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Snow Load: How Much Snow Falls on a Roof under TS 498?
Ground snow load is read from a table; roof snow load depends on the roof's shape. On an arched roof it can exceed twice the flat-roof assumption. And the standard's own text contains a typesetting error that, read literally, nearly cancels the load.
1. Ground snow load
TS 498:2021 divides Turkey into snow zones and gives the characteristic ground snow load Sk for each zone as a function of altitude (Table 3). The 81 values in the site's code pack were checked by machine against the official text of the standard. The table gives no interpolation: if the altitude falls between two rows, the next higher row is used. Above 1000 m the values are increased.
| Snow zone | Altitude (m) | Sk (kN/m²) | Note |
|---|---|---|---|
| 1 | 500 | 0.75 | — |
| 2 | 500 | 0.75 | — |
| 4 | 1000 | 1.20 | — |
| 4 | 1500 | 1.32 | above 1000 m: ×1.10 |
| 6 | 1500 | 1.49 | above 1000 m: ×1.10 |
2. From ground to roof: the shape coefficient
The roof snow load is the ground snow load times a shape coefficient: S = μ·Sk. As the slope increases snow slides off and μ decreases (TS EN 1991-1-3 Table 5.2): 0.8 up to 30°, decreasing linearly to 0 between 30° and 60°, and 0 above 60°. For zone 4 at 1000 m (Sk = 1.20 kN/m²), with the site's engine:
| Roof | μ | S = μ·Sk (kN/m²) |
|---|---|---|
| Flat roof or α ≤ 30° | 0.8 | 0.96 |
| α = 45° | 0.4 | 0.48 |
| α = 60° | 0.0 | 0.00 |
| Arched roof, h/b = 0.15 | 1.7 | 2.04 |
3. Arched roofs: more than twice
The last row of the table is the most important. On arched (cylindrical) roofs snow slides from the crown and accumulates at the sides; TS EN 1991-1-3 Clause 5.3.5 gives a separate coefficient: μ3 = 0.2 + 10·h/b ≤ 2.0 (if the maximum slope does not exceed 60°). For an arch with h/b = 0.15, μ3 = 1.7 and the snow load is 2.04 kN/m² — more than twice the flat-roof assumption.
Sports halls, covered markets and hangars often have arched roofs. Before a 2026 audit, this site's snow tool also treated an arched roof as flat with μ = 0.8, showing less than half the snow load. It was corrected in the audit.
4. Snow and wind together
TS 498 Clause 12.1 defines two cases for the combined action of snow and wind on roofs up to 45° and requires the less favourable to be used: a) S + W/2 (full snow, half wind) and b) W + S/2 (full wind, half snow).
5. Points to watch
- Drifting: on adjacent roofs of different heights, next to parapets and rooftop equipment, wind-driven snow accumulates. The simple shape coefficient does not cover this.
- Snow-free places: per the table footnote, the design value may be taken as zero where snow does not fall; this decision must rest on local knowledge.
- Intermediate altitude: using the next higher row is a conservative reading; since the standard gives no interpolation, the engine states this explicitly.
References
- TS 498:2021 — Table 3 (ground snow load), Clause 11, Clause 12.1 (snow and wind). Values from the site code pack (data/code-packs/TR/ts498-2021-loads.json); the 81 snow values were checked by machine against the official text (docs/TS498-DENETIM-RAPORU.md).
- TS EN 1991-1-3 — Table 5.2 (μ1), Clause 5.3.5 (cylindrical roofs, μ3).
- archi-civil.com, correction proposals for TS 498:2021 (standart-notlari/ts498-2021.html).
The numbers in this article were produced by the site's engines; code values are given with their clause numbers.