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

ASCE 7-22 changed the basis of the snow calculation. Ground snow loads are now strength-level values from four Risk Category maps, so the snow importance factor is gone and the load factor on snow dropped to match: $1.0S$ instead of $1.6S$ in LRFD, and $0.7S$ instead of $1.0S$ in ASD. The changes to $C_t$, $C_s$, drift, and the minimum load build on that shift.

The two halves have to move together. A 7-22 ground snow load run through 7-16 load factors comes out 60% high, and it reads as conservative, so nothing flags it.

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

ASCE 7-22 Snow Load Changes at a Glance

The rows follow the order of the sections below.

Ground Snow Loads and Load Factors Changed Together

Under ASCE 7-16, the ground snow load map gave a 50-year value and the margin was applied afterward: the snow importance factor $I_s$ for Risk Category, then a 1.6 load factor.^[1] ASCE 7-22 builds both into the map. Section 7.2 gives a strength-level ground snow load for each Risk Category from the ASCE Design Ground Snow Load Geodatabase, available through the ASCE 7 Hazard Tool.^[2] With the margin already in $p_g$, $I_s$ drops out of every snow equation and the load factors drop with it (ASCE 7-16 and 7-22 Sections 2.3.1, 2.3.6, and 2.4.1):^[2,1]

The ASD 0.7 is the inverse of the typical 1.5 ratio between LRFD and ASD design strength, rounded (ASCE 7-22 Commentary C7.2), and the 2024 IBC alternative ASD combinations in Section 1605.2 use it as well.^[2,3]

The new map values are not the old ones times 1.6. ASCE 7-22 Commentary C7.2 explains that a single 1.6 factor gave uneven reliability: too little margin in some places, where failures from underestimated ground snow loads have been observed, and more than needed in others. The 7-22 values target the same reliability everywhere. Northern and mountain sites tend to land at or below 1.6 times their old value for Risk Category II, and midlatitude sites with rare but extreme snowfalls land above it.^[2] Baltimore, in the worked example, is 2.4 times its old value.

The new maps also shrank the case study (CS) regions. The ones left are above any nearby snow measurement, and the geodatabase still returns a value there with a warning, so a site that was CS under 7-16 may now have a usable value.^[2]

The trap is mixing the two halves:

• 7-22 $p_g$ with 7-16 factors: 60% high in LRFD, 43% high in ASD. It reads as conservative, so nothing flags it.
• 7-16 $p_g$ with 7-22 factors: 38% low.

Try it against your own $p_g$:

Low-Snow Sites: A New Exemption from Chapter 7

Very low-snow sites can now skip Chapter 7, but only where nothing on the roof can drift, or the snow is very light. ASCE 7-22 Section 7.2 adds this exception with no ASCE 7-16 counterpart. The snow provisions need not be considered:^[2]

• For roofs or roof members with no potential for drift accumulation or unbalanced loading, where $p_g$ is less than the factored roof live load used in design.
• For all other roofs, where $p_g\le 10$ psf and the upwind roof length $l_u$ to any potential drifting location is ≤ 100 ft, or where $p_g\le 5$ psf and $l_u\le 300$ ft.

With a 20 psf roof live load factored at 1.6, the first condition covers $p_g$ below 32 psf, but only on roofs where nothing can drift or load unevenly. A roof step, a parapet, rooftop equipment, or a gable roof in the unbalanced slope range all fall under the second condition, which is much tighter. Commentary C7.7 notes that a single storm with high wind has produced 3 to 4 ft drifts in low-snow regions, which is why drift potential decides which test applies.^[2]

Flat Roof Snow Load: $I_s$ Drops Out, $C_t$ Changes

The flat roof equation lost $I_s$, and the thermal factor became a lookup instead of a constant.

The exposure factor, $C_e$, did not change: ASCE 7-22 Table 7.3-1 matches ASCE 7-16, from 0.7 for fully exposed roofs above the tree line to 1.2 for sheltered roofs in Surface Roughness B.^[2,1]

The thermal factor did. ASCE 7-22 Table 7.3-2 now sends heated buildings with unventilated roofs to a lookup, and moves cold ventilated roofs up a step:^[2,1]

ASCE 7-16 limited the cold ventilated value to roofs with more than R-25 between the ventilated space and the heated space; ASCE 7-22 ties it to the applicable energy code.

For heated, unventilated roofs, Table 7.3-3 reads $C_t$ off roof R-value and $p_g$:

Linear interpolation is permitted. For $R_{\text{roof}}$ above 50, the footnote says $C_t$ should be taken as 1.2. The old 1.0 survives in only three cells, R-20 or less at $p_g$ of 50 psf or more; every other heated, unventilated roof gets a higher $C_t$. An R-38 roof at $p_g=60$ psf interpolates to 1.14.

ASCE 7-22 Commentary C7.3.2 ties this to where the 32°F isotherm sits in the roof assembly. With high insulation or low $p_g$, the freezing point falls inside the insulation, no snow melts from building heat, and the roof behaves like an unheated one.^[2]

Sloped Roofs: $C_t$ Now Picks the $C_s$ Curve

A heated roof's slope factor now depends on its insulation. ASCE 7-16 treated every heated building as a warm roof ($C_t=1.0$, Figure 7.4-1a). ASCE 7-22 Section 7.4.1 and Table 7.4-1 pick the curve from the $C_t$ value: Figure 7.4-1(a) where $C_t\le 1.1$, Figure 7.4-1(b) where 1.1 < $C_t$ < 1.2, and Figure 7.4-1(c) where $C_t\ge 1.2$.^[2,1] Since Table 7.3-3 puts most heated roofs above 1.0, a heated roof can land on any of the three, which changes two things:

• Figure 7.4-1(a) lost its slippery-surface curve. ASCE 7-16 let an unobstructed slippery warm roof with $R\ge 30$ (unventilated) reduce $C_s$ from 5°. The ASCE 7-22 Figure 7.4-1(a) has one line for all surfaces; Commentary C7.4 explains that at $C_t\le 1.1$ an ice dam is expected at the eave and resists sliding.^[2]
• Figure 7.4-1(b) holds $C_s=1.0$ to steeper slopes. The solid line stays at 1.0 to 37.5° instead of 30°, and the slippery-surface reduction starts at 10° instead of 5°.

For a heated, unventilated, unobstructed metal roof at 6 on 12, with the same $p_g$ in both editions:

• R-38, $p_g=60$ psf: $C_t$ goes from 1.0 to 1.14 and $C_s$ from 0.67 to 0.72, so the sloped roof load is 23% higher.
• R-30, $p_g\ge 70$ psf: $C_t=1.10$ lands on Figure 7.4-1(a), which has no slippery reduction, so $C_s$ goes from 0.67 to 1.0 and the load is 65% higher.
• R-20, $p_g=20$ psf: ASCE 7-16 gave no slippery reduction below R-30. ASCE 7-22 gives $C_t=1.11$ and Figure 7.4-1(b), so the load is 20% lower.

Minimum Snow Load and Rain-on-Snow

The minimum roof load is now capped by Risk Category, and the cap went up (ASCE 7-16 Section 7.3.4; ASCE 7-22 Section 7.3.3 and Table 7.3-4):^[2,1]

For Risk Category II, the cap went from 20 to 30 psf. In both editions $p_m$ applies to monoslope, hip, and gable roofs under 15° as a separate uniform load case, not combined with drift, sliding, unbalanced, or partial loads. It can govern on roofs where $p_f$ is low; in the worked example it governs under 7-16 and not under 7-22.

Rain-on-snow went from 5 psf to 8 psf, and the threshold moved from $p_g\le 20$ psf (ASCE 7-16 Section 7.10) to $p_g\le p_{m,\text{max}}$ (ASCE 7-22 Section 7.10), with the same $W/50$ slope test.^[2,1] ASCE 7-22 Commentary C7.10 explains the threshold: above $p_{m,\text{max}}$, the measured ground snow data are assumed to already include rain-on-snow. It does not explain the increase from 5 to 8 psf.^[2] The surcharge applies only to the balanced load case and is separate from the Chapter 8 rain load; for that, see the rain load calculation guide.

Drift Loads: Equation (7.6-1) and $W_2$

Drift height now depends on regional winter wind. ASCE 7-16 read $h_d$ off Figure 7.6-1 from $l_u$ and $p_g$, scaled by $I_s$. ASCE 7-22 replaces that with the closed-form Equation (7.6-1), which drops $I_s$ and adds $W_2$, the winter wind parameter: the percent of time from October through April that wind speed exceeds 10 mph (Section 7.1.2).^[2,1]

Drift density is unchanged in both editions:

ASCE 7-22 Commentary C7.6.1 reports that $W_2$ influences drift height as much as $p_g$ or $l_u$, and C7.7 notes that the new form no longer needs the old 20 ft minimum $l_u$.^[2]

ASCE 7-22 Section 7.7.1 uses Equation (7.6-1) for both lower-roof cases: leeward with the upper roof length as $l_u$, windward with the lower roof length and three-quarters of the result. It adds that windward and leeward drifts be checked independently to find which controls each member, so one governing drift applied to the whole lower roof is no longer enough.^[2]

$W_2$ is not confined to roof steps. Equation (7.6-1) also sets the gable roof unbalanced surcharge in Section 7.6.1 and the parapet and projection drifts in Section 7.8.^[2]

Snow in Seismic Design and Concrete Load Combinations

Snow now enters effective seismic weight at a higher threshold and a smaller share. ASCE 7-22 Section 12.7.2 includes 15% of the uniform design snow load where $p_f$ exceeds 45 psf; ASCE 7-16 used 20% where $p_f$ exceeded 30 psf.^[2,1] Because 7-22 $p_f$ values are larger, a roof with no snow in $W$ under 7-16 can still pick it up under 7-22, as the worked example's roof does. The simplified procedure in ASCE 7-22 Section 12.14.8.1 still lists the old 30 psf and 20%.

Concrete load combinations have not caught up. ACI 318-19 Table 5.3.1 still carries the 7-16 snow factors: $1.6S$ when snow is primary (Equation 5.3.1c), $0.5S$ as a companion, and $0.2S$ with earthquake (Equation 5.3.1e). ACI 318-19 Section 5.3.1 requires concrete members to resist those combinations.^[4] The 2024 IBC adopts ACI 318-19 alongside ASCE 7-22 without modifying Table 5.3.1, while IBC Section 1605.1 points to the ASCE 7 combinations.^[3] On concrete projects, state which combinations you used and confirm them with the AHJ.

Worked Example: Baltimore Roof Step, Two Workflows

One Baltimore roof step, run through both editions, shows how the basis change, $C_t$, the minimum load, and the new drift equation combine on a real roof.

Design parameters (both editions unless noted):

• Location: Baltimore, Maryland; commercial, Risk Category II
• Roof: ordinary heated, unventilated, R-38; partially exposed, Surface Roughness C → $C_e=1.0$
• Lower roof: flat ($C_s=1.0$), at a 6 ft step down from a taller portion of the same structure
• Fetch: $l_u=80$ ft for both the upper roof (leeward) and the lower roof (windward)

Ground snow and winter wind from the ASCE 7 Hazard Tool:^[5]

• ASCE 7-16: $p_g=25$ psf (50-year, service-level); $I_s=1.0$ (Risk Category II, Table 1.5-2)^[1]
• ASCE 7-22: $p_g=60$ psf (Risk Category II, strength-level); $W_2=0.45$^[2]

Derived inputs:

• $C_t$: 1.0 under 7-16; 1.14 under 7-22 (Table 7.3-3, R-38 at $p_g=60$ psf)
• Drift density, Equation (7.7-1): $\gamma =0.13(25)+14=17.25$ pcf under 7-16 and $0.13(60)+14=21.8$ pcf under 7-22
• Clear height: $h_c=6-p_f/\gamma$ is 4.99 ft under 7-16 and 3.80 ft under 7-22, above $h_d$ in both, so the full triangular drift forms with $w=4h_d$
• Rain-on-snow: not required in either edition, because $p_g$ exceeds the threshold (20 psf and $p_{m,\text{max}}=30$ psf)

Under 7-16, $p_m=20.0$ psf exceeds $p_f=17.5$ psf, so the minimum load governs the uniform case. Under 7-22, $p_f=47.9$ psf governs.

The mismatch: run this 7-22 value through 7-16 LRFD combinations and you get $1.6(47.9)=76.6$ psf, 60% above the correct 47.9 psf. The workflow check in the ground snow section starts from this roof.

The windward case ($0.75h_d$ with the 80 ft lower-roof fetch) gives 2.26 ft and 2.58 ft, so the leeward drift governs at this step. Drift width is $w=4h_d$: 12.0 ft under 7-16 and 13.8 ft under 7-22.

The drift sits on the balanced load, $p_f$, not on $p_m$, so Step 5 uses 17.5 psf under 7-16.

This roof also crosses the 7-22 seismic weight threshold: $p_f=47.9$ psf exceeds 45 psf, so 15% of it, 7.2 psf, goes into $W$. Under 7-16, $p_f=17.5$ psf added nothing.

Summary of Results

Result: The LRFD snow term rises 50%, from 32.0 psf (7-16, where the minimum load governs) to 47.9 psf, and the peak design pressure at the step rises 77%, from 69.4 to 122.9 psf. Most of both increases comes from $p_g$: the 7-22 Risk Category II value is 2.4 times the 7-16 map value here, a site-specific ratio from the new maps, not a scale factor.

This covers the uniform load and one drift. A complete design still runs the rest of the Transition Checklist below, from windward and parapet drifts to unbalanced, sliding, and partial loads.

Transition Checklist

1. Snow basis matches on both sides. $p_g$ and the combination table on the same edition: $1.0S$ for LRFD and $0.7S$ for ASD under 7-22.
2. No $I_s$ anywhere. Not in Equation 7.3-1, the drift height, or $p_m$.
3. Low-snow exemption checked first. Section 7.2 may remove Chapter 7 entirely at very low $p_g$.
4. $C_t$ from Table 7.3-3. Heated, unventilated roofs interpolate on R-value and $p_g$; cold ventilated roofs are 1.2.
5. $C_s$ curve picked by $C_t$. Sloped and metal roofs rechecked against Figure 7.4-1(a), (b), or (c).
6. Drift carries $W_2$. Steps, parapets, projections, and gable unbalanced loads all use Equation (7.6-1).
7. Windward and leeward checked per member at every roof step (Section 7.7.1).
8. $p_m$ and rain-on-snow rechecked against the Table 7.3-4 thresholds.
9. Seismic weight rechecked wherever $p_f$ exceeds 45 psf.
10. Concrete combinations stated and confirmed with the AHJ where ACI 318-19 applies.

The Bottom Line

The ASCE 7-22 snow changes are a basis change first and a table update second. Pull $p_g$ for the project's Risk Category from the Hazard Tool, check the Section 7.2 exemption, remove $I_s$, and put the combinations on the same basis: $1.0S$ in LRFD, $0.7S$ in ASD, $0.15S$ with seismic. Then interpolate $C_t$ from Table 7.3-3, let it pick the $C_s$ curve on sloped roofs, rebuild every drift around $W_2$ and Equation (7.6-1), and recheck $p_m$ and rain-on-snow against Table 7.3-4. The rain load provisions shifted in 7-22 as well. 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

• Free snow load calculator — balanced flat and sloped roof snow loads across ASCE 7-10, 7-16, and 7-22, for checking a single roof quickly
• ASCE 7-16 vs ASCE 7-22 — how the snow changes fit with the changes in every other load chapter
• 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.