How Snow Particle Size Changes Roof Snow Loads

When snow falls in a winter storm, people often judge the threat to buildings by the depth that accumulates. Engineers monitor snowfall totals closely, as substantial snowfalls can impose immense weight on roofs.

However, new research indicates that the size of each snow particle can affect how much snow ultimately remains on a roof.

Bigger particles are less readily blown away, enabling deeper layers to form once they have settled. In comparison, smaller particles can be lifted and dispersed much more easily.

This overlooked distinction means two storms producing comparable snowfall totals may leave dramatically different roof loads.

Snow particle size determines roof patterns

As wind passes over a roof, it creates irregular snow cover: some particles remain attached while others are carried off.

By examining these distributions, Qingwen Zhang of the Harbin Institute of Technology (HIT) showed that larger snow particles are more resistant to removal by wind after landing, allowing heavier deposits to persist.

Smaller particles, on the other hand, are more readily raised and scattered as the wind speeds up across the roof, resulting in thinner remaining layers.

Consequently, the depth of snow on a roof is determined not just by the volume of snowfall, but by the particles that remain in place long enough to build up.

After snow has settled, airflow over the roof may either hold it down or pull it free. Because larger grains have greater momentum relative to their size, gusts find it more difficult to lift or displace them.

“In cold regions, snow load is a critical factor in structural design,” said Zhang. Engineers use the term snow load for the downward force exerted by snow on a roof.

Wind strength and roof width affect snow loads

High winds do not remove all snow particles uniformly across a roof. Where air accelerates over a roof edge, finer particles become airborne earlier, whereas larger particles better withstand the force.

This contrast becomes more pronounced as winds increase, meaning snow may be stripped from one section during a storm while continuing to accumulate elsewhere. When the snowfall stops, snow depth can therefore depend as much on the particles that remained as on the amount that fell.

The width of the roof makes a difference too. A larger surface provides more space for wind-driven snow to settle and stay put.

On narrower roofs, drifting snow can escape more quickly, while wider roofs hold on to a greater quantity. The strongest effect occurred for particles measuring roughly 0.5 mm, for which the additional area produced the deepest accumulation.

The result indicates that roof form and scale may heighten snow-related hazards in ways that more basic models fail to capture.

Modelling snow loads with greater realism

Modelling an entire winter storm becomes highly costly if a simulation follows every separate snow particle size.

To make the process less demanding, the team investigated whether mean snow particle size could stand in for the complete mix.

Using this one figure reproduced the behaviour of the full particle-size range while requiring far less computing power. For engineers, it provides a workable means of representing complex snow behaviour without the need for enormous simulations.

Reliable modelling is important because building regulations already recognise snow as a significant structural risk. Models that average out differences between particles may overlook areas where wind gathers weight rather than distributing it evenly.

The U.S. Federal Emergency Management Agency (FEMA) has cautioned that snow loads exceeding design limits can make buildings susceptible to failure or collapse.

Findings of this kind could enable building codes to reflect more accurately how storms deposit and remove snow over a roof.

Roof geometry and storms create further complexity

Flat roofs offered the most straightforward conditions for testing, yet the central finding extends beyond uncomplicated structures. Zhang said that snow patterns would probably differ on sloping or curved roofs, although the same size-related particle principles could still hold.

Slopes, ridges and curves alter the direction of airflow, changing the places where snow collects and those where it is swept clear. The findings should therefore be treated as an initial step rather than a definitive solution, particularly for contemporary buildings with complicated roof designs.

Natural storms bring further variables into play. Changes in temperature, melting, adhesion and evolving snow-crystal forms can all affect snow after it lands.

To focus specifically on size, the researchers conducted controlled laboratory experiments using very small, sand-like silica particles. Although the simplified arrangement enabled exact measurements, it excluded some of the variability of real snowfall.

HIT researchers intend to examine curved and sloped roofs next, where airflow changes more abruptly.

These tests should show whether the same modelling shortcut remains effective once roof geometry begins to exert a strong influence on snow accumulation.

Implications for structural designers

For structural designers, a key message may be that treating snow as evenly distributed is an appealing but potentially hazardous assumption.

“Accurately assessing snow loads for structural safety requires considering the natural variation in snowflake sizes, and ignoring this can lead to underestimation of snow accumulation in certain conditions,” said Zhang.

This underestimate may be particularly important for large roofs in exposed, windy areas, where snow distributions seldom stay even for long.

Snow on a roof is as much an issue of where it is deposited as how much falls, with particle size, wind and building geometry all shaping the result.

Improved estimates cannot remove winter hazards, but they can reduce uncertainty before a storm begins.

As builders contend with more severe weather and increasingly varied architectural forms, subtle differences in the behaviour of snow particles could decide whether safety margins are maintained or reduced.