Navigating the Nuances of Snow Loads: A Practical Look at EN 1991-1-3

Let’s be honest, when most of us think about structural loads, we jump straight to dead loads, live loads, and wind. Snow load often gets filed under “that thing the designer puts in the calculations,” and we assume it’s a simple blanket number. That’s where the confusion with EN 1991-1-3 starts. It’s not just about “how much snow”; it’s about where the snow ends up, how it drifts, and what happens when it partially melts and refreezes. The standard provides the framework, but its real-world application is highly sensitive to the specific geometry of the building and the local micro-climate. The gap between the code’s maps and the reality on your site is where mistakes happen.

 When Do Projects Actually Need to Consider EN 1991-1-3 Seriously?

You might think, “We’re in a moderate climate, we get 2cm once a year, can we skip the deep dive?” The short answer is no, and here’s why. The standard isn’t just for alpine resorts. Its critical role is in defining exceptional snow loads—those rare, once-in-50-year events that can collapse a structure that’s been fine for decades. The need for serious consideration spikes in a few key scenarios:

1. Low-Slope Roofs (≤ 5°): This is drift formation central. Snow doesn’t slide off; it just sits and accumulates, especially if the roof has parapets or adjacent higher sections.
2. Multi-Level Roofs and Roof Obstructions: Any change in roof height, or the presence of a large HVAC unit, solar array, or parapet wall, creates a wind shadow. EN 1991-1-3 gives you the tools to calculate the drift that forms in these leeward areas, which can be 3-4 times the ground snow load. Missing this is a classic error.
3. Slippery Roofs vs. Non-Slippery Roofs: The standard differentiates between materials that allow snow to slide (e.g., glass, metal sheeting) and those that don’t (e.g., rough concrete). This directly impacts whether you have to consider snow sliding onto lower roofs or into gutters, which is a massive load consideration for canopies, entrances, and multi-story atriums.
4. Industrial Sheds and Large Span Structures: The load distribution is not uniform. Wind can scour snow from the windward side and deposit it on the leeward side, creating an unbalanced load case that is often more critical than the uniform load for the design of frames and connections.

What Are the Most Common Mistakes We See on Site and in Design?

This is where the rubber meets the road. I’ve seen these crop up repeatedly in design reviews and during construction inspections:

Treating the Snow Load Map Value as Gospel: The national annex gives you a characteristic snow load value for the ground, s_k*, at a given location. The biggest mistake is using this value directly as the roof load. You must apply the shape coefficients (μ) from the standard, which account for roof slope, wind exposure, and thermal conditions. A 30° roof in an exposed area will have a much lower load than the ground snow load, while a sheltered valley next to it might have a higher one.
Ignoring Local Drifting Due to Adjacent Structures: The design might be perfect for the building in isolation. But if a taller building is planned 10 meters upwind, the snow drift pattern on your roof changes completely. This is often missed in phased developments or urban infill projects. You need to consider both the current and* the foreseeable future surrounding context.
Overlooking Exceptional Drifts at Roof Steps: The standard provides a relatively simple triangular shape for drift loads at roof steps. The mistake is not considering the sheer volume* and length of that drift for a long building. The load intensity is high, and it acts over a significant area, putting immense stress on purlins, rails, and their connections. I’ve seen purlin-to-rafter connections fail in these zones because they were designed for uniform load only.
* Forgetting About Snow Guards and Barriers on Slippery Roofs: If the architectural specs call for a sleek, standing-seam metal roof (a “slippery” surface), the calculations will show snow sliding off. But what does it slide onto? Lower roofs, parking canopies, or pedestrian walkways? The design must then include snow guards or specify the lower structure for the sliding snow load. Often, this coordination between architect, structural engineer, and facade contractor happens too late.

How Does EN 1991-1-3 Differ from Older, More Traditional Methods?

Many engineers who started with older national codes find the Eurocode approach more prescriptive in some ways and more conceptual in others.

* It’s More Geometrically Driven: Older codes sometimes gave you a simple multiplier. EN 1991-1-3 forces you to engage with the actual shape of the building. You’re not just picking a number; you’re walking through a decision tree based on roof pitch, presence of obstacles, and thermal properties. It links the load directly to the physics of snow deposition and wind action.
* The Concept of “Load Cases”: This is a fundamental shift. You don’t design for one snow load. You design for multiple load cases that must be considered simultaneously with other actions (like wind). For instance, you must check:
* Case 1: Uniform distribution.
* Case 2: Unsymmetrical distribution (wind-scoured on one side, drifted on the other) for duo-pitch roofs.
* Case 3: Drift at roof steps or obstacles.
The structure must be safe for all these configurations. The old way often just looked at a single, symmetrical load.
* Emphasis on Accidental Load Case for Exceptional Snowfall: The standard formally defines an accidental design situation for exceptional snow loads, which has a different partial safety factor (typically γ = 1.0). This acknowledges that for these extreme, rare events, we accept a different level of risk compared to the persistent design situation. Older codes sometimes baked this into a single, higher factor, which was less transparent.

In essence, EN 1991-1-3 moves you from being a clerk looking up a table to being an analyst modeling a environmental process on your structure. It demands that you visualize the wind flow over and around the building, understand where snow will settle and where it will be removed, and then translate that physical reality into meaningful load diagrams. The goal isn’t just compliance; it’s building a structure that behaves predictably under real snowfall, not just on paper.