Roof Snow Load Calculator
Work out the design snow load on a roof by ASCE 7, including the minimum that governs at low ground snow, and weigh the snow that is up there now.
How to use this calculator
- 1Get the ground snow load for your actual address from the ASCE Hazard Tool or your building department. Everything else on the page is a multiplier on that one number.
- 2Be honest about exposure. A house in trees is sheltered, and sheltered means more snow, not less.
- 3Enter the plan dimensions - the footprint seen from above - rather than the sloping roof surface.
- 4Use the "what is up there now" section in the middle of winter to compare the actual pack against the design figure.
How the calculation works
pf = 0.7 x Ce x Ct x Is x pg
pf >= Is x 20 psf where pg > 20; pf >= Is x pg where pg <= 20
ps = Cs x pf
Snow density = 0.13 pg + 14, capped at 30 lb per cubic foot- pg
- Ground snow load at the site, in psf - a mapped, site-specific value, not something to estimate
- Ce
- Exposure factor. 0.8 windswept to 1.2 sheltered - shelter increases the load
- Ct
- Thermal factor. 1.0 heated, 1.1 ventilated cold roof, 1.2 unheated, 1.3 refrigerated
- Is
- Importance factor by risk category. 0.8 minor, 1.0 ordinary, 1.1 substantial occupancy, 1.2 essential
- Cs
- Slope factor. 1.0 up to a breakpoint that depends on pitch, surface and Ct, then falling linearly to zero at 70 degrees
The 0.7 is not a safety factor being spent - it is an empirical observation that a balanced roof carries less than the ground, because wind removes snow and heat from below melts it. The minimum exists because that observation is least reliable where there is least snow.
Snow load acts on the horizontal projection of the roof, not on its sloping surface. A steeper roof has more surface but the same footprint, and it is the footprint that catches the snow.
The slope factor only applies to an unobstructed surface. Snow guards, vents, chimneys, dormers and solar arrays all hold snow in place, and a roof with any of them is treated as obstructed.
Worked example
A house in Chicago with a 6/12 shingle roof
- 1.A 25 psf ground snow load, ordinary suburban exposure, heated house, risk category II.
- 2.The equation gives 0.7 x 1.0 x 1.0 x 1.0 x 25 = 17.5 psf.
- 3.The minimum is 20 psf because the ground load exceeds 20, so 20 psf governs - the equation would have under-stated it by 13%.
- 4.A 6/12 shingle roof is 26.6 degrees, still short of the 30 degrees at which snow starts sliding off a shingled surface, so the slope factor is 1.0 and the sloped load stays at 20 psf.
Result: 20 psf - the code minimum, not the equation
The same roof in standing seam metal
- 1.Nothing changes until the slope factor. A slippery unobstructed surface starts shedding at 5 degrees rather than 30.
- 2.At 26.6 degrees the factor is (70 - 26.57) / (70 - 5) = 0.668.
- 3.So the sloped load falls from 20 psf to 13.4 psf - a third less, from the roof covering alone.
- 4.That reduction is contingent on nothing holding the snow. Fit snow guards, as many metal roofs over a doorway need, and the roof is obstructed again and the full 20 psf comes back.
Result: 13.4 psf - but only while the surface stays unobstructed
Weighing what is actually up there in February
- 1.A 40 psf site: the equation gives 28 psf and the minimum is 20, so 28 psf governs. A 4/12 shingle roof sheds nothing, so the design load is 28 psf.
- 2.On the roof there is 20 in of pack with 2 in of ice at the bottom.
- 3.The 18 in of packed snow at 22 lb per cubic foot is 33 psf, and the 2 in of ice at 57 lb per cubic foot adds 9.5 psf.
- 4.That is 42.5 psf, half again the design load, and 28 tons over the footprint. The ice layer is only a tenth of the depth and nearly a quarter of the weight.
Result: About 42 psf - over the design load, and the ice is doing more than it looks
Why roof load is less than ground load, and when it is not
Snow on the ground sits undisturbed. Snow on a roof gets blown off the windward side, melted from below by heat escaping through the ceiling, and slides off if the pitch is steep enough. ASCE 7 accounts for all three with a single 0.7 coefficient and then adjusts it for how exposed, how heated and how important the building is.
The trouble is that the reduction is an average across a great many roofs and a great many storms, and it is least dependable where there is least snow. A region with a 15 psf ground load may see one anomalous storm a decade that puts far more than the average on a roof, and a structure designed to 10.5 psf has nothing in reserve for it. So the code sets a floor, and across a wide swathe of the country that floor is the number that actually governs.
This is worth stating plainly because it is where the widely available snow load calculators go wrong. They implement the equation, which is the memorable part, and omit section 7.3.4, which is the part that matters in every mild-winter region.
Drift, and why it is the thing that fails
Almost no roof collapses under balanced snow. What collapses is the lower roof next to a higher one, the porch off the back of the house, the addition, the lean-to against the barn wall. Wind scours snow off the upper surface and drops it in the sheltered pocket at the step, and the load there can reach several times the balanced figure over a strip a few feet wide.
The geometry that governs it is the height of the step and the length of the upwind roof feeding it, and neither is something a general calculator can ask for usefully - the same house can have four different drift conditions on four different elevations. ASCE 7 devotes an entire section to it, and structural engineers spend real time on those cases.
The practical takeaway for a homeowner is about where to look rather than what to compute. If snow is going to be removed from one part of a roof, the low roof beside a high wall is where to start, not the main span. And if an addition or a porch is going on an existing house, drift is one of the reasons that work needs an engineer even when the addition itself looks trivial.
- Roof steps — A lower roof adjacent to a higher wall. The classic drift case and the classic failure.
- Parapets — Snow piles against the inside face, which is why flat commercial roofs fail at the edges rather than the middle.
- Rooftop equipment — Anything tall enough to shelter a pocket downwind of it collects a drift.
- Valleys — Snow sliding off two planes converges, and unbalanced load across a valley is its own ASCE case.
- Rain on snow — A warm rain onto an existing pack adds weight without adding depth, because the snow absorbs the water. It is why some collapses happen on days that feel mild.
Reading the pack on your own roof
Depth is a poor proxy for weight and everyone uses it anyway. Fresh powder runs around 7 lb per cubic foot, settled snow about 15, wind-packed snow above 20, and wet or partly thawed snow can reach 30. Ice is 57. Three feet of powder is lighter than a foot of wet snow, and a two inch ice layer at the bottom of a pack can outweigh a foot of what is above it.
The most reliable simple method is to cut a full-depth column out of the pack with a shovel, put it in a bucket and weigh it, then scale by area. Failing that, judging the type honestly and using published densities gets within a reasonable margin - certainly closer than judging by depth alone.
What to do about a heavy load is a different question from how heavy it is. Roof raking from the ground is the safe answer where the pitch allows. Getting on a snow-loaded roof is how people are seriously injured every winter, and chipping at ice with anything metal is how roofs that were merely overloaded become roofs that also leak.
What this assumes, and where it stops
Assumptions
- The building is a simple roof with no adjacent higher structure, no parapet and no rooftop equipment - that is, a balanced load case with no drift.
- Snow load acts on the horizontal projection of the roof, which is what ASCE 7 specifies.
- The slope factor assumes the surface is unobstructed if you have said it is slippery. Snow guards, vents, chimneys, dormers and solar arrays all make it obstructed.
- Snow density for the design depth uses the ASCE 7 relationship between ground load and density, which is a regional average rather than a measurement of your pack.
Limitations
- It does not calculate drift, sliding snow surcharge, unbalanced load across a gable or valley, or rain-on-snow surcharge. Those are the cases that fail, and they need an engineer.
- It tells you the load, not the capacity. Whether a particular roof can carry it depends on framing, span, condition and connections, none of which are on this page.
- Ground snow load in mountainous regions is not mapped by ASCE 7 at all - those areas are marked as requiring a site-specific case study, and a number pulled from a nearby town can be badly wrong.
- Existing older buildings may have been designed to an earlier and lower standard, or to no standard at all. A load within today's design figure is not automatically within what an old barn was built for.
- Solar arrays, added HVAC units and re-roofing over existing layers all add dead load that eats into whatever margin the structure had.
Common questions
How much snow can my roof hold?
This page will tell you what the roof was most likely designed for, which is a different question. A code-built modern house in a 30 psf ground snow region is designed for roughly 20 to 25 psf on the roof, which is around 14 to 18 inches of settled snow. But the capacity of your particular roof depends on its framing and its condition, and the load that fails it is nearly always a drift rather than an even blanket.
Why is my roof load lower than the ground snow load?
Because wind blows snow off a roof and heat escaping through the ceiling melts it from underneath, so a roof genuinely accumulates less than open ground. ASCE 7 captures that with a 0.7 coefficient. The catch is that the code also imposes a minimum, and where the ground load is 20 psf or less that minimum is the full ground load - so in mild-winter regions the reduction largely vanishes.
Does a steep roof mean I can ignore snow load?
Not until it is genuinely steep. On an asphalt shingle roof the load is not reduced at all below about 30 degrees, which is a 7/12 pitch, and it does not reach zero until 70 degrees. Metal and slate start shedding at 5 degrees, which is a real advantage - but only while the surface is unobstructed. One row of snow guards and the reduction is gone.
Should I rake snow off my roof?
From the ground, with a roof rake, on a roof carrying an unusual load, yes - and concentrate on the low roofs and the eaves rather than the main span. Getting onto a snow-covered roof is genuinely dangerous and it is how most snow-related injuries happen. Chipping ice with a hammer or a spade wrecks the roof covering, and clearing right down to the shingles with a metal blade does the same more slowly.
What is a rain-on-snow surcharge?
Rain falling on an existing snowpack is absorbed rather than shed, so the roof gains weight with no visible change in depth. ASCE 7 adds a surcharge for it on low-slope roofs in regions where it is plausible. It is the mechanism behind a fair number of winter collapses that happen on days people describe as a thaw rather than a storm.
Sources
- ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, chapter 7 — American Society of Civil Engineers
- ASCE Hazard Tool - site-specific ground snow load — American Society of Civil Engineers
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
Related calculators
Tools people commonly use alongside the roof snow load calculator.