The snow load calculator above applies the balanced snow load procedure from ASCE 7 to inputs you supply. It converts a mapped ground snow load into a flat roof snow load using the exposure, thermal and importance factors, applies the slope factor to get the sloped roof snow load, tests whether the low-slope minimum governs, and reports which of those controls along with the total weight sitting on the roof's horizontal projection.
This is a preliminary and teaching tool published by Arb Digital. It is not a structural design, it is not stamped, and it does not replace a licensed engineer. It also does not perform several checks that routinely govern real roofs. Read the section on what is missing before you use any number from it, because a roof that satisfies the balanced load can still be badly overloaded by drift.
What This Snow Load Calculator Does
It implements one equation and one comparison. The equation is the ASCE 7 flat roof snow load, and the comparison is against the minimum roof snow load that applies to low-slope roofs. Everything it needs beyond your roof geometry is an input, because every one of those quantities is either mapped, tabulated or read from a figure in the standard, and none of them can responsibly be typed from memory into a web page.
That applies most of all to the ground snow load. pg is a mapped, site-specific value set by the authority having jurisdiction. It varies enormously over short distances in mountainous terrain, many states publish their own maps that supersede the national one, and some regions are designated as requiring a site-specific case study rather than a mapped figure at all. There is no defensible way to publish a table of it, so this page does not. Get the number from your building department, or from the ASCE 7 Hazard Tool, which returns mapped hazard data by location for the edition you select.
The exposure factor Ce, the thermal factor Ct, the importance factor Is and the slope factor Cs are likewise inputs. They come from the tables and figures in chapter 7 of ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. Which edition applies to your project is decided by the building code adopted locally, published for much of the United States through the ICC Digital Codes library. Local amendments differ by state, province and country, and other countries use entirely different standards.
How to Use It
- Get the ground snow load for the actual site. Not the nearest city, not a remembered figure — the mapped value for the coordinates, from the authority having jurisdiction.
- Read Ce and Ct from the standard. Ce reflects terrain and how sheltered the roof is; Ct reflects how much heat escapes through the roof. Both change the answer materially.
- Set Is from the building's risk category. A hospital or emergency facility carries a higher importance factor than an ordinary building, and a minor storage structure a lower one.
- Read Cs from the roof slope factor figure that matches your roof. Warm, cold, slippery and non-slippery roofs each use a different curve, which is why the tool asks for Cs directly rather than guessing it from the slope.
- Check which line governs. On a shallow roof the minimum frequently controls, and the tool says so explicitly.
The Formula / How It's Calculated
The flat roof snow load is pf = 0.7 × Ce × Ct × Is × pg. The 0.7 is a fixed ground-to-roof conversion built into the method, reflecting that a roof holds less snow than open ground because of wind scour and because snow slides and melts. The sloped roof snow load is ps = Cs × pf.
A separate minimum applies to low-slope roofs, defined in the standard by a slope threshold, and it is not reduced by Cs, Ce or Ct. Where pg is 20 psf or less the minimum is pm = Is × pg; where pg exceeds 20 psf the minimum is pm = 20 × Is psf. The design balanced load is the larger of ps and, where applicable, pm.
Worked example with the loaded values. With pg = 30 psf and Ce, Ct, Is and Cs all at 1.0, the flat roof snow load is 0.7 × 1 × 1 × 1 × 30 = 21 psf, and the sloped roof snow load is 1.0 × 21 = 21 psf. The roof is at 4 in 12, which is 18.43 degrees, above the low-slope threshold, so the minimum does not apply. The governing balanced load is 21 psf, which is 1.01 kPa. Over a 1,200 square foot horizontal projection that is 25,200 pounds, about 12.6 US tons of snow.
What This Page Does Not Check
This is the most important section. The balanced load is only the first of several snow load cases in ASCE 7, and it is frequently not the one that governs a failure.
Drift. Where a roof steps down from a taller one, or where snow blows against a parapet, a mechanical unit or an adjoining structure, snow accumulates into a triangular drift whose peak intensity can be several times the balanced load. Drift is calculated separately, it depends on the upwind fetch and the ground snow load, and it is the single most common cause of snow-related roof collapse. This page does not compute it.
Sliding snow. Snow shedding off a steep upper roof lands on a lower roof and loads it in addition to whatever is already there. That is its own check with its own procedure, and it is not performed here.
Unbalanced load. On a gable roof, wind moves snow from the windward side to the leeward side, so the two slopes carry different loads and the structure has to be checked for that asymmetric case. Not performed here.
Rain-on-snow surcharge. In some low-slope situations a rain surcharge is added, because rain falling into an existing snowpack adds weight without draining. Not performed here.
Ponding. On a very shallow roof, deflection under load creates a depression that collects more water, which increases deflection, which collects more water. That instability is checked separately and is not addressed here.
None of that is a criticism of the balanced load calculation. It is the point: a single number for a uniform load over a whole roof is a starting point, and the cases that actually break roofs are the local, concentrated ones.
Why the Factors Matter More Than They Look
The four multipliers look like fine tuning and they are not. Take the worked example at 21 psf. A sheltered site in dense terrain and a well-insulated cold roof can push Ce and Ct upward, and combining modest increases in both can raise the load by a third. An essential facility carries a higher importance factor again. Conversely a fully exposed roof in open terrain reduces Ce, because wind scours snow off it.
The thermal factor deserves a moment on its own, because it points at something counter-intuitive. A poorly insulated warm roof loses heat, melts the underside of the snowpack and sheds load — so it carries a lower design snow load. Insulate that roof properly and the snow stays, and the design load goes up. Retrofitting insulation into an attic can therefore increase the snow load a roof must carry, on a structure that was never designed for it. It also changes the ice dam picture, because a cold roof stops the melt-and-refreeze cycle at the eaves.
The slope factor is where the geometry finally enters. Steep roofs shed snow, so Cs falls as slope increases, and it falls faster on a slippery surface such as metal or glass than on a rough one such as asphalt shingle. Warm roofs shed more readily than cold ones. That is why ASCE 7 has several Cs curves rather than one, and why this tool asks you to read the right one rather than assume.
Snow Load in Context With Everything Else
Snow is one load among several. The roof also carries its own dead load, and it may carry a roof live load, wind uplift and, in some regions, seismic demand. Codes combine those loads through prescribed load combinations, applying different factors to each, and the governing combination is not always the one with the largest single load in it. A roof that is fine under snow alone can be governed by a combination of snow and wind.
Once you have a load, sizing the members is a separate step again. Our beam load calculator turns a distributed load into reactions and moments, the section modulus calculator gives the geometric properties a member needs, and the wood beam span calculator covers timber members. For roof geometry, the rafter length calculator handles rafter lengths and cut angles and the roof pitch calculator converts between slope expressions.
Existing Roofs, Snow Removal and Judgement
A question that comes up every hard winter is whether an existing roof is at risk. This calculation cannot answer it. The design load tells you what a roof was supposed to be built for; it says nothing about what a specific existing roof, of unknown construction, in unknown condition, with unknown modifications, can actually carry today. Depth on the roof is also a poor proxy for weight, because the density of snow varies by a factor of several between fresh powder and old settled or ice-laden snow.
Removing snow from a roof is itself hazardous — falls, falling snow and ice, and unbalancing a structure by clearing one side and not the other are all real risks — and it is work for people equipped for it. If a roof is a concern, the right call is a structural engineer, not an online calculator. The factor of safety calculator is useful for understanding margins in general terms, but it is not a substitute for an assessment of a specific building.
Arb Digital builds free calculators and interactive tools that earn search traffic for engineering, roofing and construction businesses. Browse the full library, or tell us what your customers keep asking you to work out.
Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Using a ground snow load from memory or from a nearby town — pg is mapped and site-specific, it varies sharply in mountainous terrain, and some areas require a case study instead.
- Assuming every factor is 1.0 — Ce, Ct, Is and Cs are read from the standard, and combinations of them move the answer by tens of percent.
- Stopping at the balanced load — drift, sliding snow, unbalanced load and rain-on-snow are separate cases and are the ones that most often govern.
- Using sloping roof area instead of plan area — snow load acts on the horizontal projection, so the sloping surface area overstates the total weight.
- Reading depth on a roof as a load — snow density varies by a factor of several, so depth alone tells you very little about weight.
Related Free Tools From Arb Digital
Pair this with the beam load calculator for reactions and moments, the section modulus calculator for section properties, the rafter length calculator for roof geometry, the roof pitch calculator for slope conversions and the roofing calculator for covering materials. The full free online tools hub lists every calculator we publish.
Frequently Asked Questions
From the authority having jurisdiction, or from the ASCE 7 Hazard Tool, which returns mapped hazard data by location. It is site-specific, it varies sharply in mountainous terrain, and some regions require a site-specific case study rather than a mapped value.
Because pg is a mapped value set locally, many states publish their own maps that supersede the national one, and a table typed into a web page would be wrong for a great many sites. It is an input for exactly that reason.
It is the fixed ground-to-roof conversion factor in the ASCE 7 method. A roof holds less snow than open ground because wind scours it, snow slides off and some melts, and 0.7 accounts for that in the balanced load case.
Generally yes, through the slope factor Cs, and the reduction is greater on slippery surfaces such as metal than on rough ones such as asphalt shingle. But Cs must be read from the correct figure in the standard, since warm and cold roofs use different curves.
No, and drift is the most common cause of snow-related roof collapse. Where a roof steps down or meets a parapet or a taller structure, snow accumulates far above the balanced load. That is a separate calculation performed by an engineer.
Because a poorly insulated roof loses heat and melts snow from underneath, shedding load. Insulating it properly keeps the snow in place, which raises the design load through the thermal factor. A retrofit can therefore increase demand on a structure not designed for it.
No. This gives a design load for a new structure. It says nothing about what a specific existing roof of unknown construction and condition can carry, and snow depth is a poor proxy for weight. Consult a structural engineer.
Plan area, the horizontal projection. Snow load in this method is defined as acting on the horizontal projection of the roof, so using the sloping surface area would overstate the total weight.
This tool applies a published method to inputs you supply, for education and preliminary work only. It is not a structural design, it is not stamped, it omits drift, sliding, unbalanced and rain-on-snow cases, and all structural design must be carried out by a licensed engineer in accordance with local code.
