How this snow load calculator works
Roof snow load follows the ASCE 7 balanced-load method: start from the ground snow load, take 70% of it, adjust for the site, then apply the slope:
roof snow load ps = Cs × pf · Cs from the pitch
At the defaults — a 30 psf ground load, 4/12 pitch, standard factors — that is 21 psf flat and, with a slope factor of 1.00 at this pitch, 21 psf on the roof: about 21,000 lb, or 10.5 tons, on a 1,000 ft² roof. The full chain is in the worked example. It is a planning number — the design load is an engineer’s call.
Ground load, the 0.7, and the roof
Every roof snow number starts on the ground and works up — knowing each step tells you what moves the load:
- Ground snow load (pg): the mapped design value for your area — the biggest input by far.
- The 0.7 factor: a roof holds about 70% of the ground snow, the baseline conversion.
- Flat-roof load (pf): 0.7 × pg after the site factors — the load before slope.
- Slope factor (Cs): steep warm roofs shed, so Cs trims the load above 30°.
- Roof load (ps): Cs × pf — the balanced load the frame carries per square foot.
The three factors, and the slope
Four adjustments move the load off the base 0.7 × pg — set each to your roof:
- Exposure (Ce): a windswept open roof sheds snow (0.9); a sheltered one collects it (1.1).
- Thermal (Ct): a heated roof melts snow (1.0); an unheated or cold-vented roof holds it (1.1–1.2).
- Importance (Is): a shed (0.8) is designed for less than a normal building (1.0) or a hospital (1.2).
- Slope (Cs): 1.0 up to 30°, falling to 0.625 at 45° and to zero by 70° on a warm roof.
- Cold-roof caution: a slippery or unheated roof sheds sooner but also loads the eave when it slides.
Roof load by ground snow, and the slope factor
Table 1 runs the roof load at standard factors and a low slope; Table 2 is the slope factor by pitch on a warm roof.
| Ground pg | Roof ps |
|---|---|
| 20 psf | 14 psf |
| 30 psf | 21 psf |
| 40 psf | 28 psf |
| 50 psf | 35 psf |
| 60 psf | 42 psf |
| Pitch | Cs |
|---|---|
| 3/12 | 1.00 |
| 6/12 | 1.00 |
| 8/12 | 0.91 |
| 12/12 | 0.63 |
| 16/12 | 0.42 |
How to figure a roof snow load
- Get the ground snow load (pg) for your area from the building department or ASCE 7.
- Set the pitch — it sets the slope factor, which only bites above 30°.
- Pick the exposure, thermal and importance factors that match the building.
- Read the roof load in psf, and the total on your roof area.
- Hand it to an engineer for drift, sliding and the design checks below.
Worked example: a 30 psf ground load
The default, worked line by line — 30 psf ground load, 4/12 pitch, standard factors, a 1,000 ft² roof:
angle = atan(4 ÷ 12) = 18.4° → Cs = 1.00
ps = 1.00 × 21 = 21 psf
total = 21 × 1,000 = 21,000 lb ≈ 10.5 tons
Steepen the roof to 12/12 and the slope factor falls to 0.63, cutting the roof load to about 13 psf — the snow slides before it piles. But that shed snow lands somewhere, so a steep roof over a doorway or a lower roof trades a lighter deck for a drift and sliding problem below. The balanced number is only the start of the story.
What this snow load estimate doesn’t cover
It is the balanced load only; a real design adds the cases that often govern:
- Drift loads. Snow piling at walls, parapets and roof steps — often the heaviest case.
- Sliding snow. Load dumped on a lower roof from an upper slope.
- Rain-on-snow. A surcharge added to low-slope roofs in some climates.
- Unbalanced load. More snow on the lee side of a ridge than the windward.
- Code minimums. A minimum roof snow load that can exceed the calculated value.
- The structure itself. Rafter, truss and header sizing — an engineer’s work.
Snow load FAQ
How do I calculate roof snow load?
By the ASCE 7 method: the flat-roof load is pf = 0.7 × Ce × Ct × Is × pg, where pg is the ground snow load and Ce, Ct and Is are the exposure, thermal and importance factors. The sloped-roof load is ps = Cs × pf, with Cs a slope factor. At a 30 psf ground load and standard factors, pf is 21 psf.
What is the difference between ground snow load and roof snow load?
Ground snow load (pg) is what the code maps report for your location; roof snow load is what actually sits on the roof, which starts at 0.7 of the ground value and is then adjusted for exposure, heat loss, importance and slope. So a 30 psf ground load becomes about 21 psf on a standard heated roof — usually less than the ground figure, not more.
How much does snow weigh on a roof?
At the default 21 psf roof load, a 1,000 ft² roof carries about 21,000 lb — roughly 10.5 tons of snow. Snow weight varies with type: fresh light snow is only a few pounds per square foot per foot of depth, while wet or packed old snow can be 20 lb per square foot or more, which is why the ground snow load already bakes in a heavy-snow assumption.
Does roof pitch reduce snow load?
On a warm, non-slippery roof, yes — above about 30° the slope factor Cs drops, because snow slides off. At 30° or less Cs is 1.0 (no reduction); at 45° (a 12/12 pitch) it is 0.625; by 70° it reaches zero. A cold or slippery roof sheds sooner. The calculator applies the warm-roof factor automatically from the pitch.
What ground snow load should I use?
Use the value for your location from ASCE 7 or your local building code — many jurisdictions publish a design ground snow load, and mountain and lake-effect areas set their own higher figures. Ask the building department; do not guess, because the ground snow load is the single biggest number in the whole calculation.
What are Ce, Ct and Is in the snow load formula?
Ce is the exposure factor (a windswept open site sheds snow, a sheltered one collects it), Ct is the thermal factor (a heated roof melts snow, an unheated one holds it), and Is is the importance factor (a hospital is designed for more than a shed). Each nudges the load up or down from the base 0.7 × pg.
Is this an engineered snow load?
No — it is the balanced-load planning number. A full design also checks drift at walls and steps, sliding snow from an upper roof, rain-on-snow surcharge, unbalanced load on one side, and the code minimums. Those can govern the frame. Use this to plan and sanity-check; have a structural engineer and the AHJ set the design.
Sources and standards
The method is the standard; the design belongs to a professional:
- ASCE 7: Minimum Design Loads — the pf = 0.7 Ce Ct Is pg and ps = Cs pf balanced-load method.
- Ground snow load: from the ASCE 7 maps or the locally adopted design value.
- Slope factor: warm, non-slippery roof — 1.0 to 30°, linear to zero at 70°.
- The AHJ governs: the adopted code, local ground snow load, and the drift and minimum rules.
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