How Much Does the Snow on My Roof Weigh?
A foot of wet, heavy snow weighs about 21 lb per square foot. On a roof covering 1,500 square feet in plan, that is 31,500 lb, almost 16 tons. The same foot of light, dry snow weighs 3 lb per square foot, a seventh as much.
Weight of snow per square foot of roof, by depth and type:
| Depth | Light | Settled | Wet |
|---|---|---|---|
| 3 in | 0.8 | 1.6 | 5.3 |
| 6 in | 1.5 | 3.1 | 10.5 |
| 12 in | 3 | 6.2 | 21 |
| 18 in | 4.5 | 9.4 | 31.5 |
| 24 in | 6 | 12.5 | 42 |
| 36 in | 9 | 18.7 | 63 |
| Depth | Light, dry, psf | Settled 10:1, psf | Wet, heavy, psf |
|---|---|---|---|
| 3 in | 0.8 | 1.6 | 5.3 |
| 6 in | 1.5 | 3.1 | 10.5 |
| 12 in | 3 | 6.2 | 21 |
| 18 in | 4.5 | 9.4 | 31.5 |
| 24 in | 6 | 12.5 | 42 |
| 36 in | 9 | 18.7 | 63 |
Use the area of the roof in plan, the footprint plus the overhangs, not the area along the slope. Snow loads in the IRC and ASCE 7 act on the horizontal projection of the roof, and a depth measured straight down already accounts for the slope.
Weight of one foot of depth, in pounds per square foot, by snow type (FEMA P-957; settled snow by the 10:1 rule):
Ice is nearly three times as heavy as wet snow, which is why ice dams and refrozen melt water matter so much at the eaves.
Where Snow Piles Up
The weight above assumes an even layer. Real roofs collect more in some places than others:
- 1Balanced snow: the even layer the design load assumes
- 2Drifting and sliding snow on a lower roof, against the wall
- 3Melt water refreezing into ice at a cold eave
FEMA lists the spots to watch: lower roofs in the wind shadow of a taller part of the house, snow blown against dormers, parapets and rooftop equipment, valleys, and porch roofs under a gable that sheds onto them. Measure the depth at those spots as well as the open middle of the roof.
How to Calculate the Snow Load on a Roof
Multiply the depth in feet by the weight of a foot of that kind of snow, then by the plan area of the roof:
If you can measure the water in the snow, use it directly. One inch of water weighs 5.2 lb per square foot (62.4 lb per cubic foot ÷ 12). The National Weather Service's old rule of 10 inches of snow to 1 inch of water gives the settled-snow figure, but it holds only about a quarter of the time; around 12:1 fits the Upper Midwest better, and very dry snow can run 30:1.
How Much Snow Can a Roof Hold?
A roof holds the load it was designed for, plus a safety margin you shouldn't count on. Under the IRC, a house roof is designed for the greater of the roof live load, 12 to 20 lb per square foot depending on the slope and the area a member carries, and the ground snow load for the site (R301.6). Where the ground snow load is over 70 lb per square foot, the code's prescriptive framing rules stop and the house must be engineered (R301.2.3).
How deep a design load is, by snow type:
| Design load | Light, dry | Settled 10:1 | Wet, heavy |
|---|---|---|---|
| 20 psf | 80 in | 38 in | 11.4 in |
| 30 psf | 120 in | 58 in | 17.1 in |
| 40 psf | 160 in | 77 in | 22.9 in |
| 50 psf | 200 in | 96 in | 28.6 in |
| 70 psf | 280 in | 135 in | 40 in |
A roof built for 30 lb per square foot carries about 17 inches of wet snow, but about 4.8 ft of settled snow. The design load of an existing house is on its plans; if there are none, FEMA suggests having a design professional work it out before the snow season.
From Ground Snow Load to Roof Snow Load
Codes start from the ground snow load, pg, the weight of snow on the ground at the site; the 2021 code maps show a load with a 2% chance of being exceeded in any year. ASCE 7, the load standard the IBC uses, turns it into a flat-roof snow load:
| Factor | Values in ASCE 7-16 |
|---|---|
| Ce, exposure | Fully exposed 0.8–0.9, partially exposed 0.9–1.0, sheltered 1.0–1.2, by terrain (Table 7.3-1) |
| Ct, thermal | 1.0 heated, 1.1 cold ventilated roof, 1.2 unheated or open (Table 7.3-2) |
| Is, importance | 0.80, 1.00, 1.10, 1.20 for Risk Category I to IV; houses are II |
- ASCE 7-22 drops Is and maps the ground snow load separately for each Risk Category instead. The 2024 IBC lists Alaskan locations that way, with Category IV loads 25% above Category II and Category I loads 20% below.
- Minimum load: on roofs flatter than 15°, ASCE 7-16 also checks a minimum of Is × pg, or Is × 20 psf where pg is over 20, as a separate load case.
- Sloped roofs take ps = Cs × pf, with the slope factor Cs from ASCE 7; steep, slippery, warm roofs shed snow and carry less.
- Drifts, sliding snow, unbalanced loads and rain on snow are added on top, case by case.
- Trusses under the IRC are loaded with a roof snow load of 0.7 × pg (R802.10.2.1).
That chain is an engineer's job for anything beyond a simple house or shed. The IRC rafter span tables already have it built in: they are indexed by ground snow load, 30, 50 or 70 psf, and the rafter span check uses the 30 psf tables when you pick a snow load. Trusses come with their design loads from the maker; count and space the trusses on their own. For a low-slope roof, see the flat roof calculator.
Where to Find the Ground Snow Load
- 2021 IRC and IBC: ground snow load maps in Figure R301.2(3) and Figures 1608.2(1) and (2). Areas marked CS need a site-specific study, and sites above the elevations on the map need an approved value.
- 2024 IRC: an allowable-stress map, pg(asd), drawn from the ASCE 7 Hazard Tool's geodatabase of ground snow loads, with gray areas over 140 psf to be looked up there.
- 2024 IBC and ASCE 7-22: the reliability-targeted, strength-level values from the same tool, by Risk Category.
- Alaska and Hawaii: the IBC lists Alaskan locations in Table 1608.2; snow loads in Hawaii are zero except in the mountains.
The building department fills in the design ground snow load for the jurisdiction in IRC Table R301.2. Ask them first: that is the number your permit drawings must use.
When to Clear the Roof
FEMA's warning signs that a roof is overloaded: sagging ceilings or roof members, popping and creaking, bowing truss chords, cracked or split framing, and doors or windows that suddenly stick. If you see them, get everyone out and call a structural engineer.
When snow does need to come off, FEMA recommends a roofing contractor who removes snow, working from the drifts first and leaving about 2 inches on the roof to protect the covering. If you rake from the ground, use a non-metal rake and stand well back from the eave, since the snow can slide at any moment. Never use picks or other sharp tools on the roof.
Frequently Asked Questions
How much does a foot of snow weigh?
From about 3 lb per square foot for light, dry snow to 21 for wet, heavy snow, FEMA says. Settled snow at the 10:1 snow-to-water ratio weighs about 6.2 lb per square foot per foot of depth, and a foot of ice about 57.
How much does a cubic foot of snow weigh?
The same numbers in other units: 3 lb for light, dry snow, about 6.2 lb for settled snow and up to 21 lb for wet, heavy snow. A cubic foot of ice weighs about 57 lb.
How much does an inch of snow weigh?
About 0.25 lb per square foot for light snow, 0.52 for settled 10:1 snow and 1.75 for wet snow. An inch of ice weighs a little under 5 lb per square foot.
How much snow can my roof hold?
As much as it was designed for, which your building department or the house plans can tell you. A 23.1 psf roof load, for example, equals 13.2 inches of wet snow or about 44 inches of settled snow. Drifts make some spots much deeper than the average.
What does a 30 psf snow load mean?
That the roof or the ground carries 30 lb of snow on each square foot of its horizontal area. On the 2021 code maps, a 30 psf ground snow load has a 2% chance of being exceeded in any year. The roof design load is usually less: 23.1 psf in the calculator's default.
How do I convert ground snow load to roof snow load?
With ASCE 7: the flat-roof snow load is 0.7 × Ce × Ct × pg in ASCE 7-22, and the same times the importance factor Is in ASCE 7-16. With pg = 30 psf, Ce = 1.0 and Ct = 1.1, that is 23.1 psf. Sloped roofs, drifts and unbalanced loads are separate steps.
How do I find the snow load for my zip code?
Look up the ground snow load in the ASCE 7 Hazard Tool, which the 2024 IRC points to, or read it from the ground snow load map in your code. Your building department sets the value that applies, and some mountain areas need a site study.
What roof load are houses built for?
Under the IRC, at least the roof live load of 12 to 20 psf by slope and loaded area, or the ground snow load, whichever is greater. Where the ground snow load is over 70 psf, the house must be engineered.
Should I shovel snow off my roof?
Only when the load nears what the roof was designed for, FEMA says, and preferably by a roofing contractor who removes snow. Leave about 2 inches on the roof, use plastic shovels or a snow rake, and clear drifts first.
Why is snow lighter in the West?
Snow in the western U.S. is typically lighter and less dense than on the East Coast, which tends to be wetter, FEMA notes, with exceptions such as the Pacific Northwest. The same depth can weigh several times more in one place than another.
Sources
- American Society of Civil Engineers. (2017). Minimum design loads and associated criteria for buildings and other structures (ASCE/SEI 7-16, Chapter 7).
- American Society of Civil Engineers. (2022). Minimum design loads and associated criteria for buildings and other structures (ASCE/SEI 7-22, Chapter 7).
- Federal Emergency Management Agency. (2013). Snow load safety guide (FEMA P-957).
- International Code Council. (2020a). 2021 International Building Code (Section 1608).
- International Code Council. (2020b). 2021 International Residential Code (Sections R301.2.3, R301.6, R802.10.2.1).
- International Code Council. (2023a). 2024 International Building Code (Section 1608).
- International Code Council. (2023b). 2024 International Residential Code (Section R301.2.3, Figure R301.2(3)).
- National Weather Service. (n.d.-a). Weather 101: Snow. NWS Nashville, TN.
- National Weather Service. (n.d.-b). Why snow ratios? NWS La Crosse, WI.
Updated .
Cite this page as:
Daniel, P. Snow Load Calculator. Available at: https://constructioncalculators.com/snow-load-calculator/. Accessed: October 28, 2026.