Steel Roof Framing — Joist Selection, Ponding Check & Snow Drift Loading

Steel roof framing spans between primary frames to support the roof deck, insulation, and roofing membrane. The three main roof framing options are open web steel joists (OWSJ), cold-formed purlins (Z/C sections), and hot-rolled wide-flange beams. The choice depends on span, load, fire rating requirements, and the need for MEP routing through the framing depth. Roof framing design must account for three loading conditions that do not affect floor framing: rain ponding, snow drifts, and wind uplift.

Roof framing system comparison

System Typical span Depth Advantages Limitations
Open web steel joist (K-series) 20–60 ft 10–30 in Light, long spans, MEP routing through open webs Cannot support concentrated loads, limited connection options
Open web steel joist (LH-series) 40–96 ft 20–48 in Very long spans for roofs Heavy, require joist girders for support
Z/C purlins 15–35 ft 6–12 in Economical for metal building roofs, fast erection Limited to light loads, CFS design required
Hot-rolled W-beams 20–50 ft 12–24 in Supports concentrated loads, compatible with connections Heavier than joists for same span

Open web steel joist design

Steel joists are designed per SJI (Steel Joist Institute) standard specifications, not AISC 360. The engineer of record specifies the joist designation, loads, and span; the joist manufacturer designs the internal members. The EOR must specify:

K-series joists: Economical for spans to 60 ft with uniform loads. Designation example: 26K10 = 26 in deep, K-series, load table row 10.

Joist Span (ft) Safe uniform load (plf) Deflection at 240 plf (in)
18K5 30 288 0.92
22K7 35 302 1.04
26K10 45 316 1.14
30K12 50 346 1.28

(Values approximate — refer to SJI load tables for exact capacities.)

Critical joist specification requirements:

Snow drift loading (ASCE 7-22 Section 7.7–7.9)

Snow drifts form where wind carries snow from an upper (windward) roof to accumulate against a lower adjacent parapet, roof step, or obstruction. ASCE 7-22 Chapter 7 provides the drift surcharge calculation:

Drift height (leeward step):

hd = 0.43 × (lu)^(1/3) × (pg + 10)^(1/4) - 1.5

Where lu = length of the upper roof (ft) and pg = ground snow load (psf). The drift surcharge is a triangular load with peak intensity:

pd = gamma × hd    (where gamma = snow density = 0.13pg + 14 ≤ 30 pcf)

The drift extends a horizontal distance of 4hd from the step or obstruction.

Worked example — snow drift at roof step

Given: Two-level building. Upper roof: lu = 150 ft, lower roof: ll = 80 ft. pg = 40 psf. Roof step height hr = 6 ft.

Step 1 — Snow density: gamma = 0.13 × 40 + 14 = 19.2 pcf.

Step 2 — Leeward drift height: hd = 0.43 × 150^(1/3) × (40 + 10)^(1/4) - 1.5 = 0.43 × 5.31 × 2.66 - 1.5 = 6.07 - 1.5 = 4.57 ft. Check: hd ≤ hr (drift cannot exceed step height without balanced snow on lower roof). 4.57 < 6.0 — OK, drift is not truncated.

Step 3 — Drift surcharge intensity: pd = 19.2 × 4.57 = 87.7 psf (peak, triangular distribution). Drift length = 4 × hd = 4 × 4.57 = 18.3 ft from the wall.

Step 4 — Total load on lower roof beam at drift zone: Balanced snow on lower roof: pf = 0.7 × Ce × Ct × Cs × Is × pg = 0.7 × 1.0 × 1.0 × 1.0 × 1.0 × 40 = 28 psf. Total at wall: 28 + 87.7 = 115.7 psf — nearly 3× the balanced snow load. This surcharge governs the design of roof beams, joists, and purlins within the drift zone.

Ponding stability (AISC 360-22 Appendix 2)

Ponding is the progressive accumulation of rainwater on a flat or near-flat roof. If the roof deflects under water weight, it collects more water, which causes more deflection — a positive feedback loop that can lead to collapse. AISC 360-22 Appendix 2 provides the ponding stability criterion:

Cp + 0.9 × Cs ≤ 0.25    (stability criterion)

Where:

Cp = 504 × Lp^4 / (Ip × 10^7)    (primary member flexibility)
Cs = 504 × Ls^4 / (Is × 10^7)    (secondary member flexibility)

Lp, Ls = primary and secondary member spans (in), Ip, Is = moments of inertia. If the criterion is not satisfied, the roof is ponding-unstable and members must be stiffened, the roof slope increased, or secondary drainage provided.

Rule of thumb: A minimum roof slope of 1/4 in per foot (1:48) with properly located secondary (overflow) drains typically avoids ponding instability for spans under 50 ft.

Wind uplift on roof framing

Wind uplift (negative pressure on the roof surface) can exceed gravity load, putting roof framing into net upward force. Per ASCE 7-22 Figure 30.3-2A (for low-rise buildings), roof pressure coefficients for components and cladding:

Roof zone GCp (negative, uplift) Typical at V = 115 mph
Interior (Zone 1) -1.0 to -1.4 20–30 psf uplift
Edge (Zone 2) -1.7 to -2.3 35–47 psf uplift
Corner (Zone 3) -2.5 to -3.2 51–65 psf uplift

When net uplift exceeds the dead load, the roof framing and its connections must resist the difference in tension. Joist seat welds, purlin-to-frame clips, and anchor bolts at bearing walls must all be checked for uplift.

Code comparison

ASCE 7-22 + AISC 360-22 + SJI (USA): Snow loading per ASCE 7 Chapter 7. Ponding per AISC 360 Appendix 2 or SJI Technical Digest 3. Joist design per SJI Standard Specification (K, LH, DLH series). Wind uplift per ASCE 7 Chapter 30.

AS 1170.3 / AS 4100-2020 (Australia): Snow loading per AS 1170.3 (limited to alpine regions; most Australian roofs are not snow-loaded). Roof live load per AS 1170.1 (0.25 kPa minimum for access, 0.12 kPa for non-trafficable roofs). Ponding is addressed by mandating minimum roof slope (1:40 recommended) rather than a structural stability check. Wind loading per AS 1170.2, which uses regional wind speed maps and aerodynamic shape factors.

EN 1991-1-3 / EN 1993-1-3 (Eurocode): Snow loading per EN 1991-1-3, which uses characteristic ground snow load sk, exposure and thermal coefficients, and shape coefficients for drift. Drift provisions differ from ASCE 7: Eurocode uses a mu coefficient approach (mu_1 for balanced, mu_2 for drift shape factor). Ponding is addressed in EN 1991-1-3 Section 5.4 (requirement for adequate roof drainage and slope). CFS purlin design per EN 1993-1-3.

Common mistakes engineers make

  1. Omitting snow drift loads at parapets and roof steps. Snow drift surcharge can triple the balanced snow load within the drift zone. Ignoring drifts is the single most common cause of roof framing failure in snowy climates.

  2. Specifying concentrated loads on standard K-series joists. K-series joists are designed for uniform loads only. Point loads (from rooftop units, dunnage beams, suspended loads) require special joist design marked "SP" on the schedule. Applying concentrated loads to a standard joist can cause web member buckling.

  3. Not checking ponding stability on flat roofs. Engineers sometimes assume "the drains will handle it." If primary drains are blocked (debris, ice), water accumulates. Without adequate slope, secondary drains, and structural capacity for the ponding load, progressive collapse can occur. The 1999 collapse of the Martin Luther King Jr. civic center in New York was a ponding failure.

  4. Neglecting uplift anchorage at roof edge zones. ASCE 7 corner and edge zones have 2–3× the uplift pressure of interior zones. Standard joist seat welds designed for gravity may be inadequate for the net uplift at building corners. Supplemental anchorage (tie-down rods, welded angles) is required.

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Disclaimer

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.