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ASCE 7-22 Flat-Roof Snow Load

ASCE 7-22 moved to reliability-targeted, strength-level ground snow loads given separately for each risk category, removed the snow importance factor I_s, cut the LRFD snow factor from 1.6 to 1.0 and introduced a risk-category minimum load for low-slope roofs. Many tools labelled “ASCE 7-22” still apply I_s and the old 20·I_s minimum.

Site & Building

Risk category

p_g for this risk category psf

ASCE 7 Hazard Tool (strength level)

Surface roughness

Roof exposure

C_t — thermal factor from Table 7.3-3 (0.85–1.3)

In ASCE 7-22 C_t depends on the roof R-value and p_g. Read it from Table 7.3-3 — this tool does not reproduce the table.

Roof shape

Slope (curved: eave-to-crown vertical angle) °

Results

C_e Table 7.3-1—

p_f = 0.7·C_e·C_t·p_g Eq. 7.3-1—

p_m = min(p_g, p_m,max) §7.3.3—

p_g(asd) = 0.7·p_g—

Load factor on SLRFD 1.0 · ASD 0.7

ASCE 7-22 changes (v549)Ground snow loads are now reliability-targeted, strength-level values with a separate map per Risk Category (ASCE 7 Hazard Tool), so the importance factor I_s was removed and the load factor on S changed from 1.6 to 1.0 (LRFD 1.2D + 1.0S; ASD uses 0.7S). Do not combine values from this tool with ASCE 7-16 load factors. Sources: O'Rourke and Duntermann, Snow and Rain Loads in ASCE 7-22, STRUCTURE (Feb 2022); NFBA Frame Builder; ICC Demystifying Loads Part 2.

§7.2p_g is reliability-targeted and strength-level, one map per risk category (ASCE Design Snow Load Geodatabase / Hazard Tool). I_s no longer exists.

Eq. 7.3-1p_f = 0.7·C_e·C_t·p_g (ASCE 7-16 had 0.7·C_e·C_t·I_s·p_g)

§7.3.3 · Table 7.3-4Only for monoslope, hip and gable roofs with slope < 15° and curved roofs with eave-to-crown angle < 10°: p_m = p_g if p_g ≤ p_m,max, else p_m,max = 25 / 30 / 35 / 40 psf (RC I–IV); a separate uniform load case (ASCE 7-16 used 20·I_s)

§2.3 · §2.4Strength-level snow: LRFD factor 1.0 (was 1.6), ASD 0.7

SourcesASCE amplify (§7.3.3 text); STRUCTURE magazine; Tekla ASCE 7-22 notes; MiTek; Denver DBC 2025; NFBA. C_e values are the Table 7.3-1 values of ASCE 7-16; the only ASCE 7-22 source found reports the same values.

ScopeBalanced flat / low-slope roof load and minimum load only. Sloped-roof factor C_s, unbalanced loads, drifts (with the new winter wind parameter W2) and rain-on-snow surcharge (§7.10) are NOT included.

Preliminary calculation tool. Final design requires the full text of the referenced standard (see the sources on this page) and review by a licensed PE.