-------------- | -------------------- | --------------------- | ------------------------ | ---------------------------- | | Ground snow load | Figure 7.2-1 (pg) | Cl 2.2 regional (S_e) | Annex C (sk) | Table C-2 (Ss, Sr) | | Flat roof formula | 0.7CeCtIspg | Cl 2.4 (CeCtIs*s0) | Eq. 5.1 (mu_1CeCt*sk) | Eq. 4.1.6.2 (IsSsCbCwCa) | | Exposure factor | Table 7.3-1 (Ce) | Cl 3.3 | EN 1991-1-3 Cl 5.2 (Ce) | Cw in Table 4.1.6.2-B | | Thermal factor | Table 7.3-2 (Ct) | Cl 3.4 | Ct in Cl 5.2 | Included in Cb | | Slope reduction | Figure 7.4-1 (Cs) | Cl 4.4 | Eq. 5.3 (mu_1) | Ca in Table 4.1.6.2-A | | Drift loads | Section 7.7, 7.8 | Cl 4.3 | Annex B | Eq. 4.1.6.2-3 (Ca) | | Rain-on-snow | Section 7.10 (5 psf) | Not explicit | EN 1991-1-3 Annex B | Sr in ground snow |

Key difference: ASCE 7 uses a 0.7 ground-to-roof conversion factor. Eurocode EN 1991-1-3 uses mu_1 shape coefficients (0.8 for flat roofs). Canadian NBCC separates ground snow (Ss) and rain (Sr), adding them before applying roof factors. All codes require separate drift analysis for stepped roofs.

Step-by-Step Example

Problem: Calculate balanced and drift snow loads for a two-level commercial building in Minneapolis, MN. Lower roof: 20 ft above grade, 80 ft wide. Upper wall height above lower roof: 12 ft. Risk Category II, sheltered site (Exposure B), heated building.

Step 1 -- Ground snow load: Minneapolis, MN: pg = 50 psf (ASCE 7-22 Figure 7.2-1).

Step 2 -- Balanced flat roof snow load: Ce = 1.2 (sheltered, Exposure B, Table 7.3-1). Ct = 1.0 (heated building, Table 7.3-2). Is = 1.0 (Risk Category II). pf = 0.7 _ 1.2 _ 1.0 _ 1.0 _ 50 = 42 psf.

Step 3 -- Snow density: gamma = 0.13 * 50 + 14 = 20.5 pcf.

Step 4 -- Leeward drift (from upper roof snow blowing onto lower roof): lu = upwind fetch on upper roof. Assume upper roof is 60 ft long: lu = 60 ft. hd = 0.43 _ (60)^(1/3) _ (50+10)^(1/4) - 1.5 = 0.43 _ 3.91 _ 2.78 - 1.5 = 4.68 - 1.5 = 3.18 ft. Check: hd must not exceed the wall height = 12 ft. 3.18 < 12. OK. Drift surcharge at peak: pd = 3.18 _ 20.5 = 65.2 psf. Drift width = 4 _ hd = 4 * 3.18 = 12.7 ft.

Step 5 -- Windward drift (from lower roof snow pushed toward wall): lu = 80 ft (lower roof fetch). hd = 0.43 _ (80)^(1/3) _ (60)^(1/4) - 1.5 = 0.43 _ 4.31 _ 2.78 - 1.5 = 5.15 - 1.5 = 3.65 ft. Windward drift uses 75% of hd: 0.75 * 3.65 = 2.74 ft. Leeward drift (3.18 ft) controls.

Step 6 -- Total load at drift peak: p_total = pf + pd = 42 + 65.2 = 107.2 psf. This is over 2.5 times the balanced load.

Result: Balanced roof load = 42 psf uniform. Drift load at wall = 107.2 psf peak, tapering over 12.7 ft. The drift condition governs beam and connection design near the roof step.

Common Design Mistakes

• Ignoring drift loads at roof steps: Drift loads can double or triple the balanced snow load. Missing this check is the single most common snow load error and has caused numerous roof collapses at stepped buildings.
• Using the wrong exposure category: Many buildings that appear sheltered (suburban, trees) actually qualify for partial exposure or even full exposure under ASCE 7 definitions. Using "sheltered" when "partially exposed" applies overestimates pf by the Ce ratio (typically 10-20%).
• Not checking rain-on-snow surcharge: For sites with pg ≤ 20 psf, an additional 5 psf must be added. This 5 psf surcharge can increase the design load by 25-50% at low-snow sites and is frequently overlooked.
• Applying slope reduction to drift loads: The slope factor Cs reduces only the balanced snow load, not the drift surcharge. Drift loads are computed independently from the ground snow load and applied as triangular surcharges regardless of roof slope.
• Using pg = 0 for warm climates: Even in moderate climates, ASCE 7 may specify pg > 0. Sites in Tennessee, Virginia, and North Carolina have pg = 10-25 psf. Assuming pg = 0 without checking the map is an error.
• Forgetting to check both windward and leeward drift: Both drift sources must be computed; the larger one governs. Windward drift uses 75% of hd computed from the lower roof fetch; leeward drift uses 100% of hd from the upper roof fetch.

Frequently Asked Questions

What is the ground snow load for Denver, CO, and how does that convert to a flat roof load? Denver, CO has a ground snow load pg = 30 psf per ASCE 7-22 Figure 7.2-1. For a Risk Category II building with Terrain Category B (partially exposed roof), Ce = 1.0; a heated building with Ct = 1.0; and Is = 1.0: flat roof snow load pf = 0.7 × Ce × Ct × Is × pg = 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21 psf. For a fully exposed roof in Category B (Ce = 0.9), pf drops to 18.9 psf. For unheated storage (Ct = 1.3), pf rises to 27.3 psf with Ce = 1.0.

How does the importance factor Is affect snow load across risk categories? The importance factor Is scales snow loads based on risk category per ASCE 7-22 Table 1.5-2. Risk Category I (low-hazard, minor storage) uses Is = 0.8 — a 20% reduction from baseline. Category II (ordinary occupancy) uses Is = 1.0. Category III (substantial hazard — assembly buildings, schools, utilities) uses Is = 1.1. Category IV (essential facilities — hospitals, emergency response) uses Is = 1.2. Using Is = 1.0 for a Category IV hospital underestimates snow load by 20% relative to the correct value.

Why can snow drift control the design even when the balanced roof load looks modest? Drift loads near parapets, roof steps, and equipment screens can be 2–3× the balanced roof load locally. ASCE 7-22 Section 7.7 drift surcharge height hd is a function of lu (upwind fetch length) and pg. For a 100 ft drift source length with pg = 25 psf, hd ≈ 3.5 ft, giving a drift surcharge of 3.5 × γ ≈ 3.5 × 17.6 = 61.6 psf at the peak — nearly three times the balanced flat roof load of 0.7 × 25 = 17.5 psf. This drift peak governs beam and connection design near the step.

What is the difference between ground snow load and roof snow load? Ground snow load (pg) is the reference value from the ASCE 7 snow map for the building site. Roof snow load (pf or ps) is derived from the ground value by applying exposure factor (Ce), thermal factor (Ct), and importance factor (Is). For flat roofs, pf = 0.7 × Ce × Ct × Is × pg. The 0.7 factor accounts for the statistical tendency of roofs to accumulate less snow than the ground due to wind, heat loss, and roof slope effects.

When does roof slope eliminate snow load? ASCE 7-22 Section 7.4 allows the balanced snow load to be reduced to zero for sufficiently steep warm roofs. For slippery warm roof surfaces, the balanced snow load reaches zero at a roof slope of about 70°. For unobstructed slippery roofs, reduction begins at 15° and reaches zero at 70°. Cold roofs (Ct = 1.3) reach zero reduction only at steeper slopes. Always check whether the roof is classified as warm or cold, and whether the surface is slippery per ASCE 7 definitions before applying slope reductions.

What is an unbalanced snow load and when must it be checked? Unbalanced snow load occurs when wind redistributes snow to the leeward side of a sloped roof or causes drift accumulation near obstructions. ASCE 7-22 Section 7.6 requires unbalanced checks for gable and hip roofs with slopes between 2.39° and 30.2° (1/2 on 12 to 7 on 12). The unbalanced load places a higher snow load on the leeward slope and a reduced load on the windward slope, creating asymmetric demand on the roof framing. This check can govern ridge beam and rafter design even when the balanced load is modest.

Related pages

• Wind load calculator
• Load combinations calculator
• Seismic load calculator
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• Snow load calculation — ASCE 7 design procedure
• Load combinations — ASCE 7 LRFD & ASD reference
• Live load reference — IBC and ASCE 7 occupancy table
• How to verify calculator results
• Disclaimer (educational use only)
• beam analysis for snow-loaded rafters
• structural steel material properties
• EN 1990 load combinations with snow
• CSA load combinations for snow regions

Disclaimer (educational use only)

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