ASCE 7-22 Snow Load Changes: What's Different from 7-16

If you carry a 7-16 snow workflow into ASCE 7-22 without changing the load basis, you can get a number that looks conservative for the wrong reason. The map value is no longer a service-level ground snow load waiting for a 1.6 snow factor.

The 7-22 snow chapter changed the basis of the calculation, not just a few table values. Ground snow loads are now strength-level, reliability-targeted values from four Risk Category maps. The flat roof equation no longer includes the snow importance factor. Heated-building thermal factors now come from an R-value and pg grid. Drift heights use a winter wind parameter, W2, and lower-roof members must be checked for windward and leeward drift cases independently. Rain-on-snow went from 5 psf to 8 psf where it applies.

Not all jurisdictions have adopted ASCE 7-22. Verify the edition in force before applying these provisions.

Seven Changes at a Glance

The main trap is treating the 7-22 map value like the old 7-16 map value. The rest of the changes matter because they compound with that basis shift.

Ground Snow Loads: Service to Strength, One Map to Four

ASCE 7-16 used a single ground snow load map based on a 50-year mean recurrence interval. Risk Category entered later through the snow importance factor, Is, in the flat roof equation. ASCE 7-22 changed that structure. Section 7.2 sends the user to the ASCE Design Ground Snow Load Geodatabase, Figures 7.2-1A through 7.2-1D, or Table 7.2-1 for Alaska. The four figures correspond to Risk Categories I, II, III, and IV.^[1]

That is why Is disappears from the Chapter 7 snow equation. The Risk Category adjustment is already embedded in the ground snow load selection. Under ASCE 7-16, RC I/II/III/IV used snow importance factors of 0.80, 1.00, 1.10, and 1.20 from Table 1.5-2.^[2] Under ASCE 7-22, Table 1.5-2 is no longer the snow importance factor table; the snow Risk Category effect moved into the maps.

The Commentary explains the reason for the change: reliability. A 50-year ground snow load plus a single 1.6 strength factor produced nonuniform reliability because annual snow variability is not the same everywhere. ASCE 7-22 uses reliability-targeted ground snow loads tied to Table 1.3-1, with nearly 30 additional years of snow data behind the maps.^[1]

Do not reduce that to "7-22 snow loads are 1.6 times higher." In northern and mountain regions, Risk Category II reliability-targeted loads are often less than or equal to 1.6 times the prior 50-year load. In regions with higher year-to-year snow variability, particularly midlatitude locations that rarely see large accumulations but can experience extreme ones, the ratio can be greater than 1.6. The map value changed because the reliability basis changed, not because the committee applied a uniform multiplier.^[1]

ASCE 7-22 also significantly reduced CS regions. The Commentary supports that qualitative statement; it does not support a clean percentage or state count. For publication, the safe statement is simple: many locations that were CS under prior editions now have mapped or geodatabase values, while remaining CS areas are generally tied to elevations above locally available measurement points.

In practice: Treat the 7-22 pg as a strength-level, Risk Category-specific value. Do not enter it into a workflow that assumes a 7-16 service-level pg.

Flat Roof Snow Load: The Formula Lost a Factor

The two editions side by side show the change: the snow importance factor, Is, drops out of the 7-22 equation because Risk Category is now embedded in the ground snow load itself.

Ce did not materially change. Table 7.3-1 still ranges from 0.7 for fully exposed sites in severe exposure conditions to 1.2 for sheltered Exposure B roofs. The common judgment issue is not the table itself; it is classifying the roof exposure correctly. Sheltered is a narrower condition than many engineers assume, and the table note depends on obstructions within 10ho, where ho is the obstruction height above the roof.^[1,2]

Ct did change. For ordinary heated buildings, ASCE 7-16 made this easy to memorize: heated structure, Ct = 1.0. ASCE 7-22 does not preserve that shortcut.

Load Combinations: 1.0S Replaces 1.6S

The strength load combinations changed because S changed basis. In ASCE 7-16 Section 2.3.1, combination 3 used 1.6S when snow was the principal variable load. In ASCE 7-22 Section 2.3.1, combination 3a uses 1.0S when snow is principal.^[1,2]

The companion snow factors changed too. ASCE 7-16 used 0.5S when snow was a companion term in combinations 2 and 4. ASCE 7-22 uses 0.3S in the analogous 2a and 4a combinations.

Edition note: ASCE 7-16 does not use 1.6S every time snow appears in Section 2.3.1. It uses 1.6S when S is the principal variable load in combination 3. When snow is a companion load in combinations 2 and 4, the factor is 0.5S.^[2]

This is the easiest place to double-count. If you enter ASCE 7-22 pg into a program that still applies ASCE 7-16 LRFD combinations internally, the snow term can be 60% high before any member design begins. That may sound conservative, but it is still wrong. It distorts load comparisons, connection design, uplift checks, and serviceability decisions.

There is also an unresolved material-code coordination issue in the 2024 IBC adoption cycle: ACI 318-19 still contains load combinations with 1.6S. That conflict is not resolved by ASCE 7 Chapter 7 alone. On concrete projects governed by ASCE 7-22 and ACI 318-19, document the S basis being used and coordinate with the AHJ before treating a 1.0S substitution as settled code language.

In practice: Check both sides of the workflow: the ground snow load source and the load combination table. They have to be from the same edition basis.

Thermal Factor Ct: Tables 7.3-2 and 7.3-3

ASCE 7-22 split the thermal factor workflow into two tables. Table 7.3-2 routes the thermal condition. For "all structures except as indicated as follows," it sends you to Table 7.3-3. The other rows retain special cases: unheated/open-air/cold ventilated roof conditions at Ct = 1.2, freezer buildings at Ct = 1.3, and qualifying continuously heated greenhouses at Ct = 0.85.^[1]

Table 7.3-3 is the new piece most likely to be missed. Ct is now a two-dimensional lookup based on roof R-value and pg:

Linear interpolation is permitted for intermediate R-values and pg values. For Rroof greater than 50, the table footnote says Ct should be taken as 1.2; note the "should" language.

The Commentary basis is thermodynamic. Ct depends on where the 32°F isotherm falls within the roof assembly, which depends on both the insulation level and the snow load. Do not explain the change as "energy codes made insulation higher" or as a generic sustainability update. Energy code practice may make the change show up more often, but the provision itself is tied to the roof assembly temperature profile.^[1]

In practice: Stop defaulting heated commercial roofs to Ct = 1.0 on 7-22 projects. A modern R-38 roof with moderate pg will usually interpolate to a Ct above 1.0.

Drift Loads: Equation (7.6-1) and W2

ASCE 7-22 changed the drift height calculation by introducing W2, the winter wind parameter. Section 7.1.2 defines W2 as the percent of time the wind speed is above 10 mph during the winter season, October through April. The functional form changed entirely: 7-16 read hd off the graph/equation in Figure 7.6-1 as a function of lu and pg (with the snow importance factor Is), while 7-22 uses the closed-form Equation (7.6-1) with W2 and drops Is.^[1,2]

Drift density remains:

For lower roofs, Section 7.7.1 uses Equation (7.6-1) for both leeward and windward drift cases. Leeward drift uses the upper roof length for lu. Windward drift uses the lower roof length for lu, then takes three-quarters of hd. The larger of the two heights is used in design, and 7-22 adds an explicit requirement that windward and leeward drifts be checked independently to determine which controls the structural design of each member.^[1]

That last sentence matters. A single "governing drift" applied globally to the lower roof is not the full 7-22 workflow if different members see different controlling cases.

The Commentary explains the reason for W2 plainly: prior drift provisions used a national-average winter wind climate. The actual amount of wind-transported snow varies by region. W2 brings that regional winter wind climate into the equation. The Commentary also says the prior functional form produced "unrealistic results" for very short upwind fetch distances and very low ground snow loads; the new equation made the old minimum lu workaround unnecessary.^[1]

In practice: Do not run the old 7-16 drift formula with a 7-22 pg. Use Equation (7.6-1), get W2 from the 7-22 source, and check windward and leeward cases at the member level.

Minimum Snow Load and Rain-on-Snow

ASCE 7-16 Section 7.3.4 used a 20Is framework for minimum snow load on low-slope roofs. Where pg was 20 psf or less, pm = Is pg. Where pg was greater than 20 psf, pm = 20Is.^[2]

ASCE 7-22 Section 7.3.3 replaced that with Table 7.3-4:

Where pg is less than or equal to pm,max, pm = pg. Where pg is greater than pm,max, pm = pm,max.^[1]

Rain-on-snow also changed. ASCE 7-16 used a 5 psf surcharge where pg was 20 psf or less, but not zero, and the roof slope in degrees was less than W/50. ASCE 7-22 uses 8 psf where pg is less than or equal to pm,max from Table 7.3-4, but not zero, and the same slope expression applies.^[1,2]

The provision applies only to the sloped roof balanced load case. It is not combined with drift, sliding, unbalanced, minimum, or partial loads. Note that this rain-on-snow surcharge is a Chapter 7 snow provision and is separate from the ASCE 7 Chapter 8 rain load — for the blocked-drainage rain load itself, including the ponding term ASCE 7-22 added, see the rain load calculation guide.

Do not invent a rationale for the 5 psf to 8 psf change. The mandatory provision is clear; the Commentary does not state a specific reason for the magnitude increase.

Worked Example: Baltimore Roof Step, Two Workflows

This example runs one real site — Baltimore, Maryland — through both editions.

Design parameters (both editions unless noted):

• Location: Baltimore, Maryland
• Occupancy: commercial — Risk Category II
• Roof: ordinary heated, unventilated; R-38
• Exposure: partially exposed, Surface Roughness C → Ce = 1.0
• Lower roof: flat (Cs = 1.0), adjacent to a taller portion of the same structure, with a 6 ft roof step
• Fetch: upper-roof length (leeward) lu = 80 ft; lower-roof length (windward) lu = 80 ft

Ground snow and winter wind (edition-specific — this is the basis change the example exists to show):

• ASCE 7-16: pg = 25 psf from the 50-year service-level map; snow importance factor Is = 1.0 (RC II, Table 1.5-2).^[2]
• ASCE 7-22: pg = 60 psf from the Risk Category II geodatabase (strength-level), reflecting the high mid-Atlantic year-to-year snow variability captured by the new maps; winter wind parameter W2 = 0.45 from Figure 7.6-1 (enters the 7-22 drift equation only).^[1]

For project work, confirm pg and W2 in the ASCE 7 Hazard Tool for your coordinates.^[3]

Derived inputs:

• Thermal factor Ct: 1.0 under 7-16 (the "heated structure" shortcut); 1.14 under 7-22, interpolated from Table 7.3-3 for R-38 at pg = 60 psf.
• Drift geometry: with the 6 ft step, the clear height above the balanced snow exceeds the drift height in both editions (7-22: hc = 6 − pf/γ = 6 − 2.20 = 3.80 ft ≥ hd = 3.44 ft; 7-16: hc = 6 − 1.01 = 4.99 ft ≥ hd = 3.01 ft), so the full triangular leeward drift forms and w = 4hd.

Scope: This example covers a single load case — the balanced flat-roof snow load plus the leeward drift on a lower roof at a roof step. It is not a complete snow design. See the checklist after the results for the other conditions a full check must cover.

The two workflows are worked side by side below, step for step.

The trap: if you carry the 7-22 value into software still applying 7-16 combinations, you get 1.6(47.9) = 76.6 psf — 60% high on the snow term before any member design begins, because the 7-22 S value is already strength-level. This is not a step in either workflow; it is the error these articles exist to prevent.

Windward drift height before member-specific comparison (0.75hd, computed with the 80 ft lower-roof fetch): 2.26 ft under 7-16, 2.58 ft under 7-22. Because the upper and lower fetches are equal here, the leeward case (hd = 3.01 ft / 3.44 ft) governs at this step.

Summary of Results

Result: At this Baltimore site, a correct ASCE 7-16 workflow produces a principal snow term of 28.0 psf, while ASCE 7-22 produces 47.9 psf — a 71% increase driven by the pg basis change and Ct interpolation. Here the 7-22 pg is 2.4 times the 7-16 map value; that ratio is site-specific and comes straight from the new maps, not from scaling the old value. The leeward drift heights land close (3.01 ft vs 3.44 ft) only because the two editions use different equations and different pg values that happen to nearly offset here; the 7-22 number comes from W2 and Equation (7.6-1), not the old empirical figure with a new pg inserted. Once the drift surcharge is added, the peak design pressure at the step rises from 69.4 psf under 7-16 to 122.9 psf under 7-22 — a 77% increase at the controlling location, set by the pg basis change rather than any single factor.

This was one load case — a lower-roof step drift. A complete snow design still has to address, at minimum:

1. Windward drift at the same step, checked independently from the leeward case at the member level (Section 7.7.1)
2. Drifts at parapets and roof projections — including rooftop equipment with a side 15 ft or longer (Section 7.8)
3. Drifts from adjacent structures within 20 ft of the building (Section 7.7.2)
4. Rain-on-snow surcharge on the low-slope balanced case (Section 7.10)
5. Minimum roof snow load, pm (Section 7.3.3)
6. Unbalanced, sliding, and partial loads as the roof geometry requires (Sections 7.6, 7.9, 7.5)

Prose generates this documentation automatically — see a sample code-referenced load report to see what the output looks like.

Transition Checklist

1. Snow basis matches on both sides of the workflow. Confirm the ground snow load source and the load combination engine are on the same edition basis. This is the transition item most likely to hide in plain sight: a 7-22 pg entered into software that still applies 1.6S internally produces a larger, conservative-looking number while the snow term runs 60% high. The output looks fine; nobody notices the combination engine is still on a service-level snow assumption.

2. Ct computed from the R-value/pg grid, not defaulted to 1.0. Confirm heated buildings get Ct from Tables 7.3-2 and 7.3-3 rather than the old "heated structure, Ct = 1.0" shortcut. That shortcut was workable under 7-16; under 7-22, Table 7.3-3 makes Ct a function of both R-value and pg, and modern commercial roof insulation can push it materially above 1.0.

3. Drift formula carries W2. Confirm the drift calculation uses Equation (7.6-1) with W2 from the 7-22 source. A drift spreadsheet with no place for W2 is a prior-edition calculation, not a 7-22 one — and it tends to surface in review precisely because the rest of the snow package looks updated while the drift tab still runs the old empirical figure with a new pg inserted.

4. Windward and leeward drift checked at the member level. Confirm each lower-roof member is checked against the controlling drift case, not a single governing shape applied globally. Section 7.7.1 still compares windward and leeward drift heights, but 7-22 adds the member-level instruction: different members can be controlled by different cases, and stopping at one global drift shape misses that.

5. Formerly CS locations rechecked against the 7-22 source. Confirm whether a location mapped CS under a prior edition now carries a 7-22 map or geodatabase value before scoping a site-specific study. Many former CS locations now have mapped values, and the old habit can send a project into unnecessary site-specific work. Check the adopted 7-22 source first.

6. ACI 318-19 conflict documented, not buried. Confirm the S basis is documented and coordinated with the AHJ on concrete projects. ASCE 7-22 moved snow to a strength-level basis and uses 1.0S when snow is principal, while ACI 318-19 still contains 1.6S combinations. That is a material-code coordination issue, not a Chapter 7 snow provision — resolve it on the record rather than inside a calculation template.

The Bottom Line

The ASCE 7-22 snow changes are a basis change first and a table update second. Start with the ground snow load source: four Risk Category maps, strength-level values, no Is in the flat roof equation. Then confirm the load combinations are on the 7-22 basis, compute Ct from Tables 7.3-2 and 7.3-3, and rebuild drift calculations around W2 and Equation (7.6-1). The provisions are not hard to apply, but a half-updated 7-16 template can produce very polished wrong answers. The same edition-basis traps show up in the other ASCE 7-22 load chapters — the rain load provisions, for instance, also shifted in 7-22. If you want that edition logic handled consistently across snow, rain, wind, seismic, dead, and live loads, that is what Prose is built to do.

Related Resources

• Rain Load Calculation per ASCE 7 — static head, hydraulic head, ponding, and the SDSL changes in 7-22
• Replace your snow load spreadsheets — why edition-specific load development belongs in a tool that cites its own work

If you read this provision differently or have run into it applied another way on a real project, reach out — support@prose-eng.com. I'm a practicing engineer, not an infallible one.

If you're developing loads for a real project, Prose produces code-referenced reports covering all six ASCE 7 load types across editions 7-10, 7-16, and 7-22. Review a sample report →