The weight of winter, measured to the letter of the code.
RoofHelm is an interactive monograph on roof engineering. Start with the ASCE 7-22 snow load method below (flat, sloped, minimum, rain-on-snow and §7.6.1 unbalanced loads, every factor shown), then the roof geometry, insulation, HVAC and cost calculators further down the page.
- 5
- load cases
- 7
- ASCE factors
- 51
- states + DC
- $0
- to use
Run your roof
Adjust any input and the section drawing, factor table and governing load update live. Your inputs stay in the URL, so a result is shareable and bookmarkable.
The worksheet
ASCE 7-22At 18.4° the §7.3.4 minimum does not apply, so the balanced load of 21 psf governs.
≈ 31,500 lb total on 1,500 sq ft of roof plan
Large gable roof (W > 20 ft): windward side at 0.3·Ps, leeward side at Ps plus a drift surcharge from the ridge.
surcharge 19.2 psf over 8.6 ft · hd 1.86 ft · γ 17.9 pcf
Flat-roof load Pf = 0.7 × Ce(1) × Ct(1) × Is(1) × Pg(30) = 21 psf. Sloped-roof balanced load Ps = Cs(1) × Pf = 21 psf.
| Ground snow load, Pg | 30 psf | site input |
| Exposure factor, Ce | 1 | Table 7.3-1 |
| Thermal factor, Ct | 1 | Table 7.3-2 |
| Importance factor, Is | 1 | Table 1.5-2 |
| Slope factor, Cs | 1 | Fig. 7.4-1 |
| Snow density, γ | 17.9 pcf | Eq. 7.7-1 |
| Settled depth at design load | 14 in | design ÷ density |
- 01Estimated snow density ≈ 17.9 pcf, so 21 psf is roughly 14 in of settled snow.
- 02The §7.6.1 unbalanced (leeward-drift) case is computed above for this gable/hip roof; at roof steps, parapets and walls also check drift loads (§7.7) with the drift calculator, plus sliding snow (§7.9) onto anything below.
- 03Ground snow load Pg is set by your local building department / the ASCE 7 Hazard Tool. Always confirm the value for your exact site before you build or submit.
Reduce the load on an existing roof
Your design roof snow load is 21 psf. If the design load is close to your roof's capacity, the cheapest fix is to keep snow off it. These are the tools that do that.
A long-handled rake pulls snow off from the ground, cutting the load before it builds up. The first thing to buy in a heavy winter.
Shop roof rakesSelf-regulating heat cable along eaves and valleys stops the ice dams that trap meltwater and add weight.
Shop heat cableSome links are affiliate links: if you buy through them we may earn a commission at no extra cost to you. These are starting points, not engineering endorsements. Always confirm a product's rated snow load meets the design load above.
From a ground snow number to a defensible roof load.
Set the ground snow load
Pg comes from the ASCE 7 Hazard Tool or your building department. The per-state appendix gives a planning range to start from.
Describe the roof
Slope, shape, warmth, surface, exposure and importance. Each maps to a published ASCE 7 factor, never a guess.
Read every load case
Flat, sloped, minimum, rain-on-snow and the §7.6.1 unbalanced load, with the governing value and a settled-depth section.
A working drawing, not a single number.
The calculator returns the whole load picture: a labelled roof section, the factor breakdown and every load case named, so you can check it, defend it and hand it to a reviewer.
- •The governing case is named, so you know which load controls.
- •Every ASCE 7 factor is shown with its table reference.
- •The leeward drift profile is drawn when §7.6.1 governs.
Read any result at a glance
Fig. 2· the ASCE 7 ground snow load scale, from a mild Gulf-coast roof to the high Sierra.
An index of calculators
Each runs the same engine with the right defaults for that roof, from a flat membrane to a slippery standing-seam metal building.
Roof geometry, insulation, HVAC & cost
The same transparent, real-formula approach applied to the rest of a roof project: pitch and framing, the building envelope, and what it costs to replace.
Roof geometry & framing
Insulation & HVAC
Ground snow load, state by state
Planning ranges for the populated parts of every state, with mountain case-study zones flagged. Confirm the exact value for your site with your building department.
Snow load, explained plainly
Practical, accurate walkthroughs for builders, engineers, solar installers and homeowners.
A plain-English walkthrough of the ASCE 7-22 roof snow load calculation: ground snow load, the flat-roof equation, the slope factor and the minimum load.
What ground snow load (Pg) means, why it varies so much by location and elevation, and exactly where to look up the value for your site.
Planning ranges for ground snow load across all 50 states, with the mountainous case-study states flagged. Always confirm the exact value with your AHJ.
Notes & questions
01How do you calculate roof snow load?+
Start from the ground snow load (Pg) for your site, then apply ASCE 7-22 Eq. 7.3-1: flat-roof load Pf = 0.7 × Ce × Ct × Is × Pg, where Ce is the exposure factor, Ct the thermal factor and Is the importance factor. For a sloped roof multiply by the slope factor Cs. Low-slope roofs must also carry the §7.3.4 minimum load.
02What is the difference between ground snow load and roof snow load?+
Ground snow load (Pg) is the weight of snow on the open ground at your site. It is fixed by your location and set by the ASCE 7 Hazard Tool or your building department. Roof snow load is what your roof actually has to carry: it is the ground value adjusted for exposure, the roof's warmth, its importance and its slope, and it is almost always less than Pg on a normal heated building.
03Where do I find my ground snow load?+
Use the ASCE 7 Hazard Tool (asce7hazardtool.online) or ask your local building department for the adopted design ground snow load. Many jurisdictions publish a county or town value in their building-code amendments. Our per-state pages give a planning range, but the legally-binding number for a permit is the one your AHJ adopts.
04Is this calculator accurate enough for a permit?+
It implements the ASCE 7-22 balanced-load equations exactly and shows every factor, so it is a strong design aid and a good check. For a permit submittal you still need the correct site ground snow load, the unbalanced/drift/sliding load cases where they apply, and a licensed engineer's review. The Pro report packages the calculation for exactly that.
Sizing a permit submittal?
The Pro report sets your calculation as a clean, referenced PDF offprint: every input, every factor, the governing case and the cases a reviewer will ask about.