for the unheated condition. The flat-roof load rises to 0.7 × 1.0 × 1.2 × 1.0 × 30 = 25.2 psf — over 4 psf more than the heated home purely because there's no heat loss helping melt the snow underneath, which is exactly the kind of difference this calculator is built to surface instantly.

Who This Calculator Is For

This tool is useful at several stages of a building or framing project:

• Homeowners and DIY builders sizing up a carport, pergola, shed, or addition before talking to an engineer or truss supplier.
• Contractors putting together a fast planning estimate or sanity-checking a number on an engineered drawing.
• Anyone comparing how exposure, building heat, or roof pitch shifts the design snow load for a project in a snowy climate.
• Students and apprentices learning how ground snow load data turns into an actual roof design number, step by step.