In winter, snow lies on roof windows and rooflights, and on the façade the wind pushes and pulls. Both affect how strong a window actuator must be and when it is better to keep it closed. Here you can see how to take snow and wind loads into account when selecting an actuator.
In brief: Snow on the sash has to be lifted when opening. As a rough estimate, add the snow load (N/m² × sash area) to the sash weight and use the familiar approximation F ≈ 0.54 × load. Wind is mainly a question of closing safely: a wind and rain sensor closes open windows in good time.
How snow increases the force required
With a rooflight or a flat roof sash, the actuator lifts almost the entire weight. If snow lies on it, its weight is added. You know the basic formula from our guide Calculate thrust: F ≈ 0.54 × G. With snow it becomes F ≈ 0.54 × (G + S), where S is the snow load on the sash in newtons.
Worked example: rooflight 1.2 × 1.2 m
| Snow load | Load on 1.44 m² | Calculation | Force without reserve |
|---|---|---|---|
| no snow | 0 N | 0.54 × 392 N | approx. 212 N |
| 250 N/m² | 360 N | 0.54 × (392 + 360) N | approx. 406 N |
| 500 N/m² | 720 N | 0.54 × (392 + 720) N | approx. 600 N |
| 1000 N/m² | 1440 N | 0.54 × (392 + 1440) N | approx. 989 N |
A sash weight of 40 kg (392 N) is assumed. Even a moderate layer of snow doubles the force required. Also plan a clear reserve. For rooflights with snow load, a Linear actuator VS-LA with 600 to 5,000 N is usually the safe choice.
Which snow load should be assumed?
For structural design in Germany, DIN EN 1991-1-3 with the National Annex applies. Germany is divided into snow load zones, and the ground snow load (sk) increases with the zone and the altitude. On the roof, s = μ × sk applies, and for roof pitches up to 30° the shape coefficient is μ = 0.8. An example: in snow load zone 2, the minimum value is sk = 0.85 kN/m². On a flat roof this gives around 0.68 kN/m², i.e. about 680 N/m².
For the North German Plain, the National Annex also specifies an exceptional snow load. The exact values for your location are determined by the structural engineer. For smoke ventilation devices, the snow load is stated as class SL in the declaration of performance, see smoke ventilation standards.
For everyday ventilation: with a thick layer of snow, a ventilation window should not open at all. If the sash becomes too heavy, the Load cut-off protects the actuator from overload.
Wind load: keep closed and close in good time
On a closed window, wind acts as pressure or suction. The sash must then be held securely by the locking or the actuator. Check the actuator’s holding force in the data sheet. With open sashes, a gust can put heavy strain on the sash. That is why a Wind-rain-light sensor should automatically close open windows above a set wind speed.
For building design, DIN EN 1991-1-4 sets the wind loads by wind zone, and they are higher on the coast than inland. For smoke ventilation devices, class WL describes which wind load the closed device withstands. For exposed façades, actuators with a high protection rating are recommended, such as the VS-ZA rack-and-pinion actuator with IP65.
Checklist for roof and façade
- Determine sash weight and area
- Take the snow load from the structural design or fire safety concept
- Estimate the force with F ≈ 0.54 × (G + S) and add a reserve
- Check holding force and protection rating in the data sheet
- Provide a wind and rain sensor for ventilation windows
Frequently asked questions
May a ventilation window open in snow?
Better not. In snow, the sash can be overloaded and snow can fall into the building. For ventilation, the window should stay closed when there is snow.
Does a stronger actuator help against wind?
Only to a limited extent. More important are sufficient holding force, secure locking and a sensor that closes the window in wind.
Which snow load class do I need for smoke ventilation?
This is specified in the fire safety concept or the tender. The smoke exhaust device must meet the required class SL.
Not sure which actuator is enough? Send us the dimensions, weight and snow load to be assumed via the Project enquiry. We check the force and suggest the right actuator.