Snow Load for Roof Purlins Explained

Lightweight roof purlins are highly sensitive to snow loads; therefore, designs must account for the most unfavorable scenarios involving non-uniform snow distribution. Below is a brief overview of common unfavorable snow load conditions:

1. Single-span gable roof

Load characteristics: As wind blows across the roof, snow is swept away from the windward side and accumulates significantly on the leeward side (leeward slope), resulting in a non-uniform distribution. The snow load on the leeward side typically ranges from 1.25 to 2.0 times the uniform snow load (specific values ​​depend on roof slope and code-specified coefficients).

Design considerations: Verification calculations must consider unfavorable distribution scenarios for both the left and right halves of the span; design based solely on a uniform full-span distribution is insufficient.

2. Multi-span gable roof (two or more spans)

Load characteristics: The concave valley formed at the junction of two spans is the area of ​​greatest snow concentration. Wind-blown snow deposits in the valley, while snow sliding from the adjacent roof slopes also converges there, creating a distinct triangular or trapezoidal accumulation zone.

Design considerations: Purlins near the valley require special reinforcement; the snow load in this area can range from 2.0 to 2.5 times the basic snow pressure (S₀), depending on roof slope, span ratio, and wind direction. It is recommended to reduce purlin spacing or increase section sizes.

3. Gable roof with parapets

Load characteristics: Parapets obstruct airflow, causing snow to pile up at the base of the parapet and form a triangular snowdrift. The accumulation zone is triangular, with peak load occurring at the wall base—typically taken as 2.0 × S₀—and tapering to zero at the far end.

Design considerations: Purlins within 2 to 4 meters of the parapet require reinforcement. The length of the accumulation zone is calculated using formulas specified in the code, based on parapet height and basic snow pressure.

4. Gable roof with skylights

Load characteristics: Skylights protruding from the roof alter local wind patterns and snow sliding paths. Snow accumulates on the windward side of the skylight (the side facing the wind), while the skylight structure itself obstructs the downward sliding of snow. Design considerations: Purlins within a range of approximately 2–3 m on either side of the skylight should be verified against increased snow loads. If the skylight projects significantly above the roof surface (≥ 1 m), the accumulation effect becomes more pronounced.

5. Snow accumulation on stepped roofs (sliding accumulation)

Load characteristics: Snow slides from the high roof onto the low roof under the influence of gravity, forming an accumulation zone on the low roof adjacent to the high wall. This is a gravity-driven load effect.

Design considerations: The length of the accumulation zone is generally taken as twice the height difference (not exceeding 15 m), with a triangular distribution; the peak value at the base of the wall is approximately 2.0 × S₀, tapering to zero at the far end. Purlins on the low roof near the high wall require specific verification.

6. Snow drifting on stepped roofs (wind-induced drifting)

Load characteristics: Wind lifts snow from the high roof or open ground and deposits it on the low roof near the high wall. Unlike sliding accumulation, this is a load effect driven entirely by the wind.

Design considerations: The length of the drift zone is typically calculated as 5–10 times the snow drift height; the load distribution is trapezoidal or triangular, with the peak at the base of the wall. It is important to note that sliding accumulation and snow drifting loads should not be superimposed; the greater of the two values ​​is adopted as the design condition.

7. Snow accumulation around roof projections

Load characteristics: Projections (such as ventilation ducts, equipment bases, exhaust vents, outdoor AC units, etc.) obstruct snow sliding and wind scouring, causing localized snow accumulation on the windward side of the projection.

Design considerations: Purlins within a range of approximately 2–4 m on either side of the projection require reinforcement. The extent and height of accumulation depend on the dimensions of the projection and the basic snow pressure; values ​​should be determined based on the local accumulation coefficients specified in the code.

Each load case must be verified individually, with the most unfavorable result serving as the basis for the final design. For lightweight roof purlins, particular attention must be paid to reinforcement in localized accumulation zones (valleys, bases of parapets, and areas around skylights); these details must not be overlooked or oversimplified.