Snow load in Hungary

Where the value comes from

The base value of the snow load comes from the snow zone, set by the map in the national annex. The further factors build on it:

• the shape coefficient, depending on the pitch and form of the roof, because snow slides off a steep roof,
• exposure: how far the wind blows the snow off,
• thermal effects: the insulation of the roof affects melting.

What is most often underestimated: drifting

Snow does not lie evenly. In certain places it drifts, giving a substantially larger load there:

• at a roof step, where a lower roof sits beside a taller part of the building,
• at an abutment to a wall,
• behind rooftop structures, chimneys and plant units,
• in valleys, where roof planes meet,
• behind a parapet or snow guard.

At these places the local load can be several times the uniform value. On a light gauge steel roof that matters particularly, because there is little reserve from self-weight.

Why this matters in light gauge steel

On a heavy roof structure the snow is a smaller load relative to the self-weight of the structure. On a light roof the snow can exceed the weight of the structure — in other words the governing load case is the snow-covered state.

That is not a problem in design, but it means taking the snow load accurately is critical.

The practical consequences

Roof form is a structural decision

A complex, many-planed roof means not only more connections but more places for drifting.

The question of extensions

If a taller part is built beside an existing building, a new drift location arises on the old roof. That affects the existing structure, not only the new one.

A snow guard is not neutral

It stops snow sliding off, and so increases the load remaining on the roof.

What the design should record

The snow zone taken, the shape coefficient and the drift cases considered. That information is crucial at a later alteration or extension.

The point

It is not the uniform snow that is dangerous but the snow that drifts. The roof form and the neighbouring parts of the building together determine where that surplus arises.