Snow Load Calculator

Snow is a load case, and codes price it in two steps: the ground snow load for your location, converted to the flat-roof load by exposure, thermal and importance factors, then adjusted for slope. Enter those figures and this calculator returns the design snow load in pounds per square foot — and the total weight, in pounds and tons, that it parks on your roof.

The ASCE Pipeline, Simplified Honestly

The standard method (ASCE 7, the load code behind US building codes) converts weather into structure in three moves. First, the ground snow load pg — a mapped, statistical quantity: the load with a 2% annual chance of being exceeded at your site, read from the code’s maps or a site study. Second, the flat-roof load pf = 0.7 × Ce × Ct × I × pg: roofs hold less than the ground (wind scours them, heat melts them), discounted by exposure (Ce), the building’s thermal regime (Ct — a heated roof sheds its blanket; a cold warehouse keeps it), and importance (I, the consequence dial). Third, for sloped roofs, a slope factor Cs discounts further as pitch lets snow slide. This calculator performs exactly those three moves with the warm-roof slope line, and discloses the simplification on its face — drifting, unbalanced and rain-on-snow cases are separate chapters of the standard, deliberately out of scope here.

Worked Example: Thirty Pounds of Ground Snow

The defaults run a typical heated house in a moderate snow belt: pg 30 psf, all factors 1.0, a 30-degree roof over 1,200 square feet. Flat-roof load: 0.7 × 30 = 21.0 psf. At exactly 30 degrees the slope factor is still 1.0, so the design load stands at 21.0 psf — and the total comes to 21 × 1,200 = 25,200 lb, twelve and a half tons of winter parked overhead. Steepen the same roof to 50 degrees in the calculator and Cs falls to 0.5, halving the load to 10.5 psf: the arithmetic reason alpine roofs are steep, stated as a factor. Shallow roofs earn no such mercy, which is why flat commercial roofs in snow country are engineered like bridges and inspected like them after every big storm.

Weight, Not Depth: The Misreading to Avoid

Every winter, collapses are preceded by someone measuring snow depth against a load rating, and the two do not convert by eye. Snow’s density spans an order of magnitude: fresh powder may run 3–6 lb per cubic foot, settled snow 12–20, wet spring snow and refrozen layers far beyond. The code’s psf is a weight — a foot of wet snow can outweigh three feet of powder — and the ground load maps were built from water-equivalent measurements for exactly this reason. The calculator’s totals speak in tons for the same purpose: a roof’s question is never “how deep” but “how heavy”, and any shovelling-or-waiting decision made on depth alone is a guess dressed as a measurement. When in doubt, the sample is cut, weighed, and converted — or the structural engineer is called, which is the genuinely professional answer at loads anywhere near design values.

Rain on Snow: The Surcharge Nobody Sees

The standard hides one more case that has collapsed more roofs than depth ever has: a spring rainstorm falling on an existing snowpack. The snow acts as a sponge, holding the rain’s weight in place — the code adds a flat 5 psf rain-on-snow surcharge in regions where the ground load is modest but the pattern occurs, precisely because the combined weight arrives invisibly, with no new snow to measure. It belongs to the same family of lessons as the density section: the load is a weight, the weight can change without the depth changing, and the figures this page computes are the design grammar in which those hidden cases are also written. A roof nearing its design load in a thaw is a roof to clear, whatever the ruler says.

The Lower Roof’s Problem: Slides and Drifts From Above

Multi-level roofs add the case homeowners discover first: the lower roof receives not only its own sky’s snow but the upper roof’s — sliding down in slabs where pitches and slippery surfaces allow, and drifting deep in the calm pocket against the upper wall. The standard prices this as the drift surcharge of the FAQ plus, for sliding, a portion of the upper slope’s load relocated onto the lower. Porches, garage roofs under a main roof, and additions tucked against a taller house are the canonical victims, and their failures share the signature of being local — one bay, at the wall — while the uniform load everywhere else looks innocent. The uniform figure this calculator returns is the floor of the lower roof’s problems, not their ceiling.

After the Storm: Reading a Loaded Roof

The homeowner’s version of load assessment runs on warning signs rather than instruments: doors that suddenly stick in their frames, ceiling cracks that open along new lines, unfamiliar creaks and pings from the structure under a settled pack, and a visible sag along a ridge or eave that memory says was straight. Any of these under a load approaching this page’s figures converts arithmetic into instruction — clear the roof (from the ground, with a roof rake, or by hiring the crews who do it harnessed) or leave the building and call the engineer if the signs are advanced. The design load is a planning number in October; in February it is a threshold with a building’s behaviour as its gauge, and the wise read both. When the signs and the arithmetic disagree, believe the building and act first.

Related Tools

More from the Construction Calculators hub: the Gambrel Roof Calculator — whose shallow upper slope is this page’s problem child — and the Heat Loss Calculator, since the thermal factor above is your roof’s insulation confessing.

Frequently Asked Questions

Where do I find my ground snow load?

From the snow load map in your adopted building code (ASCE 7’s figure, or your state or municipal amendment of it), increasingly via online hazard tools that return pg for an address. Values run from zero across the deep south and coastal California to 100 psf and beyond in the lake-effect belts and high mountains, where site-specific case studies replace the map outright for real design work. The calculator takes pg as an input because there is no honest default — a number pulled from the wrong state’s map is worse than no number.

Why is the roof load only 70% of the ground load?

The 0.7 is the standard’s empirical core conversion: roofs, on average and before the other factors, accumulate about seventy percent of what the ground beside them holds, through wind scour, heat loss from below and the geometry of deposition. The factors then adjust the average to your roof — a sheltered, cold, important building can climb back past the ground figure (Ce 1.2 × Ct 1.2 × I 1.1 overwhelms the 0.7) — which is precisely why the conversion exists as factors rather than a single discount.

What about snow drifting against a wall or parapet?

Drift loads are a separate, often governing, case: wind piles snow against vertical obstructions in a triangular surcharge that can locally double or triple the uniform load, computed from the drift height formulas (a function of the upwind fetch and ground load) in the standard’s drift chapter. This page deliberately prices only the uniform balanced load and says so; a roof with parapets, rooftop units, valleys or a taller neighbour has drift cases that belong on the engineer’s sheet, and in real collapses they are the usual defendant.

What is the unbalanced snow load?

The code’s recognition that wind strips one slope and loads the other: gable roofs above gentle pitches are checked with the windward side partially unloaded and the leeward side carrying an increased share. It governs many real rafter designs despite sounding like a refinement. The simplified calculator returns the balanced figure only; the unbalanced case is one more reason the page’s outputs feed a conversation with the code rather than replace it.

My roof is metal and slippery — does the slope factor improve?

The full standard does give unobstructed slippery surfaces a steeper Cs discount (the slope line starts falling from a lower angle), balanced by the requirement to consider where the sliding snow lands — a consideration that has injured pedestrians and buried exits, which is why snow guards exist. This page implements the standard warm-roof line for non-slippery assumptions and keeps the conservative side; use the code’s slippery-surface line, with its snow-retention detailing, in real design work.

Can I compare this psf against my roof’s rating?

Only if you know the rating in the same dialect: the design load this page computes is the code’s demand side; a structure’s capacity was set (if it was engineered) against the code edition of its day, often with different maps and factors. Older roofs in regions whose mapped loads have risen are a recognised concern after extreme winters. The comparison a homeowner can honestly make is shallower: if measured snow weight approaches the figures here, it is time for the engineer and the snow rake, in that order of preference.