How do you calculate roof snow load?

Under ASCE 7-16, the flat-roof snow load is pf = 0.7 × Ce × Ct × Is × pg, with pg the ground snow load, Ce the exposure factor, Ct the thermal factor and Is the importance factor. The sloped-roof load is ps = Cs × pf. For the example roof, pf = 0.7 × 30 = 21 psf and ps is 21 psf because Cs is 1 at 6/12.

Where do I find my ground snow load pg?

Ask your local building department. Some places set pg in the local code, and many mountain areas are case-study sites where the ASCE map shows no value. This calculator has no map on purpose: use the number the department gives you, and check which edition of ASCE 7 it comes from.

What changed in ASCE 7-22?

ASCE 7-22 replaced the single 50-year ground snow load and the snow importance factor Is with reliability-targeted ground snow loads for each risk category, and it changed some coefficients. This calculator follows ASCE 7-16. If your department gives you a 7-22 ground snow load, do not put it in a 7-16 formula without asking how the department wants it used.

What is the minimum snow load on a low-slope roof?

For roofs under 15°, ASCE 7-16 sets a minimum: pm = Is × pg where pg is 20 psf or less, and pm = 20 × Is where pg is above 20 psf. The design load is the larger of ps and pm. A 24 × 20 ft low-slope roof at pg 15 psf has pf of 10.5 psf but a minimum of 15 psf, so 15 psf governs.

Does a metal roof carry less snow?

It can, if the surface is unobstructed and slippery so snow slides off the eave. An 8/12 warm metal roof at pg 50 psf has Cs 0.56 and ps of 19.6 psf, against 35 psf for the flat-roof load. Snow guards, gutters that hold snow and a lower roof or walkway below take that credit away, and the sliding snow itself has to land somewhere safe.

How much does snow weigh per inch?

Typically 0.25 psf per inch for light dry fresh snow up to 1.75 psf for wet heavy snow, from FEMA P-957's 3 to 21 psf per foot. Packed or old snow is heavier, perhaps 1.7 to 2.5 psf per inch (an estimate). The weight of a given depth varies a lot, so use the design load, not the depth, for design checks.

How is snow depth related to the load?

Divide the load by the snow density. With γ = 0.13 pg + 14 pcf (at most 30 pcf), the 21 psf example weighs the same as 14.1 in of snow at 17.9 pcf. Real snow depth on your roof will differ, so treat this as an estimate.

Is this enough to size my rafters or truss?

No. This is a first estimate of the uniform snow load. Rafters, trusses and beams are sized for snow plus dead and other loads, drifting, unbalanced and sliding snow, and the load combinations in the code, which an engineer or the span tables your jurisdiction adopts handle. Roofs with solar panels, valleys, parapets or taller roofs nearby need a drift check.

Roof Snow Load Calculator: What Snow Load Does My Roof Carry?

A 40 × 30 ft roof (1,200 sq ft in plan) with a ground snow load of 30 psf, a heated house in Terrain B with partial exposure (Ce 1.0, Ct 1.0, Is 1.0) and a 6/12 pitch has a 21 psf design roof snow load: pf = 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21 psf, and at 26.6° the slope factor Cs is 1, so ps = 21 psf. Over the roof plan that is about 25,200 lb, and it is the weight of roughly 14 in of snow at 17.9 pcf. This follows ASCE 7-16, and the ground snow load your building department gives you governs.

Figures based on the sources listed belowUpdated 2026-10-18How we calculate

Roof plan length (horizontal)

Roof plan width (horizontal)

Ground snow load pg (psf)

Ask your building department, or read it from the ASCE 7-16 map or your local code: it is the number that governs, and this tool has no map. Many mountain and some Alaskan sites are case-study values set by the local jurisdiction.

Terrain categoryThe ASCE 7-16 surface roughness category of the site (Table 7.3-1). Most houses are Terrain B.

Roof exposureSheltered means taller obstructions within about 10 times their height above the roof give shelter, per the Table 7.3-1 notes. Fully exposed lowers the load because wind blows snow off; sheltered raises it. When unsure, use partially exposed.

Thermal condition CtTable 7.3-2. A house is 1.0 unless it has a cold ventilated roof with more than R-25 between attic and living space. Ct also selects the Cs curve: 1.0 or less is a warm roof; 1.1 and 1.2 or more move the slope breakpoints out.

Risk categorySnow importance factor Is from ASCE 7-16 Table 1.5-2. ASCE 7-22 dropped Is and builds the risk category into the ground snow load instead.

Roof pitch (in 12)

Rise for every 12 in of run: 6 is a 6/12 roof (26.6 degrees). Zero is a flat roof. Under 15 degrees (about 3.2 in 12) the minimum load pm also applies.

Roof surfaceSlippery surfaces earn a lower Cs only where snow can slide off the eave freely: no snow guards, no gutters that trap it, no lower roof below. For a cold roof the slippery curve also assumes enough ventilation and insulation; check Figure 7.4-1. When in doubt, choose Other.

21 psf design roof snow load

Design roof snow load: 21 psf
Exposure factor Ce: 1
Importance factor Is: 1
Flat-roof snow load pf: 21 psf, from 0.7 × Ce × Ct × Is × pg
Roof slope factor Cs: 1 at 26.6°
Sloped-roof snow load ps: 21 psf, Cs × pf
Minimum snow load pm: 20 psf, not used: roof is 15° or steeper
Total snow load on the roof plan: 25,200 lb (≈ 12.6 tons), 21 psf × 1,200 sq ft of plan
Snow depth that weighs the design load: 14.08 in, at 17.9 pcf (0.13 pg + 14)
Fresh snow, per inch (typical): 0.25 psf light and dry to 1.75 psf wet and heavy
Packed or old snow, per inch (typical): about 1.7 to 2.5 psf

Shopping list

Roof rake for snow
Snow guards for metal roofs
Tape measure (25 ft)
Digital angle finder
Snow shovel, roof-safe

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Exposure factor Ce (ASCE 7-16 Table 7.3-1)

By terrain category and roof exposure. Sites above the tree line in windswept mountains and Alaskan sites with no trees within 2 miles use lower values that this calculator does not offer.
Terrain category	Fully exposed	Partially exposed	Sheltered
B: suburbs, towns, wooded areas	0.9	1	1.2
C: open terrain, scattered obstructions	0.9	1	1.1
D: flat, unobstructed, near open water	0.8	0.9	1

Thermal factor Ct and snow importance factor Is

ASCE 7-16 Tables 7.3-2 and 1.5-2. A house is normally Ct 1.0 and Risk Category II.
Item	Condition	Factor
Ct	Heated structures (all except those below)	1.0
Ct	Kept just above freezing, or cold ventilated roof with R over 25 between attic and heated space	1.1
Ct	Unheated and open-air structures	1.2
Ct	Freezer buildings	1.3
Ct	Continuously heated greenhouse with roof R under 2	0.85
Is	Risk Category I (low hazard)	0.8
Is	Risk Category II (most buildings)	1
Is	Risk Category III (large occupancy, schools)	1.1
Is	Risk Category IV (essential facilities)	1.2

Roof slope factor Cs by pitch

Worked by this calculator from the Figure 7.4-1 lines. Cs is 1.0 up to the breakpoint slope and 0 at 70°. Warm is Ct up to 1.0; the other two are Ct = 1.1 and Ct of 1.2 or more. Slippery means an unobstructed surface that sheds snow off the eave.
Pitch	Angle	Warm, other	Warm, slippery	Ct 1.1, other	Ct 1.1, slippery	Ct 1.2+, other	Ct 1.2+, slippery
Flat (0 in 12)	0°	1	1	1	1	1	1
2 in 12	9.5°	1	0.93	1	1	1	1
3 in 12	14°	1	0.86	1	0.93	1	1
4 in 12	18.4°	1	0.79	1	0.86	1	0.94
6 in 12	26.6°	1	0.67	1	0.72	1	0.79
8 in 12	33.7°	0.91	0.56	1	0.61	1	0.66
10 in 12	39.8°	0.75	0.46	0.93	0.5	1	0.55
12 in 12	45°	0.63	0.38	0.77	0.42	1	0.45

Flat-roof load by ground snow load

For a heated house in Terrain B with partial exposure (Ce 1.0, Ct 1.0, Is 1.0). Minimum pm applies to roofs under 15°. Depth is the snow that weighs pf at 0.13 pg + 14 pcf, an estimate.
Ground snow load pg	Flat-roof pf	Minimum pm (under 15°)	Snow density	Depth that weighs pf
10 psf	7 psf	10 psf	15.3 pcf	5.5 in
20 psf	14 psf	20 psf	16.6 pcf	10.1 in
30 psf	21 psf	20 psf	17.9 pcf	14.1 in
40 psf	28 psf	20 psf	19.2 pcf	17.5 in
60 psf	42 psf	20 psf	21.8 pcf	23.1 in
80 psf	56 psf	20 psf	24.4 pcf	27.5 in

Weight of snow per inch of depth (typical)

Typical figures, not measurements. Fresh snow from FEMA P-957 (3 to 21 psf per foot); settled snow is an estimate that rises to the 30 pcf ceiling in the ASCE density formula.
Snow	Density	Weight per inch	Weight per foot
Fresh, light and dry	3 pcf	0.25 psf	3 psf
Fresh, wet and heavy	21 pcf	1.75 psf	21 psf
Settled or packed (estimate)	20 to 30 pcf	1.7 to 2.5 psf	20 to 30 psf
ASCE density at pg 30 psf	17.9 pcf	1.49 psf	17.9 psf

Methodology & assumptions

This calculator follows ASCE 7-16 Chapter 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures), the edition most currently adopted codes still refer to. ASCE 7-22 changed to reliability-targeted ground snow loads with a separate map for each risk category and no snow importance factor Is, and it changed some coefficients, so do not mix 7-22 ground snow loads into these formulas. You enter the ground snow load pg yourself: the building department's value governs, and this page has no map. The flat-roof snow load is pf = 0.7 × Ce × Ct × Is × pg (Eq. 7.3-1). Ce is the exposure factor (Table 7.3-1) by terrain category and exposure, Ct the thermal factor (Table 7.3-2) and Is the snow importance factor (Table 1.5-2 by risk category). The sloped-roof snow load is ps = Cs × pf (Eq. 7.4-1). Cs comes from the piecewise lines of Figure 7.4-1: it is 1.0 up to a breakpoint slope, then falls in a straight line to 0 at 70°, so Cs = 1 − (slope − breakpoint) ÷ (70 − breakpoint). The breakpoints are 5° (slippery) and 30° (other surfaces) for a warm roof with Ct up to 1.0, 10° and 37.5° for Ct = 1.1, and 15° and 45° for Ct of 1.2 or more. The slope is atan(rise ÷ 12) from the pitch you enter. A slippery surface counts only where the roof is unobstructed and snow can slide off the eave; snow guards, gutters that hold snow or a lower roof below make it an ordinary surface. For roofs under 15° the minimum snow load also applies: pm = Is × pg where pg is 20 psf or less, and pm = 20 × Is where pg is above 20 psf, and the design load is the larger of ps and pm. The total load is the design load times the plan (horizontal) area, because snow load in ASCE 7 is per square foot of horizontal projection, so enter the building's footprint with overhangs and not the sloped surface. The depth equivalent divides the design load by a snow density of γ = 0.13 pg + 14 pcf, capped at 30 pcf: that formula is the ASCE 7 relationship between ground snow load and snow density (Section 7.7.1 for drifts) and is used here as a rule of thumb for the depth that would weigh the design load, not a measurement of the snow on your roof. The per-inch weights are typical: FEMA P-957 gives a foot of fresh snow as 3 psf for light dry snow to 21 psf for wet heavy snow, which is 0.25 to 1.75 psf per inch, and settled or old snow is typically in the 20 to 30 pcf range (about 1.7 to 2.5 psf per inch), an estimate. Not covered: drifts, sliding snow, unbalanced loads on gable and curved roofs, rain-on-snow surcharge (7.10), ponding, roof valleys, solar panels and other obstructions. Those can control, so this is a first estimate and not a structural design. The metric toggle changes lengths and the total weight; snow loads stay in psf because that is how the code states them.

How to calculate roof snow load

Get the ground snow load. Ask your building department for pg in psf and enter it. Do not guess it from a map.
Choose the site and building factors. Pick the terrain category and roof exposure for Ce, the thermal condition for Ct and the risk category for Is.
Find the flat-roof load. Multiply 0.7 × Ce × Ct × Is × pg to get pf in psf.
Apply the roof slope. Enter the pitch and surface. Cs is 1.0 up to a breakpoint slope and drops to 0 at 70°. Multiply it by pf for ps.
Check the minimum. Under 15°, the design load is the larger of ps and the minimum pm. Multiply the design load by the plan area for the total.

Worked examples

Project	Result
40 × 30 ft roof, pg 30 psf, Terrain B partially exposed, 6/12 shingles	21 psf design roof snow load: pf 21 psf, Cs 1, about 25,200 lb over the plan
36 × 28 ft slippery metal roof, 8/12, pg 50 psf	19.6 psf design roof snow load: pf 35 psf, Cs 0.56, about 19,708 lb over the plan
24 × 20 ft low-slope roof (1/12), pg 15 psf: the minimum governs	15 psf design roof snow load: pf 10.5 psf is below the minimum pm of 15 psf
50 × 40 ft Risk Category IV building, 4/12, pg 60 psf	50.4 psf design roof snow load: pf 50.4 psf with Is 1.2 and Cs 1
30 × 20 ft unheated flat-roof shed, sheltered, Terrain B, pg 40 psf	40.3 psf design roof snow load: Ce 1.2, Ct 1.2, so pf 40.3 psf

Frequently asked questions

How do you calculate roof snow load?: Under ASCE 7-16, the flat-roof snow load is pf = 0.7 × Ce × Ct × Is × pg, with pg the ground snow load, Ce the exposure factor, Ct the thermal factor and Is the importance factor. The sloped-roof load is ps = Cs × pf. For the example roof, pf = 0.7 × 30 = 21 psf and ps is 21 psf because Cs is 1 at 6/12.
Where do I find my ground snow load pg?: Ask your local building department. Some places set pg in the local code, and many mountain areas are case-study sites where the ASCE map shows no value. This calculator has no map on purpose: use the number the department gives you, and check which edition of ASCE 7 it comes from.
What changed in ASCE 7-22?: ASCE 7-22 replaced the single 50-year ground snow load and the snow importance factor Is with reliability-targeted ground snow loads for each risk category, and it changed some coefficients. This calculator follows ASCE 7-16. If your department gives you a 7-22 ground snow load, do not put it in a 7-16 formula without asking how the department wants it used.
What is the minimum snow load on a low-slope roof?: For roofs under 15°, ASCE 7-16 sets a minimum: pm = Is × pg where pg is 20 psf or less, and pm = 20 × Is where pg is above 20 psf. The design load is the larger of ps and pm. A 24 × 20 ft low-slope roof at pg 15 psf has pf of 10.5 psf but a minimum of 15 psf, so 15 psf governs.
Does a metal roof carry less snow?: It can, if the surface is unobstructed and slippery so snow slides off the eave. An 8/12 warm metal roof at pg 50 psf has Cs 0.56 and ps of 19.6 psf, against 35 psf for the flat-roof load. Snow guards, gutters that hold snow and a lower roof or walkway below take that credit away, and the sliding snow itself has to land somewhere safe.
How much does snow weigh per inch?: Typically 0.25 psf per inch for light dry fresh snow up to 1.75 psf for wet heavy snow, from FEMA P-957's 3 to 21 psf per foot. Packed or old snow is heavier, perhaps 1.7 to 2.5 psf per inch (an estimate). The weight of a given depth varies a lot, so use the design load, not the depth, for design checks.
How is snow depth related to the load?: Divide the load by the snow density. With γ = 0.13 pg + 14 pcf (at most 30 pcf), the 21 psf example weighs the same as 14.1 in of snow at 17.9 pcf. Real snow depth on your roof will differ, so treat this as an estimate.
Is this enough to size my rafters or truss?: No. This is a first estimate of the uniform snow load. Rafters, trusses and beams are sized for snow plus dead and other loads, drifting, unbalanced and sliding snow, and the load combinations in the code, which an engineer or the span tables your jurisdiction adopts handle. Roofs with solar panels, valleys, parapets or taller roofs nearby need a drift check.

Handy tools

Slope Calculator: Turn a rise and run, an angle or a percent grade into the x/12 pitch the field above takes.
Feet and Inches Calculator: Add up building sides measured in feet, inches and fractions, and get decimal feet for the plan length and width.