Calculating a roof snow load takes one formula and four inputs. The formula is pf = 0.7 times Ce times Ct times Is times pg, from ASCE 7, the engineering standard behind the building code. Plug in the ground snow load for your area and the three factors for your roof, and the answer is the flat-roof snow load in pounds per square foot. This guide works the formula through six real roofs in Waukegan and Lake County, so you can check the roof snow load calculator against hand math.
Step 1: Find the Ground Snow Load (pg)
The ground snow load, pg, comes from the ASCE 7 maps, and for Chicago and Lake County it is 25 pounds per square foot. That is the figure in the standard ASCE 7 worked example for a building in Chicago, Illinois, and it matches the maps for the whole Chicago area. If it is within a mile or two of the lake, use 30 psf to be safe. Local code governs for your exact address. The zip-code guide on roof snow load by zip code explains how the number changes across northern Illinois.
Step 2: Pick the Three Factors (Ce, Ct, Is)
Ce is the exposure factor, and it describes how much wind reaches the roof. A partially exposed roof, the usual case for a suburban home with some trees and neighbors, is 1.0. A sheltered roof, surrounded by tall trees or taller buildings that block the wind, is 1.2, because the snow stays where it falls instead of blowing off. A fully exposed roof on open ground is 0.9. Ct is the thermal factor: 1.0 for a heated home and 1.1 for an unheated garage. Is is 1.0 for an ordinary home.
Step 3: Work the Flat-Roof Formula
Multiply it out: pf = 0.7 times Ce times Ct times Is times pg. For a typical heated home in Waukegan, that is 0.7 times 1.0 times 1.0 times 1.0 times 25, which is 17.5 psf. Then apply the minimum: ASCE 7 sets a minimum flat-roof snow load of 20 psf for an ordinary home, and the minimum governs, so the design load is 20 psf, not 17.5. A sheltered roof with Ce = 1.2 gives 0.7 times 1.2 times 25, which is 21.0 psf, above the minimum, so the design load is 21 psf. An unheated garage gives 19.3 psf, below the minimum, so the design load is again 20 psf.
Step 4: Adjust for the Pitch
A sloped roof carries less than a flat roof, and the slope factor Cs does the adjustment: ps = Cs times pf. Cs is 1.0 for slopes up to 30 degrees, which covers a 6/12 pitch at about 27 degrees, so a 6/12 gable carries the full 17.5 psf. Above 30 degrees the factor falls by 1/40 per degree: an 8/12 pitch at about 34 degrees has Cs = 0.9, so it carries 0.9 times 17.5, which is 15.9 psf. The pitch adjustment is the reason a steep roof and a flat roof of the same size can have different design loads by several psf.
| Example | Roof | Ground load | Design load | Total weight (1,500 sq ft unless noted) |
|---|---|---|---|---|
| 1 | Flat roof, Chicago | 25 psf | 20.0 psf (minimum governs) | 30,000 lb (15.0 tons) |
| 2 | 6/12 gable, Waukegan | 25 psf | 17.5 psf | 26,250 lb (13.1 tons) |
| 3 | 8/12 steep roof | 25 psf | 15.9 psf | 23,828 lb (11.9 tons) |
| 4 | Sheltered flat roof, 2,000 sq ft | 25 psf | 21.0 psf | 42,000 lb (21.0 tons) |
| 5 | Unheated garage, flat, 600 sq ft | 25 psf | 20.0 psf (minimum governs) | 12,000 lb (6.0 tons) |
| 6 | Lake-effect flat roof, 1,200 sq ft | 30 psf | 21.0 psf | 25,200 lb (12.6 tons) |
The pattern is clear. Example 1 is the baseline: a flat roof in Chicago at the 20 psf minimum, 30,000 pounds on 1,500 square feet. Example 2 is the common Waukegan gable at 17.5 psf, 26,250 pounds. Example 3 is the steep roof at 15.9 psf, 23,828 pounds. Example 4 is the sheltered flat roof: the trees block the wind, the factor rises to 1.2, and a 2,000 square foot roof carries 42,000 pounds. Example 5 is the unheated garage at the 20 psf minimum, 12,000 pounds on 600 square feet. Example 6 is the lake-effect roof at 30 psf ground load, 25,200 pounds on 1,200 square feet. Every figure here comes from the same calculation the roof snow load calculator runs, cross-checked in Node and Python with identical results.
Step 5: Turn Psf Into Snow Depth
The design load is in psf, but the snow on the roof is in inches, so the last step is the conversion. Fresh, light snow weighs about 5 psf per foot of depth, wet packed snow weighs about 20 psf per foot, and ice weighs about 57 psf per foot. At the 20 psf design load, that means 4.0 feet of fresh snow, 1.0 foot of wet snow, or 4.2 inches of ice. After a storm, measure the depth, judge whether the snow is fresh or wet, and divide. If the snow is wet and the depth is anywhere near a foot, the roof is near its design load, and it is time to clear it or call someone who can. The guide on how much snow can a roof hold explains the depth limits in more detail.
What the Formula Does Not Cover
The balanced load is the starting point, not the whole story. Wind also piles snow into drifts against parapets, chimneys, and taller walls, and the drifted load there can be double the balanced load. Old ice from a previous thaw starts the next storm with part of the margin already used. If the roof shows any warning sign, a sagging ceiling, new cracks, sticking doors, or leaks during a thaw, do not rely on the formula alone: get the roof inspected, and keep people out of the rooms underneath until it is checked.
Related reading: roof snow load calculator, plus our guides on how much snow can a roof hold and roof snow load by zip code, and the roofing calculators hub.
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Frequently Asked Questions
What is the formula for roof snow load?
The ASCE 7 formula for a flat roof is pf = 0.7 times Ce times Ct times Is times pg. Pf is the flat-roof snow load in pounds per square foot, Ce is the exposure factor, Ct is the thermal factor, Is is the importance factor, and pg is the ground snow load from the ASCE 7 maps. For a sloped roof, ps = Cs times pf, where Cs is the slope factor: 1.0 for slopes up to 30 degrees, falling to 0 at 70 degrees. The minimum flat-roof snow load is 20 psf for an ordinary home.
What is the ground snow load for Chicago?
The ASCE 7 ground snow load for Chicago is 25 pounds per square foot. That is the figure in the standard ASCE 7 worked example for a building in Chicago, Illinois, and it matches the ASCE 7 maps for the area. The Great Lakes region runs 25 to 40 psf, with lake-effect areas higher. Waukegan and Lake County sit at 25 psf, with lake-shore areas up to about 30 psf. Local code governs, so the building department is the final word for any address.
What is Ce, Ct, and Is in the snow load formula?
Ce is the exposure factor: 1.0 for a partially exposed roof, 1.2 for a sheltered roof surrounded by trees or buildings, and 0.9 for a fully exposed roof on open ground. Ct is the thermal factor: 1.0 for a heated building, 1.1 for an unheated building like a garage, and 1.2 for a building kept just above freezing. Is is the importance factor: 1.0 for an ordinary home, 1.1 for a building where many people gather, and 1.2 for an essential facility like a hospital. Most homes in Waukegan use 1.0 for all three.
How do I calculate the total weight of snow on my roof?
Work out the design load in psf first, then multiply by the roof area in square feet. A 1,500 square foot flat roof in Chicago at the 20 psf minimum carries 30,000 pounds, or 15 tons. A 6/12 gable of the same size at 17.5 psf carries 26,250 pounds, or 13.1 tons. An 8/12 steep roof at 15.9 psf carries 23,828 pounds, or 11.9 tons. Divide the pounds by 2,000 to get tons. The roof snow load calculator does this arithmetic for you, but the formula above is the whole method.
Does roof pitch change the snow load?
Yes. The slope factor Cs is 1.0 for roofs up to 30 degrees, which covers a 6/12 pitch at about 27 degrees, so a 6/12 gable carries the full flat-roof load. Above 30 degrees the factor falls: an 8/12 pitch at about 34 degrees has Cs = 0.9, so it carries about 15.9 psf against 20 psf on a flat roof. A very steep roof above 70 degrees carries almost nothing, because the snow slides off. Flat and low-slope roofs get no reduction at all, which is why they carry the highest design loads.
Zayn Robert is a content specialist focused on roofing, construction, and home improvement topics. He creates easy-to-understand guides that help homeowners learn about roof measurements, materials, maintenance, and repair decisions. His work combines detailed research with practical insights from roofing professionals at Zuniga Roofing Inc.