Roof Snow Load Calculator

Calculate the design roof snow load from the ground snow load, roof pitch, exposure and thermal factors using the ASCE 7 method, plus the total snow weight on the roof.

By CalcBuilt Editorial TeamUpdated September 10, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To find the roof snow load, multiply the ground snow load by 0.7 and by the exposure, thermal and importance factors for the flat roof load, then multiply by the slope factor for your pitch. A 30 psf ground snow load on a partially exposed, heated 6/12 roof with shingles gives a flat roof load of 21 psf and a design roof snow load of 21 psf, about 15.75 tons on a 1,500 sq ft roof.

Your measurements

Gable end: pitch is the rise over a 12 in run; the factor from that pitch turns footprint into roof area.

Ground snow load (psf)Use the value your building department adopted; the ASCE 7 Hazard Tool gives the ASCE 7-22 number by address. Mountain counties set their own by elevation. Over 70 psf the roof must be engineered (IRC R301.2.3).Roof pitchExposureCe 0.9 is only for a roof with no trees or taller buildings on any side; a house in a subdivision with trees is 1.0, and one tucked into conifers is 1.2.Thermal conditionCt 1.1 applies to a vented attic or cold roof with more than R-25 between the heated space and the vent space, which is most houses insulated to current code; a detached garage or barn is 1.2.Roof surfaceSnow guards, solar panels, a chimney in the slide path or a parapet make a metal or slate roof obstructed; pick the first option for those.Roof area (horizontal projection)sq ftFootprint of the roof including overhangs, measured flat, not along the slope.

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Design roof snow load21 psfAll results

Results

Design roof snow load: 21 psf; Slope factor Cs 1 at 6/12 (26.6 degrees), non-slippery surface
Flat roof snow load: 21 psf; 0.7 x Ce 1 x Ct 1 x Is 1 x 30 psf ground snow. IRC rafter tables R802.4.1 are entered with the 30 psf ground value, not this number
Total snow weight on the roof: 15.75 tons; 31,500 lb over 1,500 sq ft of horizontal roof area
Equivalent snow depth: 12.6 in; Settled snow at 20 lb per cubic foot; fresh powder is lighter, wet snow and ice far heavier
Important: Planning estimate; Uniform load only. A drift against a taller wall or parapet can triple the load in a strip along that wall; sliding snow, unbalanced loads and rain-on-snow also add to this. A structural engineer and the local building code govern.

Roof Snow Load formula

Flat roof load pf = 0.7 x Ce x Ct x Is x pg  (Is = 1.0 for houses)
Sloped roof load ps = Cs x pf
Cs = 1.0 up to 30 degrees (5 degrees for slippery roofs), then straight line to 0 at 70 degrees
Total weight = ps x horizontal roof area;  Depth = ps / 20 lb per cu ft

Worked example

A fully exposed, heated house with a 50 psf ground snow load and a standing-seam metal roof at 8/12 (33.7 degrees): pf = 0.7 x 0.9 x 1.0 x 1.0 x 50 = 31.5 psf. Metal is slippery, so Cs = 1 - (33.7 - 5) / 65 = 0.56, and ps = 31.5 x 0.56 = 17.6 psf.

The same house with asphalt shingles: Cs = 1 - (33.7 - 30) / 40 = 0.91, so ps = 31.5 x 0.91 = 28.6 psf. On a 2,000 sq ft horizontal roof area that is 57,200 lb, or 28.6 tons, equal to about 17.2 in of settled snow at 20 lb per cubic foot.

Quick reference

Design roof snow load by pitch for a 30 psf ground snow load (Ce 1.0, Ct 1.0, pf = 21 psf)
Pitch	Angle	Cs shingles	Load shingles	Cs slippery	Load slippery
Flat	0 deg	1.00	21.0 psf	1.00	21.0 psf
3/12	14.0 deg	1.00	21.0 psf	0.86	18.1 psf
4/12	18.4 deg	1.00	21.0 psf	0.79	16.7 psf
6/12	26.6 deg	1.00	21.0 psf	0.67	14.0 psf
8/12	33.7 deg	0.91	19.1 psf	0.56	11.7 psf
10/12	39.8 deg	0.75	15.9 psf	0.46	9.8 psf
12/12	45.0 deg	0.63	13.1 psf	0.38	8.1 psf

How the ASCE 7 snow load works

Every code snow calculation starts with the ground snow load, pg. The ASCE 7 Hazard Tool gives the ASCE 7-22 value by address, but the number the building department adopted governs; mountain counties set it by elevation and it can be double the map. The flat roof load is 0.7 x Ce x Ct x Is x pg. Ce is lower for windswept sites and higher for sheltered ones, and Ct rises for unheated buildings that never melt snow off. Is is 1.0 for houses. One trap: the IRC rafter and truss tables (R802.4.1) are entered with the ground snow load, not the roof load from this page.

Sloped roofs shed snow, so the flat roof load is multiplied by a slope factor Cs. Shingle and tile roofs keep their full load up to 30 degrees (about 7/12) and taper to zero at 70, so most gable roofs get no reduction. Slippery, unobstructed surfaces such as standing-seam metal start shedding at 5 degrees on a heated building; on an unheated one (Ct 1.2) the knees move to 45 and 15 degrees. Snow guards, solar panels, chimneys and parapets make a slippery roof obstructed.

What this estimate leaves out

This is the uniform balanced load only. ASCE 7 also checks unbalanced loads on gable and hip roofs, and drifts against parapets, taller walls and rooftop equipment. It also checks snow sliding from a higher roof onto a lower one, and rain on snow. Drift is what fails porches and additions. A lower roof beside a 40 ft upper roof in a 30 psf area gets a drift about 2 ft high and 9 ft wide. That adds roughly 39 psf at the wall, about 60 psf there against 21 in the field. Roofs under 15 degrees have a minimum of the smaller of pg or 20 psf, and a 5 psf rain-on-snow surcharge applies where pg is 20 or less.

Use the number here to compare roof options. Call a structural engineer when ground snow exceeds 70 psf (IRC R301.2.3) and when an addition or porch roof lands below a taller wall. Call one before a solar array goes on and before a truss is cut or a ceiling vaulted. Call the same day if a loaded roof shows distress: sticking doors, cracks at drywall corners, a sagging ridge or popping sounds. To lighten a roof, rake from a ladder, take both slopes down evenly, and leave 2 in over the shingles.

Code limits that apply

IRC R301.2.3checked
Ground snow over 70 psf is outside the IRC rafter and truss tables; the roof is engineered
If you miss it: Rafters pulled from a span table in an 80 psf county are rejected at plan review; the job waits on a stamped design
ASCE 7-22 7.3
Flat roof load pf = 0.7 x Ce x Ct x Is x pg; Ce 0.9 to 1.2 by exposure, Ct 1.0 to 1.3 thermal
If you miss it: A detached garage figured as a heated house (Ct 1.0) is 20% light; a truss order on that number gets rejected
ASCE 7-22 7.3.4
Roofs under 15 degrees carry a minimum of pg or 20 psf, whichever is less, times Is
If you miss it: A flat porch roof reduced for exposure and heat lands under the minimum; plan review bumps the load and the joists
ASCE 7-22 7.4
Cs = 1.0 to 30 degrees (5 degrees slippery and unobstructed), then a straight line to 0 at 70
If you miss it: Solar panels on a standing-seam roof make it obstructed; the Cs credit taken at design goes away when the array goes on
ASCE 7-22 7.6
Unbalanced case on gable and hip roofs from 1/2:12 to 7:12: one slope bare, the other loaded
If you miss it: Raking one slope bare and leaving the other loaded puts the unbalanced case on a roof sized for the balanced load only
ASCE 7-22 7.7
Drift load on a lower roof against a taller wall; about 2 ft high and 9 ft wide at pg 30
If you miss it: An addition roof under a taller wall carries about triple the field load in a strip at the wall; that is the roof that fails
IRC Table R802.4.1
Rafter span tables are entered with the ground snow load pg (30, 50, 70 psf), not the roof load
If you miss it: Rafters picked from the 30 psf column with a 21 psf roof load in a 50 psf county are a size light

Provision	Limit	If you miss it
IRC R301.2.3checked	Ground snow over 70 psf is outside the IRC rafter and truss tables; the roof is engineered	Rafters pulled from a span table in an 80 psf county are rejected at plan review; the job waits on a stamped design
ASCE 7-22 7.3	Flat roof load pf = 0.7 x Ce x Ct x Is x pg; Ce 0.9 to 1.2 by exposure, Ct 1.0 to 1.3 thermal	A detached garage figured as a heated house (Ct 1.0) is 20% light; a truss order on that number gets rejected
ASCE 7-22 7.3.4	Roofs under 15 degrees carry a minimum of pg or 20 psf, whichever is less, times Is	A flat porch roof reduced for exposure and heat lands under the minimum; plan review bumps the load and the joists
ASCE 7-22 7.4	Cs = 1.0 to 30 degrees (5 degrees slippery and unobstructed), then a straight line to 0 at 70	Solar panels on a standing-seam roof make it obstructed; the Cs credit taken at design goes away when the array goes on
ASCE 7-22 7.6	Unbalanced case on gable and hip roofs from 1/2:12 to 7:12: one slope bare, the other loaded	Raking one slope bare and leaving the other loaded puts the unbalanced case on a roof sized for the balanced load only
ASCE 7-22 7.7	Drift load on a lower roof against a taller wall; about 2 ft high and 9 ft wide at pg 30	An addition roof under a taller wall carries about triple the field load in a strip at the wall; that is the roof that fails
IRC Table R802.4.1	Rafter span tables are entered with the ground snow load pg (30, 50, 70 psf), not the roof load	Rafters picked from the 30 psf column with a 21 psf roof load in a 50 psf county are a size light

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Entering this page's roof load into the IRC rafter tables: Tables R802.4.1 are indexed by ground snow load, so use pg, not ps.
Reading pg off the old IRC map instead of the value the building department adopted; mountain counties set their own by elevation, often double.
Ignoring the drift where an addition meets a taller wall: a 2 ft drift adds about 35 to 40 psf there, triple the field load.
Solar panels on a slippery metal roof make it obstructed; Cs goes from 0.56 to 0.91 at 8/12, and the array adds its own weight.
Shoveling one slope bare while the other stays loaded unbalances the roof; rake from a ladder, evenly, leaving 2 in over the shingles.
Ground snow over 70 psf is outside the IRC prescriptive tables (R301.2.3); the rafters or trusses need an engineer's stamp, not a span table.

Key facts

ASCE 7 reduces the ground snow load by 30% for a flat roof (the 0.7 factor) because wind removes some snow and roofs shed heat.
Settled snow weighs about 20 lb per cubic foot, fresh powder 5 to 7 lb, and solid ice 57 lb, so 1 ft of settled snow is about 20 psf.
Ground snow loads run from 0 psf along the Gulf Coast to 40 to 70 psf across the upper Midwest and New England and over 100 psf in mountain areas.
The IRC prescriptive rafter and truss tables are based on ground snow loads of 30, 50 and 70 psf; higher loads require engineered design.
An unheated building carries 20% more design snow (Ct 1.2) because no heat escapes through the roof to melt it.

Frequently asked questions

How do I calculate snow load on a roof?

Multiply the ground snow load by 0.7 and by the exposure, thermal and importance factors, then by the slope factor for the pitch. A 30 psf ground load on a heated, partially exposed 6/12 shingle roof gives 21 psf.

When do I need an engineer for roof snow load?

When the ground snow load is over 70 psf (IRC R301.2.3), and when a new porch or addition roof sits below a taller wall where snow will drift. Also before solar panels go on and before any truss is cut or a ceiling is vaulted. Call the same day if a loaded roof shows distress: sticking doors, cracked drywall corners, a sagging ridge or popping sounds.

Does a steeper roof reduce snow load?

Yes above 30 degrees for shingle roofs and above 5 degrees for slippery metal roofs on heated buildings. A 12/12 shingle roof carries 63% of the flat roof load; a 12/12 metal roof carries 38%.

What ground snow load should I use?

The value adopted by your local building department, which may differ from the national map for elevation or local records. Typical values are 20 to 30 psf in the mid-Atlantic, 40 to 70 psf in the upper Midwest and New England, and 100 psf or more in mountain towns.

Sources and references

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