Snow loads
Roof snow loads to ASCE 7-22 Chapter 7: the flat and sloped (balanced) roof snow loads, the minimum load for low-slope roofs, the rain-on-snow surcharge, the unbalanced load of hip and gable roofs, drifts at a higher roof or an adjacent structure, at parapets and at roof projections, sliding snow from an upper roof and ice dams on overhangs. ASCE 7-22 snow loads are strength-level loads: the load combinations use 1.0S in LRFD and 0.7S in ASD (calcs/loads.py). Copy the results into the member calculations as load type S.
Inputs of the snow loads #
- Ground snow and site: the risk category (it selects the ground snow load map and the minimum load limit pm,max of Table 7.3-4), the ground snow load pg for that risk category (7.2: the ASCE Hazard Tool, Figs. 7.2-1A to D, Table 7.2-1 for Alaska), the winter wind parameter W2 (Fig. 7.6-1 or the Hazard Tool), the surface roughness (26.7, plus the two special rows of Table 7.3-1) and the exposure of the roof (fully exposed, partially exposed, sheltered, per the Table 7.3-1 footnotes: a parapet taller than the balanced snow or large equipment makes a roof partially exposed). Ce comes from Table 7.3-1 or is entered.
- Thermal condition: heated with an unventilated roof (Table 7.3-3 by pg and the roof's thermal resistance Rroof, linearly interpolated as note a permits; Rroof above 50 gives 1.2, note b), cold ventilated or unheated (1.2), freezer (1.3), continuously heated greenhouse (0.85), or Ct entered (Table 7.3-2).
- Roof: flat or monoslope, gable or hip; the slope θ; the width across the ridge (W, the eave to ridge distance, is half of it for gable and hip roofs and the full width for a monoslope); the length; an unobstructed slippery surface (7.4: metal, slate, glass, smooth membranes, with room below the eaves for the sliding snow and no snow guards); simple prismatic rafters from eave to ridge (7.6.1: with W ≤ 20 ft the unbalanced load is pg on the leeward side); the eave overhang and whether an unventilated roof drains over its eaves (7.4.4).
- Parapet and roof projection: the parapet height above the roof and the roof length upwind of it (7.8); a roof projection (penthouse, equipment, screen wall) with its height, the length of its side (no drift under 15 ft) and the clear opening under it (no drift with 2 ft clear above the balanced snow), and the greater roof length upwind or downwind of it. Windward drift taller than hc: 7.7.1 gives the windward drift width as eight times the windward drift height but says nothing for a windward drift cut off at the clear height hc. The engineer chooses: height hc and width 8hc (as the AISC Design Examples v16.0, Part III, Table III-1), or height hc with the full width 8 × 0.75hd (conservative).
- Higher roof or adjacent structure: the height of the step h (to the upper roof edge, including any parapet), the upper roof length upwind of the step, this roof's length from the step (the fetch of the windward drift, and the 0.6 cap of the leeward drift), and the horizontal separation s (0 for the same structure; 7.7.2 for adjacent structures). Sliding snow from the upper roof: its slope, eave to ridge distance W, slipperiness and its own Ce and Ct (Table 7.3-1, Tables 7.3-2 and 7.3-3 for that roof).
Checks performed #
| Check | Basis |
|---|---|
| Balanced load | pf = 0.7 Ce Ct pg (Eq. 7.3-1); ps = Cs pf (Eq. 7.4-1); Cs from Fig. 7.4-1(a) for Ct ≤ 1.1, (b) for 1.1 < Ct < 1.2, (c) for Ct ≥ 1.2, the dashed lines for unobstructed slippery roofs of (b) and (c), written as the straight lines of Commentary C7.4; γ = 0.13pg + 14 ≤ 30 pcf (Eq. 7.7-1); hb = ps / γ |
| Minimum load | 7.3.3: pm = pg ≤ pm,max (Table 7.3-4) for monoslope, hip and gable roofs under 15°, a separate load case |
| Rain-on-snow | 7.10: 8 psf on the balanced case where 0 < pg ≤ pm,max and θ < W/50 |
| Partial loading | 7.5: stated with 0.5ps for the member analysis; gable members from 1/2 to 7 on 12 exempt |
| Unbalanced (hip, gable) | 7.6.1, Fig. 7.6-2: between 1/2 and 7 on 12, 0.3ps windward, ps leeward plus hd γ/√S over 8hd√S/3 from the ridge (hd from Eq. 7.6-1 with lu = W; the extent is limited to W); short simple rafters: pg leeward, windward unloaded |
| Drift at a roof step | 7.7.1: leeward hd from the upper roof length (≤ 0.6 of the lower roof length), windward 0.75hd from the lower roof length; w = 4hd, or 4hd²/hc ≤ 8hc with height hc; windward w = 8 × its height; none where hc/hb < 0.2; the larger height governs and the other is reported; a drift wider than the lower roof tapers to zero at its far end |
| Adjacent structure | 7.7.2: leeward drift only for s < 20 ft and s < 6h, height ≤ (6h − s)/6, width min(6hd, 6h − s); the windward drift truncated in the separation (Commentary Fig. C7.7-4) |
| Parapet, projection | 7.8: windward drift 0.75hd with lu as defined there; the projection exceptions |
| Sliding snow | 7.9: 0.4 pf W per foot of eave over 15 ft (less over a narrower lower roof), slippery upper roofs over 1/4 on 12, others over 2 on 12; separated structures with h/s > 1 and s < 15 ft over 15 − s |
| Ice dams | 7.4.4: 2pf on the overhang of an unventilated roof with Ct ≤ 1.1 draining over its eaves |
| Low ground snow | 7.2 exception: stated (pg ≤ 10 psf with lu ≤ 100 ft, or ≤ 5 psf with lu ≤ 300 ft); the loads are always shown |
The type is an analysis: a result reads Calculated. The drawings show the roof section with the balanced load, the unbalanced load of a gable or hip roof, each drift on its lower roof (load band from ps + pd at the step to ps at the drift width, the step height and the drift width dimensioned; a long lower roof cut with a break line) and the sliding load. The roof is drawn schematically one foot deep; the framing is not part of the calculation.
Limitations #
- Not covered: curved, sawtooth, folded plate, barrel vault and dome roofs (7.4.2, 7.4.3, 7.6.2 to 7.6.4), air-supported structures (7.4.5), intersecting drifts (7.7.3), roof steps in series (Commentary C7.7), ponding (7.11 and Chapter 8), open-frame equipment structures (7.13), wind tunnel or flume studies (7.14).
- One drift of each kind per calculation (one step, one parapet, one projection); run a calculation per location.
- Fig. 7.4-1(a) carries a 5° mark with no curve: there is no slippery line for Ct ≤ 1.1 (Commentary C7.4: the ice dam at the eave obstructs sliding), so the unobstructed slippery input has no effect there.
- The density of a lower roof drift uses the ground snow load entered (the roof the drift sits on).
- Sliding snow is not capped by the upper roof's height above the lower roof (7.9 has no such cap).
Verification examples #
Verification status: 59 values compared in 14 examples (9 published, 5 hand calculations from the cited equations): 55 within tolerance, 4 recorded deviations. Code basis: the provisions it uses were read in the target edition.
Published worked examples to ASCE 7-22:
| Example | What it checks | Published | ILX Calcs |
|---|---|---|---|
| AISC-III-ROOF | AISC Design Examples v16.0 Part III roof: pg 30 psf, Rroof 40, partially exposed C (pp. III-5, III-6) | Ct 1.17, pf 24.6, pm 30, ps + 8 = 32.6 psf, γ 17.9 pcf, hb 1.37 ft | same |
| AISC-III-1 | Table III-1 drifts: side parapet (lu 121 ft), end parapet (211 ft), both 2 ft, 6 ft screen wall (60.5 ft) | 0.630 ft, 11.2 psf, 5.02 ft (both parapets); 2.37 ft, 42.4 psf, 18.9 ft | same (within 0.3 %) |
| C7-EX1 | ASCE 7-22 Commentary Example 1: gable 6 on 12, Rroof 30 | Ct 1.14, pf = ps 23.9 psf, hd 2.46 ft, surcharge 31.1 psf over 9.3 ft | same |
| C7-EX2 | Commentary Example 2: fully exposed B, Rroof 20, pg 27.5, slope 41° (the curved-roof distribution is not computed) | Ct 1.07, pf 18.5 psf, Cs 0.72, ps 13.3 psf | same |
| C7-ROS | Commentary C7.10 rain-on-snow example, 1/4 on 12, 100 ft, risk category IV | ps 24.6, 32.6 with rain-on-snow, pm 32 psf | same |
| C7-EX3 | Commentary Example 3: upper roof (Ct 1.13, pf 28.5) and lower unheated roof with a 10 ft step | pf 27, pm 25, hc 8.6 ft, pd 64.8 psf, peak 92 psf | same; hb, the drift heights and the drift width differ by 4 to 5 % because the example takes the density from the upper roof's ground snow load (deviation ASCE7-C7-EX3-GAMMA) |
Hand calculations in closed form:
| Example | What it checks | Hand calculation | ILX Calcs |
|---|---|---|---|
| H1 | Sliding from a 30° slippery upper roof, W = 20 ft | pf upper 30.8 psf, 246.4 plf, 16.43 psf over 15 ft, total 50.03 psf | same |
| H2 | Adjacent structure 6 ft away, h = 12 ft, upper roof 200 ft, pg 50 psf | leeward 5.83 ft, width 34.98 ft, pd 119.5 psf; windward truncated in the gap 2.68 ft | same |
| H3 | Gable 4 on 12, W = 18 ft, simple rafters | leeward pg = 30 psf, windward 0 | same |
| H4 | Table 7.3-3 interpolation (Rroof 45, pg 25) and Fig. 7.4-1(b) slippery line at 30° | Ct 1.1875, Cs 0.667, pf 20.78, ps 13.85 psf | same |
| H5 | 3 ft step, 300 ft upper roof, 10 ft lower roof | hc 1.663 ft, drift cut at hc, width tapered to 10 ft, pd 29.76 psf | same |