How this calculator works
No black box - this is the arithmetic the tool runs, written out.
roof load (psf) = ground load × 0.7 × slope factor total load (lbs) = roof load × roof area
- ground load
- Your ground snow load in pounds per square foot - a mapped value set by your jurisdiction.
- 0.7
- The basic exposure coefficient. Less snow sits on a roof than on open ground, because wind scours it and the building loses heat through it.
- slope factor
- Steeper roofs shed snow, so they carry less. Flatter roofs hold it.
This follows the shape of the standard ground-to-roof conversion: start from the mapped ground load, reduce it for the fact that it is on a roof, then adjust for slope.
The result is a uniform load - snow spread evenly. Real snow does not do that. It drifts against walls and dormers and piles up in valleys, and those local concentrations are what actually break roofs.
What it looks like with real numbers
Starting from
- Roof area
- 2,000 sq ft
- Ground snow load
- 40 psf
- Pitch
- Medium 4/12-6/12 - factor 0.9
The working
Roof load: 40 × 0.7 × 0.9 = 25.2 psfTotal: 25.2 × 2,000 = 50,400 lbs
≈25.2 psf - about 50,400 lbs total
Reading it: That total is a uniform load, spread evenly. Real snow drifts against walls and dormers and slides into valleys, and those concentrations are what actually break roofs - they can be several times this figure locally.
How the calculation runs
Four operations, in this order. Nothing is hidden behind a button.
- Start from your mapped ground snow load.
- Apply the exposure coefficient, because less snow sits on a roof than on open ground.
- Apply the slope factor - steeper roofs shed.
- Multiply the resulting load per square foot by the roof area.
Getting your inputs right
The answer is only as good as what goes in. This is where estimates usually go wrong.
Get your ground snow load from the jurisdiction
It is a mapped value set by your building department, not something to estimate. They will tell you the design figure for your address.
Use the roof area of the section you care about
For a screening figure, the plan area of the roof section under consideration is what you want.
Identify drift locations
Note where a lower roof meets a wall, where dormers interrupt a slope, and where valleys collect. Those are the places the uniform number does not describe.
What changes your number
A calculator applies an average. These are the things that decide where in the range you actually land.
- Ground snow load The mapped design value for your location - the dominant input, and it varies enormously by region and elevation.
- Roof slope Steep roofs shed. Low-slope roofs hold everything that falls on them.
- Exposure and warmth A sheltered roof holds more snow than a windswept one, and a well-insulated roof stays cold and keeps its snow longer than a leaky one.
- Drift and sliding Not in this calculation at all, and frequently the governing case in a real structural check.
Common mistakes
Every one of these is something people genuinely do, and most of them cost real money.
- Treating this as a structural check It is a screening figure. The code method adds exposure, thermal, and importance factors and drift surcharges, and the result can be several times higher locally.
- Adding load without an engineer If you are putting solar, a heavier covering, or equipment on a roof in snow country, that changes the load path. That is an engineering question.
- Ignoring ice dams Ice damming is a different failure mode from load - it is about meltwater refreezing at a cold eave, and it is a ventilation and insulation problem.
Why snow load matters
Wet snow can weigh over 20 lbs per cubic foot, and a foot on a whole roof adds up fast. The load on your roof is roughly the ground snow load × an exposure factor × a slope factor - steeper roofs shed snow, so they carry less. If you’re near or over your roof’s rated capacity, clear it safely or call a pro.
Cost ranges reflect 2026 U.S. installed prices (BLS producer/labor data, Remodeling Cost vs. Value, and manufacturer pricing). Your real price depends on your roof, region, and pro - see the full cost guide.
Where this stops being accurate
This is a screening figure, not a structural calculation. The real code method applies separate exposure, thermal, and importance factors, and adds drift and sliding-snow surcharges that can multiply the local load several times over. If you are adding solar, a new heavy roof covering, or anything else that changes the load path, this number does not substitute for an engineer.
This is a free estimating tool, not a quote. When you want a number somebody will stand behind, get matched with local pros - they measure the roof before they price it.
The terms on your quote, explained
A roofing quote is written in trade vocabulary. These are the words that actually change what you are paying for.
- Square
- 100 square feet of roof surface. The unit the entire trade orders, prices, and quotes in - a 2,400 sq ft roof is 24 squares.
- Pitch
- The slope, written as rise over a 12-inch run. A roof climbing 6 inches per 12 horizontal inches is "6/12".
- Decking (sheathing)
- The structural panels over the rafters that everything else fastens to. Its condition is unknown until the old covering is off.
- Underlayment
- The water-resistant layer between decking and covering. Synthetic or felt; it is the roof's second line of defense.
- Ice-and-water shield
- A self-adhering membrane at eaves and valleys that seals around fasteners. Commonly required by code in freeze-thaw climates.
- Flashing
- Metal that seals the joins - around chimneys, walls, valleys and vents. Most roof leaks are flashing failures, not covering failures.
Roof Snow Load Calculator questions
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