Roof snow load design requirements start with a number that is not the roof load. The ground snow load (pg) you read off a code map or the ASCE 7 Hazard Tool is a ground measurement. ASCE 7 Chapter 7 converts it to a flat roof snow load with the equation pf = 0.7 Ce Ct Is pg, and the three factors can move the answer by a factor of three for the exact same map value. Then the slope factor Cs, drift surcharges, and a rain-on-snow surcharge get applied on top.
Two of those steps, drift at a roof step and rain-on-snow on a low slope, are where roofs actually collapse. And as of ASCE 7-22 the equation itself changed: the importance factor was deleted. So the answer to “what is my roof snow load” now depends on which edition your jurisdiction has adopted.
By The Roofing Brief Team. Last reviewed: August 2026.
Ground snow load is not roof snow load
Ground snow load (pg) is the design snow weight on open, level ground at a site, expressed in pounds per square foot. Roof snow load is what the structure is designed for after adjusting for how wind, roof heat, building use, and slope change what actually sits up there. In most of the United States the roof number is lower than the ground number. At a roof step it can be several times higher.
Under the International Building Code, Section 1608.2 sends you to Chapter 7 of ASCE 7 or to code figures for ground snow loads, with site-specific case studies subject to approval by the building official. Snow loads are taken as zero for Hawaii except in mountainous regions as approved by the building official. The code gives you pg. It does not give you the roof load.
The ASCE 7 flat roof snow load equation
The flat roof snow load pf is the balanced load on a low-slope roof. In ASCE 7-16, Equation 7.3-1 is pf = 0.7 Ce Ct Is pg. The 0.7 is a ground-to-roof conversion factor, not a safety reduction: ASCE commentary notes it arises from the wider set of exposure and thermal classifications available compared with earlier standards. Ce, Ct, and Is then adjust for site and building conditions.
What each factor actually represents
- Ce, exposure factor. Wind scours snow off a roof that sits in open terrain with nothing around it. A roof tucked among conifers or taller buildings keeps what falls on it, and also catches what blows off neighbors.
- Ct, thermal factor. Heat escaping through a roof melts the underside of the snowpack. A heated building holds less snow than an unheated barn, and a freezer building holds more than either.
- Is, snow importance factor (ASCE 7-16 and earlier). A hospital or emergency operations center is designed for a rarer snow event than an agricultural shed. This factor was eliminated in ASCE 7-22.
Exposure factor Ce values
The values below are ASCE 7-16 Table 7.3-1 as reproduced by the structural engineering reference site StructEd. ASCE paywalls the standard text, so treat this as a secondary reproduction and confirm against the edition your jurisdiction has adopted before using it on a permit set.
| Surface roughness / terrain | Fully exposed | Partially exposed | Sheltered |
|---|---|---|---|
| Terrain B (urban, suburban, wooded) | 0.9 | 1.0 | 1.2 |
| Terrain C (open terrain, scattered obstructions) | 0.9 | 1.0 | 1.1 |
| Terrain D (flat unobstructed, smooth mud flats, ice) | 0.8 | 0.9 | 1.0 |
| Above the tree line (windswept mountains) | 0.7 | 0.8 | Not applicable |
| Alaska, treeless areas (no trees within 2 miles) | 0.7 | 0.8 | Not applicable |
ASCE 7-16 defines “fully exposed” as a roof exposed on all sides with no shelter from terrain, higher structures, or trees, and “sheltered” as a roof sitting tight among conifers that qualify as obstructions. Everything else is “partially exposed”, which is where most suburban houses land at Ce = 1.0.
Thermal factor Ct values
Same caveat: these are the ASCE 7-16 Table 7.3-2 values as reproduced by StructEd, not a first-party quote from the standard.
| Thermal condition | Ct |
|---|---|
| Heated structures (all not listed below) | 1.0 |
| Cold, ventilated roofs with R-value above 25 between the ventilated space and the heated space | 1.1 |
| Unheated and open-air structures | 1.2 |
| Structures intentionally kept below freezing (freezer buildings) | 1.3 |
| Continuously heated greenhouses with roof R-value below 2.0 | 0.85 |
Note what this does to a well-insulated, well-ventilated attic. Building it right for ice damming and moisture control pushes Ct from 1.0 to 1.1, a 10 percent increase in design snow load. That is not an error in the standard. A cold roof genuinely holds more snow because it is not melting it from below. Snow load is also additive to dead load, so a heavy assembly such as composite or concrete tile consumes structural capacity before any snow lands.
Snow importance factor Is (ASCE 7-16 and earlier)
| Risk category | Typical occupancy | Is |
|---|---|---|
| I | Low hazard to human life: agricultural, minor storage | 0.80 |
| II | Ordinary buildings: most homes, offices, retail | 1.00 |
| III | Substantial hazard: schools, large assembly | 1.10 |
| IV | Essential facilities: hospitals, fire stations | 1.20 |
One ground snow load, two very different roof loads
Take pg = 40 psf, the ground snow load used in Example 7-1 of ASCE Press’s Snow Loads: Guide to the Snow Load Provisions of ASCE 7-16. That example has a heated building next to a lower unheated storage facility. Same site, same map value, two roofs:
| Step | Upper roof (heated, exposed) | Lower roof (unheated storage) |
|---|---|---|
| Ground snow load pg | 40 psf | 40 psf |
| Exposure factor Ce | 0.9 | 1.0 |
| Thermal factor Ct | 1.0 (heated) | 1.2 (unheated) |
| Importance factor Is | 1.0 (Risk Category II) | 1.0 |
| pf = 0.7 Ce Ct Is pg | 0.7 x 0.9 x 1.0 x 1.0 x 40 = 25 psf | 0.7 x 1.0 x 1.2 x 1.0 x 40 = 34 psf |
The ASCE guide confirms the lower roof balanced load at 34 psf. So the unheated roof carries 36 percent more design snow than its heated neighbor, from the identical map value, before anything else is applied.
Push the factors to their published limits and the spread gets wider. Running the same pg = 40 psf through the extreme ends of the tables (our arithmetic, ASCE’s factors):
- Best case: Risk Category I shed above the tree line, fully exposed, heated. pf = 0.7 x 0.7 x 1.0 x 0.8 x 40 = 15.7 psf.
- Worst case: Risk Category IV essential facility, terrain B, sheltered, unheated. pf = 0.7 x 1.2 x 1.2 x 1.2 x 40 = 48.4 psf.
That is a 3.1x spread from one number on one map. Anyone quoting “our ground snow load is 40, so design for 40” is guessing in both directions at once.
Sloped roof snow load: applying Cs
Once you have pf, the sloped roof snow load is ps = Cs pf (ASCE 7-16 Equation 7.4-1). Cs comes from Figure 7.4-1 and depends on two things: the thermal classification of the roof, and whether the surface is slippery and unobstructed enough for snow to slide off the eave. Slippery means metal, slate, and smooth membranes. Obstructions include snow guards, vents, and anything else that holds the pack.
Cs stays at 1.0 up to a breakpoint slope, then falls linearly to zero at 70 degrees:
| Roof condition | Cs = 1.0 up to | Cs = 0 at |
|---|---|---|
| Warm roof (Ct 1.0 or less), slippery and unobstructed | 5 degrees | 70 degrees |
| Warm roof (Ct 1.0 or less), all other surfaces | 30 degrees | 70 degrees |
| Cold roof (Ct = 1.1), slippery and unobstructed | 10 degrees | 70 degrees |
| Cold roof (Ct = 1.1), all other surfaces | 37.5 degrees | 70 degrees |
| Cold roof (Ct = 1.2 or more), slippery and unobstructed | 15 degrees | 70 degrees |
| Cold roof (Ct = 1.2 or more), all other surfaces | 45 degrees | 70 degrees |
There is a trap here for retrofits. Installing snow guards on a metal roof to stop sliding avalanches removes the “unobstructed” condition, which moves the roof from the slippery curve to the other-surfaces curve. On a 20 degree cold metal roof that can take Cs from roughly 0.83 back up to 1.0, adding about 20 percent to the design snow load on a structure that was never checked for it. Use the roof pitch chart to convert an X in 12 pitch to the degrees these curves are keyed to.
Minimum roof snow load: the floor under the calculation
ASCE 7-16 Section 7.3.4 sets a separate minimum load case for low-slope roofs so the conversion factors cannot drive the design to an unsafe number. Where pg is 20 psf or less, the minimum roof snow load pm equals Is x pg. Where pg exceeds 20 psf, pm equals 20 psf x Is. This is a separate uniform load case, not a floor applied to the balanced case, and it does not combine with drift, sliding, unbalanced, or partial loading.
In the pg = 40 psf example above, pm = 20 x 1.0 = 20 psf, which is below both the 25 psf and 34 psf balanced loads, so it does not govern. In low-snow regions it frequently does govern, which is why a roof in a 12 psf ground snow zone is often still designed for a 12 psf minimum snow case rather than the 8 or 9 psf the equation would produce.
Drift loading: where roofs actually fail
Snow drift surcharge at a roof step or against a parapet is the single most common cause of localized snow-related structural failure, because it concentrates several times the balanced load on a narrow strip. ASCE 7-16 handles it with a triangular surcharge whose height comes from the upwind fetch and the ground snow load, and whose weight comes from a snow density that rises with pg.
The pieces:
- Snow density (Equation 7.7-1): gamma = 0.13 pg + 14, capped at 30 pcf. At pg = 40 psf that gives 19.2 pcf, rounded to 19 pcf in the ASCE guide.
- Leeward drift height: hd = 0.43 x (lu)^(1/3) x (pg + 10)^(1/4) minus 1.5, with lu the upwind fetch in feet.
- Windward drift height: 0.75 times the leeward expression, using the fetch on the windward side. Whichever is larger governs.
- Peak surcharge: pd = hd x gamma.
- Drift width: w = 4 hd when the drift is not full (drift height below the clear height at the step).
Run it for Example 7-1 in the ASCE guide: pg = 40 psf, upper roof length lu = 100 ft. Leeward hd = 0.43 x (100)^(1/3) x (50)^(1/4) minus 1.5 = 3.8 ft. Windward, with a 170 ft fetch, gives 3.6 ft, so the leeward drift governs. Peak surcharge pd = 3.8 ft x 19 pcf = 72 psf, over a width of 4 x 3.8 = 15 ft.
| Load at the roof step (ASCE guide Example 7-1, pg = 40 psf) | psf | Ratio to lower-roof balanced load |
|---|---|---|
| Upper roof balanced load | 25 | 0.7x |
| Lower roof balanced load | 34 | 1.0x |
| Drift surcharge peak | 72 | 2.1x |
| Total at the step (balanced plus drift) | 106 | 3.1x |
106 psf against a 40 psf map value. A framing member sized for the balanced 34 psf and placed at the step is loaded to roughly three times its design snow load. This is why our winter roof collapse and snow-load losses report finds failures clustering at additions, dormers, mechanical wells, and parapets rather than in the middle of open roof planes.
Drift also governs against parapets and rooftop projections, where the “upper roof” is simply the wall. A short parapet plus a long upwind fetch produces a real surcharge on a roof that looks completely flat and uniform from the ground.
Rain-on-snow surcharge on low slopes
ASCE 7-16 Section 7.10 adds a 5 psf rain-on-snow surcharge to the balanced load where the ground snow load is greater than zero but no more than 20 psf, and the roof slope in degrees is less than W/50, with W the horizontal eave-to-ridge distance in feet. Above pg = 20 psf, ASCE commentary treats the rain-on-snow effect as already captured in the measured ground snow data, so no separate surcharge applies.
The W/50 test catches long, nearly flat roofs. For an 80 ft eave-to-ridge distance the threshold is 80/50 = 1.6 degrees, and a 1/4 in 12 pitch is 1.19 degrees, so the surcharge applies. For a short 18 ft eave-to-ridge run the threshold drops to 0.36 degrees, and almost no real roof qualifies. Longer drainage path means more chance for rain to pond in a saturated pack instead of draining away.
In Example 7-3 of the ASCE guide, at pg = 15 psf, the sloped roof loads come out at 10 psf and 13 psf, and the 5 psf surcharge takes them to 15 psf and 18 psf respectively. The surcharge is not a rounding error at that scale: it is a 38 to 50 percent increase. It applies only to the balanced load case and is not combined with drift, sliding, unbalanced, or partial loads. On low-slope assemblies this ties directly into drainage capacity and overflow scupper code requirements, because a blocked primary drain converts a rain-on-snow event into a ponding problem.
Unbalanced snow load on hip and gable roofs
Wind moves snow from the windward slope to the leeward slope, so ASCE 7-16 requires an unbalanced case for hip and gable roofs with slopes between 2.38 degrees (1/2 in 12) and 30.26 degrees (7 in 12). Below and above that band, unbalanced loading need not be considered. Inside it, the leeward slope is loaded and the windward slope may carry nothing.
For narrow roofs the ASCE 7-16 unbalanced leeward load is Is x pg, the full importance-adjusted ground snow load, with zero on the windward side. At pg = 46 psf and Is = 1.0, that is 46 psf on one slope against a 36.5 psf balanced load on both. The unbalanced case frequently governs ridge beams and individual rafters even when the balanced case does not.
Why “look it up on the map” is now edition dependent
ASCE 7-22 rebuilt the ground snow load provisions. The old national contour map was replaced by a geocoded ground snow load geodatabase of reliability-targeted values, published through the ASCE 7 Hazard Tool at asce7hazardtool.online, which sharply reduced the number of case-study regions where no mapped value existed. The equation changed with it.
| Provision | ASCE 7-16 | ASCE 7-22 |
|---|---|---|
| Flat roof snow load | pf = 0.7 Ce Ct Is pg | pf = 0.7 Ce Ct pg |
| Snow importance factor | Is from 0.80 to 1.20 by risk category | Eliminated: separate ground snow maps per risk category |
| Ground snow load basis | Uniform-hazard mapped values | Reliability-targeted (strength based) geodatabase values |
| Snow load factor in strength combinations | 1.6 | 1.0 |
| Thermal factor Ct | Fixed values 0.85 to 1.3 (Table 7.3-2) | Function of roof R-value and pg (Table 7.3-3) |
| Minimum roof snow load | Is x pg, or 20 x Is where pg exceeds 20 psf | Values by risk category (Table 7.3-4) |
| Rain-on-snow surcharge | 5 psf | 8 psf |
| Drift height | Function of fetch and pg | Adds winter wind parameter W2 |
The W2 parameter is defined as the percentage of time during the winter (October through April) when wind speed is at or above 10 mph, which makes drift height site-specific in a way it never was before. Because the ground snow values, the load factor, and the drift formula all moved together, ASCE 7-16 and ASCE 7-22 numbers are not directly comparable. A 7-22 pg that looks alarmingly high next to a 7-16 value is being multiplied by a 1.0 load factor instead of 1.6.
The net effect is not uniform. In a worked comparison published by the structural firm SK and A for a Washington, DC roof step (40 ft fetch, 4 ft step, 30 ft lower roof), the peak drift load rose from about 53 psf under ASCE 7-16 to about 89 psf under ASCE 7-22, an increase of roughly 67 percent for that geometry. Other geometries and regions move less, and some move down.
How the IBC and IRC adopt all of this
Neither model code contains the snow calculation itself. Both point at ASCE 7 and then set the boundary of where prescriptive design stops.
- IBC Section 1608.2 requires ground snow loads to be determined from Chapter 7 of ASCE 7 or from the code’s own ground snow load figures, with a separate table for Alaska. Site-specific case studies must be approved by the building official.
- IRC Section R301.2.3 allows the prescriptive tables in IRC Chapters 5, 6, and 8 for wood-framed, cold-formed steel, masonry, concrete, and structural insulated panel construction in regions with ground snow loads of 70 psf or less. Above 70 psf, buildings must be designed in accordance with accepted engineering practice.
- IRC Section R301.6 requires roofs to be designed for the snow load indicated in Table R301.2(1), the jurisdiction-filled climatic and geographic design criteria table.
That 70 psf threshold matters commercially. Below it a builder can use span tables. Above it, every roof in the jurisdiction needs engineering, which is why mountain-town permit sets look different from valley permit sets 30 miles away. The same split shows up in IRC roof deck fastening requirements, where prescriptive tables cover the ordinary case and engineered design takes over at the edges.
Practical checks before anyone quotes a number
- Confirm the adopted edition. Ask the building department which ASCE 7 edition their code cycle references. A pg from the 7-22 geodatabase used inside a 7-16 equation produces a badly wrong answer in the unconservative direction on the load factor side.
- Classify exposure honestly. “Fully exposed” means no shelter from terrain, structures, or trees on any side. A treeline that will mature over the building’s life pushes the roof toward partially exposed or sheltered.
- Classify thermal condition by what the building will be, not what it is. An unheated garage that gets finished later moves Ct from 1.2 down to 1.0. A heated space converted to cold storage moves it up to 1.3.
- Find every step, parapet, and projection. Drift is a local load. Mark each one and compute a fetch for both wind directions.
- Check the rain-on-snow trigger. If pg is 20 psf or less and the eave-to-ridge run is long, the surcharge is likely to apply, and under ASCE 7-22 it is 8 psf rather than 5.
- Do not treat removal as a design strategy. Loading a shovel crew onto a marginal roof adds live load at the worst moment. Our guidance on flat roof snow removal covers the sequencing that avoids creating an unbalanced case while clearing.
Frequently asked questions
Is ground snow load the same as roof snow load?
No. Ground snow load (pg) is the design snow weight on open level ground at a site, taken from a code map or the ASCE 7 Hazard Tool. Roof snow load is derived from it using ASCE 7 Chapter 7, adjusting for wind exposure, roof heat loss, building risk category, and slope. On most roofs the design load is lower than pg, but at a roof step it can be three times higher.
How do you convert ground snow load to roof snow load?
Under ASCE 7-16 the flat roof snow load is pf = 0.7 Ce Ct Is pg, where Ce is the exposure factor, Ct the thermal factor, and Is the snow importance factor. The sloped roof load is then ps = Cs pf. ASCE 7-22 dropped the importance factor, so the equation there is pf = 0.7 Ce Ct pg with risk category built into the mapped ground snow value.
What is the exposure factor Ce in snow load design?
Ce accounts for wind scouring snow off the roof. It ranges from 0.7 for a fully exposed roof above the tree line to 1.2 for a sheltered roof in terrain category B, per ASCE 7-16 Table 7.3-1. Most suburban houses are partially exposed at Ce = 1.0. A more exposed roof gets a lower design load because wind removes part of the snowpack.
What is the thermal factor Ct?
Ct reflects how much heat escapes through the roof and melts snow from below. ASCE 7-16 assigns 1.0 to ordinary heated buildings, 1.1 to cold ventilated roofs with R-values above 25, 1.2 to unheated and open-air structures, 1.3 to freezer buildings, and 0.85 to continuously heated greenhouses with roof R-values below 2.0. A well-insulated cold roof genuinely carries more snow.
Does ASCE 7-22 still use a snow importance factor?
No. ASCE 7-22 eliminated the snow importance factor and published separate reliability-targeted ground snow load maps for each risk category instead. The flat roof equation became pf = 0.7 Ce Ct pg. The load factor on snow in strength combinations also changed from 1.6 to 1.0, so 7-22 ground snow values look much larger than 7-16 values without representing an equivalent design increase.
How much larger is a snow drift load than the balanced roof load?
Frequently two to three times larger at the peak. In Example 7-1 of ASCE Press’s guide to the ASCE 7-16 snow provisions, a 40 psf ground snow load produces a 34 psf balanced load on the lower roof and a 72 psf drift surcharge peak, for 106 psf total at the step. That is 3.1 times the balanced load, concentrated across a 15 ft wide strip.
When does the rain-on-snow surcharge apply?
Under ASCE 7-16 Section 7.10 it applies where the ground snow load is above zero but no more than 20 psf and the roof slope in degrees is less than W/50, with W the horizontal eave-to-ridge distance in feet. The surcharge is 5 psf in ASCE 7-16 and 8 psf in ASCE 7-22. It applies only to the balanced load case, not in combination with drift or unbalanced loads.
What ground snow load can the IRC prescriptive tables handle?
IRC Section R301.2.3 permits prescriptive wood-framed, cold-formed steel, masonry, concrete, and structural insulated panel construction in regions with ground snow loads of 70 psf or less. Above 70 psf, buildings must be designed in accordance with accepted engineering practice. Roofs are designed for the snow load in Table R301.2(1), which each jurisdiction fills in with local values.
Do snow guards increase the design snow load on a roof?
They can. The ASCE 7 slope factor Cs uses a more favorable curve for slippery surfaces that are unobstructed, meaning snow can slide free at the eave. Adding snow guards removes that condition and moves the roof to the other-surfaces curve, which raises Cs. On a moderately pitched cold metal roof that can add roughly 20 percent to the design snow load.
Sources
- ASCE/SEI 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Chapter 7 (equations 7.3-1, 7.4-1, 7.7-1; Tables 7.3-1, 7.3-2; Sections 7.3.4, 7.6, 7.10). Standard text is paywalled by ASCE; factor tables above are reproduced from the structural reference site StructEd and cross-checked against worked examples.
- ASCE Press, Snow Loads: Guide to the Snow Load Provisions of ASCE 7-16, Chapter 7 worked examples 7-1 and 7-3 (first-party ASCE publication).
- ASCE/SEI 7-22 changes as reported in STRUCTURE magazine, “Snow and Rain Loads in ASCE 7-22” and “ASCE 7-22 Flat Roof Snow Load Versus Minimum Snow Load”, and in Kirby Building Systems engineering guidance.
- SK and A Structural Engineers, “Snow Drift Loads: ASCE 7-16 vs 7-22”, worked Washington, DC roof step comparison.
- International Building Code Section 1608.2 and International Residential Code Sections R301.2.3 and R301.6, via UpCodes.
- ASCE 7 Hazard Tool, asce7hazardtool.online, for geocoded ground snow load values.
This article explains published design provisions and is not a substitute for a structural engineer’s analysis of a specific building. Adopted code editions, local amendments, and site-specific case study requirements vary by jurisdiction. Confirm requirements with your building official before relying on any figure here.