Roof Loading — ASCE 7 Snow, Rain, Wind Uplift & Ponding

Roof loading is unique in structural design because four load types act simultaneously or in critical alternation: snow (gravity), rain/ponding (gravity, progressive), wind uplift (reversed gravity), and roof live load (construction and maintenance). ASCE 7-22 Chapters 7 (Snow), 8 (Rain), and 26–30 (Wind) provide the loading provisions. The critical design condition often involves the interaction between these loads — particularly wind uplift opposing dead load, and rain accumulation on deflected roofs.

Balanced snow load (ASCE 7-22 Section 7.3)

The flat-roof snow load pf represents the uniform snow accumulation on a roof without drift effects:

pf = 0.7 × Ce × Ct × Cs × Is × pg

Where:

pg = ground snow load (psf) from ASCE 7 Figure 7.2-1 (ranges from 0 in southern states to 300+ psf in mountain regions)
Ce = exposure factor (0.7 for fully exposed windy sites, 1.0 for partially exposed, 1.2 for sheltered)
Ct = thermal factor (1.0 for heated, 1.1 for unheated closed, 1.2 for unheated open, 1.3 for continuously below freezing)
Cs = roof slope factor (1.0 for slopes ≤ 30 degrees on warm roofs, reduces for steeper slopes)
Is = importance factor (1.0 for Risk Category II, 1.1 for RC III, 1.2 for RC IV)

Example: pg = 30 psf, heated warehouse (Ct = 1.0), suburban terrain (Ce = 1.0), flat roof (Cs = 1.0), Risk Category II (Is = 1.0): pf = 0.7 × 1.0 × 1.0 × 1.0 × 1.0 × 30 = 21 psf.

Roof live load (ASCE 7-22 Section 4.8)

Roof live load Lr per ASCE 7-22 Table 4.3-1 accounts for maintenance workers, equipment, and temporary storage:

Lr = 20 × R1 × R2    (psf, 12 psf ≤ Lr ≤ 20 psf)

Where R1 reduces for tributary area (R1 = 1.0 for At ≤ 200 sf, R1 = 1.2 - 0.001At for 200 < At < 600, R1 = 0.6 for At ≥ 600) and R2 reduces for steep slopes (R2 = 1.0 for F ≤ 4 in/ft). For most structural roof framing: **Lr = 12 psf** (tributary area > 600 sf, flat slope).

Snow vs roof live load: ASCE 7 load combinations 3 and 4 treat snow (S) and roof live load (Lr) as separate loads that do not occur simultaneously:

1.2D + 1.6S + 0.5W (or 0.5Lr)
1.2D + 1.6Lr + 0.5S (or 0.5W)

For locations with pg > 30 psf, snow typically governs over Lr. For locations with pg < 15 psf, Lr often governs.

Rain load and ponding (ASCE 7-22 Section 8)

Rain load R is the weight of water that accumulates on a roof assuming the primary drainage is blocked and only the secondary (overflow) drainage functions:

R = 5.2 × (ds + dh)    (psf)

Where ds = depth of water at the secondary drain inlet (in) and dh = additional depth from hydraulic head at the secondary drain (in). For a typical secondary scupper at 2 in above the roof surface with 1 in of hydraulic head: R = 5.2 × 3 = 15.6 psf.

Ponding instability occurs when the accumulated rainwater causes enough deflection to collect more water in a progressive feedback loop. AISC 360-22 Appendix 2 provides the stability criterion:

Cp + 0.9Cs ≤ 0.25

Where Cp = 504 × Lp^4 / (Ip × 10^7) and Cs = 504 × Ls^4 / (Is × 10^7). If this criterion fails, the roof is ponding-unstable.

Worked example — roof beam design for combined loads

Given: Roof beam spanning 40 ft, tributary width 6 ft, flat metal deck roof. Location: Chicago (pg = 25 psf). Dead load = 20 psf (deck + insulation + roofing + beam self-weight distributed).

Step 1 — Calculate loads: pf = 0.7 × 1.0 × 1.0 × 1.0 × 1.0 × 25 = 17.5 psf (balanced snow). Lr = 12 psf (tributary area > 600 sf). R = 5.2 × 3 = 15.6 psf (rain — assuming 2" secondary drain + 1" head).

Step 2 — Governing load combination (ASCE 7-22 Section 2.3.1): LC3: 1.2D + 1.6S + 0.5Lr = 1.2(20) + 1.6(17.5) + 0.5(12) = 24 + 28 + 6 = 58 psf LC4: 1.2D + 1.6Lr + 0.5S = 1.2(20) + 1.6(12) + 0.5(17.5) = 24 + 19.2 + 8.75 = 52 psf LC5 (rain): 1.2D + 1.6R = 1.2(20) + 1.6(15.6) = 24 + 25.0 = 49 psf

LC3 governs at 58 psf.

Step 3 — Beam design: wu = 58 × 6 / 1000 = 0.348 klf. Mu = 0.348 × 40² / 8 = 69.6 kip-ft. Required Zx = 69.6 × 12 / (0.90 × 50) = 18.6 in³. Minimum: W10x19 (Zx = 21.6 in³).

Step 4 — Deflection check (L/240 for total load): I_required = 5 × wservice × L^4 / (384 × E × delta_allow). wservice = (20 + 17.5) × 6 / 1000 = 0.225 klf. delta_allow = 480/240 = 2.0 in. I = 5 × 0.225 × (480)^4 / (384 × 29000 × 2.0) = 5 × 0.225 × 5.31 × 10^10 / (2.23 × 10^7) = 5.97 × 10^10 / 2.23 × 10^7 = 2679 in^4? This seems too high — recalculate in kip-in units: w = 0.225/12 = 0.01875 kli. I = 5 × 0.01875 × 480^4 / (384 × 29000 × 2.0) = 5 × 0.01875 × 5.31 × 10^10 / (22,272,000) = 4.977 × 10^9 / 2.23 × 10^7 = 223 in^4. W12x26 has I = 204 in^4 — close but insufficient. Use W14x22 (I = 199 in^4) — still insufficient. Use W12x30 (I = 238 in^4) — OK. Deflection controls over strength (as is typical for roof beams).

Wind uplift load combinations

The critical roof load combination for net uplift is:

0.9D + 1.0W    (ASCE 7-22 LC7, where W is negative/uplift)

For a roof with D = 20 psf and wind uplift = -35 psf (Zone 1, interior): Net = 0.9(20) + 1.0(-35) = 18 - 35 = -17 psf (net uplift).

The roof framing, connections, and anchorage must resist this net uplift. Joist seat welds, purlin clips, and deck attachments all need to be checked for this reversed loading direction.

Code comparison

ASCE 7-22 (USA): Snow per Chapter 7, Rain per Chapter 8, Wind per Chapters 26–30. Separate load combinations for each. Ponding per AISC 360 Appendix 2 or SJI Technical Digest 3. Ground snow load mapped by location.

AS 1170.1/AS 1170.2/AS 1170.3 (Australia): Snow per AS 1170.3 (alpine regions only, sk = 0.3–4.5 kPa). Most Australian roofs are governed by wind uplift (cyclonic regions) or roof live load. Rain loading is addressed through drainage design standards (AS/NZS 3500.3) rather than structural load provisions. Wind provisions in AS 1170.2 are more detailed for cyclonic regions than ASCE 7.

EN 1991-1-3/EN 1991-1-4 (Eurocode): Snow per EN 1991-1-3 with characteristic ground snow load sk, shape coefficients mu_1 and mu_2. Rain ponding addressed by requiring adequate drainage and minimum slope. Wind per EN 1991-1-4 with peak velocity pressure qp and external/internal pressure coefficients. Eurocode applies partial factors on actions (gamma_Q = 1.5 for variable loads) rather than the LRFD load factors used in ASCE 7.

Common mistakes engineers make

Applying snow load and roof live load simultaneously. ASCE 7 treats S and Lr as non-concurrent variable loads. They appear in different positions in the load combinations (one as the primary 1.6 factor, the other as 0.5 companion). Adding them together overestimates the demand.
Ignoring rain loading because "the roof has drains." Rain load R per ASCE 7 Section 8 assumes the primary drains are fully blocked. The design rain depth is measured to the secondary drainage level. Engineers who delete R from the load combinations are ignoring a load that has caused real collapses.
Using floor live load reduction for roof members. ASCE 7 Section 4.7 live load reduction applies to floor live loads, not roof live loads. Roof live load reduction uses the separate R1/R2 factors from Section 4.8. Applying the 15-psf-per-tributary-area floor reduction to roof framing is incorrect.
Designing roof framing for gravity only without checking net uplift. At building edges and corners, wind uplift can exceed twice the dead load. If the dead load is light (15–20 psf metal roof) and the wind uplift is high (40–65 psf in Zones 2 and 3), the net uplift is substantial and governs connection design.

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This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.