Snow Load Calculator

• How to Use the Snow Load Calculator
• Worked Example: A 6/12 Shingle Roof at 30 psf Ground Snow Load
• Ground Snow Load vs Roof Snow Load
• The ASCE 7 Formula, Step by Step
• Roof Snow Load at Common Ground Snow Loads
• How Roof Pitch Changes the Load
• How Deep Is the Snow on Your Roof?
• What This Calculator Does Not Cover
• FAQs

Snow weighs far more than it looks. In fact, a foot of settled snow on a 1,500 square foot roof can weigh as much as a couple of cars, and every winter some roofs — usually flat or low-slope roofs on older buildings, carports, and sheds — give way under it. However, engineers handle this with a standard method from ASCE 7, the loading standard behind US building codes, which converts the ground snow load for your area into the roof snow load your roof actually carries. Finally, this snow load calculator runs that method for you: enter your ground snow load, pick your exposure and roof type, and it returns the flat roof load, the slope-adjusted load, the snow depth that load represents, and the total weight of snow sitting on your roof.

How to Use the Snow Load Calculator

Start with the ground snow load, pg, for your location — the design snow load on the ground, in pounds per square foot (psf). If you do not know yours, the free ASCE Hazard Tool gives the mapped value for any US address, and your local building department can tell you the value your jurisdiction has adopted, which is the one that legally governs.

Then choose the exposure factor for your site (sheltered, partially exposed, or fully exposed), the thermal factor for your building (heated, cold ventilated roof, or unheated), and your roof pitch as a rise over 12. Pick whether the roof surface is non-slippery like shingles or slippery like smooth metal, and enter the roof’s horizontal plan area — the footprint it covers, not the sloped surface. If you need the sloped surface area itself rather than the plan area, our surface area calculator will work it out from the roof’s dimensions. The importance factor is fixed at 1.0, the value for an ordinary house. The snow load calculator then applies the ASCE 7 equations explained below.

Worked Example: A 6/12 Shingle Roof at 30 psf Ground Snow Load

For example, take a heated house in a suburban area with a ground snow load of 30 psf, a shingle roof pitched at 6/12, and a roof plan area of 1,500 square feet. Next, enter pg = 30, exposure partially exposed (Ce = 1.0), thermal heated (Ct = 1.0), rise = 6, surface non-slippery, area = 1,500:

So the design load on this roof is 21 pounds on every square foot — the weight of roughly 14 inches of settled snow — and the total snow load the structure carries is about 31,500 pounds, nearly 16 tons. Also, note how much work the 0.7 factor and the factors do: the roof load is 30 percent lower than the ground load before slope is even considered, because wind scours some snow off a roof and heat escaping through it melts some from beneath.

Ground Snow Load vs Roof Snow Load

Overall, the two numbers are often confused, and they are not interchangeable. However, the ground snow load is a climate statistic for your location: roughly, the weight of snow on the ground in a severe winter, mapped by ASCE from decades of weather records. In practice, it varies enormously across the United States — single figures across much of the deep South, 20 to 50 psf across the Midwest and Northeast, and well over 100 psf in mountain areas where local rules set it by elevation.

In contrast, the roof snow load is what your particular roof is expected to hold, and it is almost always lower, because a roof is not the ground: wind blows snow off it, heat melts it, and slope sheds it. As a result, building codes start from the ground value and reduce it through the exposure, thermal, importance, and slope factors. Finally, one trap to avoid: the rafter and truss span tables in the residential code are entered with the ground snow load, not the reduced roof load, so do not use the reduced figure where a table asks for the ground value.

The ASCE 7 Formula, Step by Step

Overall, the method has two equations. Next, the flat roof snow load:

pf = 0.7 × Ce × Ct × Is × pg

In short, the 0.7 is the ground-to-roof conversion. Also, Ce, the exposure factor, reflects how windswept the site is: 0.9 for a fully exposed roof in open terrain, 1.0 for the typical partially exposed suburban site, and 1.2 for a sheltered roof surrounded by trees or taller buildings that keep wind from scouring it. Then, Ct, the thermal factor, reflects how much heat the roof loses: 1.0 for a heated building, 1.1 for a cold, ventilated roof over heated space, and 1.2 for an unheated structure where nothing melts. Finally, Is, the importance factor, is 1.0 for an ordinary house.

Next, for a sloped roof, multiply by the slope factor:

ps = Cs × pf

There is also a floor under the flat roof value for low-slope roofs (15 degrees or less): pf cannot be less than the ground snow load itself when pg is 20 psf or below, or less than 20 psf when pg is above 20. For example, that minimum is why a flat roof in a 10 psf area is still designed for 10 psf, not 7 — on a flat roof in a light-snow climate, a single heavy storm can load the roof almost exactly as it loads the ground. In practice, the calculator applies this minimum automatically.

Roof Snow Load at Common Ground Snow Loads

Overall, with all factors at 1.0 — a heated, partially exposed house — the flat roof load at common ground snow loads is:

In short, the depth column uses the ASCE 7 snow density formula, and it carries a lesson: depth alone is a poor guide to danger. For example, twenty inches of light, fluffy snow can weigh less than eight inches of wet, saturated snow. That is why codes work in pounds per square foot, not inches, and why a modest-looking depth of heavy spring snow deserves more respect than a deeper fall of powder.

How Roof Pitch Changes the Load

Overall, slope is the roof’s own snow-removal system. On a warm roof (Ct = 1.0) with a non-slippery surface like asphalt shingles, ASCE 7 keeps the slope factor at 1.0 — no reduction at all — until the pitch passes 30 degrees (about 7/12), then reduces it in a straight line to zero at 70 degrees. A slippery surface such as smooth metal starts shedding much earlier, at just 5 degrees:

Overall, two cautions belong with this table. First, snow that slides off a steep roof has to land somewhere — sliding loads on a lower roof, a porch, or a walkway below are a separate design case, and snow guards exist precisely because a roof-full of snow leaving at once is dangerous. Second, a cold roof (an unheated building, or a well-ventilated cold roof deck) holds on to its snow longer, so its slope factor starts falling later than the warm-roof lines above; the calculator accounts for this when you select the cold or unheated thermal options with a slippery surface.

How Deep Is the Snow on Your Roof?

In practice, the calculator converts load back into depth using the ASCE 7 density relationship, γ = 0.13 × pg + 14 pounds per cubic foot, capped at 30. Snow gets denser in snowier climates because deeper packs compress. In fact, depth is the number most homeowners actually want — it tells you what the design load looks like in inches you can compare with the drift outside your window.

Use it as a rough gauge, not a safety certificate: density varies through the season, rain falling on snow adds weight without adding depth, and drifts against a chimney, a parapet, or a taller wall next door can be several times deeper than the uniform depth on the open roof. If your roof is carrying noticeably more than the design depth, if doors start sticking, if you hear creaking, or if the ceiling shows new cracks or sagging after a storm, treat those as warning signs: keep people out from under the suspect span and call a professional rather than climbing up to shovel — roof snow removal injures people every year, and an unbalanced shoveling pattern can overload one side of a roof worse than the snow did.

If the same winter has you stacking wood as well as clearing snow, the firewood calculator measures the pile in cords, face cords, and weight.

What This Calculator Does Not Cover

Overall, ASCE 7’s full snow chapter includes several cases a simple balanced-load calculator leaves out, and honesty about them matters. For example, drift loads — snow piled by wind against parapets, rooftop units, and the wall of a taller adjoining building — can locally reach several times the uniform load. Similarly, unbalanced loads on gable roofs, where wind strips one slope and piles the other, are checked separately in real design. In addition, rain-on-snow surcharge, sliding snow from an upper roof, and ponding on flat roofs are additional cases. The result here is the uniform balanced roof snow load: the right number for understanding what your roof carries, comparing depths, and deciding whether to worry. In short, it is a planning estimate, not a structural design — a building, an addition, or a solar panel installation needs the full method and, where the code requires it, a licensed engineer.

FAQs

How do I calculate the snow load on my roof?

Use the ASCE 7 method: multiply your ground snow load by 0.7, then by the exposure factor (0.9 to 1.2), the thermal factor (1.0 to 1.2), and the importance factor (1.0 for a house) to get the flat roof load. For a sloped roof, multiply by the slope factor, which stays at 1.0 on a shingle roof up to 30 degrees and then falls toward zero at 70 degrees. The calculator on this page runs the whole method for you.

Where do I find my ground snow load?

In fact, the free ASCE Hazard Tool returns the mapped ground snow load for any US address, and your local building department publishes the value your jurisdiction has adopted, which is the legally governing number. Mountain jurisdictions often set the value by elevation, and it can be far higher than the general map suggests, so a local source always wins over a national map.

How much snow is too much for a roof?

A roof built to code is designed for the roof snow load this calculator estimates, so problems usually start when actual snow passes that figure — or on older, flat, or lightly built roofs such as carports and sheds that were never designed to a modern code. For example, warning signs include sagging ceilings, new cracks, doors and windows that suddenly stick, creaking sounds, and severe ice dams. In short, depth by itself is not the measure; weight is.

How much does snow weigh per cubic foot?

In fact, ASCE 7 estimates snow density as 0.13 times the ground snow load plus 14, in pounds per cubic foot, up to a maximum of 30. In a 30 psf area that gives 17.9 pounds per cubic foot, so a foot of settled snow weighs about 17.9 pounds per square foot. Fresh powder can weigh as little as 3 to 5 pounds per cubic foot, while wet spring snow and ice weigh far more.

Does a metal roof hold less snow than a shingle roof?

In the design method, yes: a smooth metal roof is treated as a slippery surface, so its slope factor starts dropping at just 5 degrees instead of 30, and the design load falls faster as pitch increases. In practice metal roofs really do shed snow in sudden slides, which is why they are often fitted with snow guards above doors and walkways.

Is roof snow load the same as ground snow load?

No. In fact, the ground snow load is the design weight of snow on the ground for your location. In contrast, the roof snow load is what your specific roof is expected to carry after reductions for wind exposure, heat loss, and slope — usually about 70 percent of the ground value or less. Some code tables, such as the residential rafter span tables, are entered with the ground value, so check which number a table asks for before using it.

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