Can a roof access hatch fall shut?

No, a roof access hatch cannot fall shut if this is opened completely.

All roof access hatches are equipped with a security device to prevent them from falling shut. However, the security device for a manually controlled roof access hatch is different from the security device for electrically operated roof access hatches. 

Manually controlled roof access hatch

Manually controlled roof access hatches are equipped with two gas pressure springs. If the roof access hatch is opened fully, the gas pressure spring slots automatically into a red security bracket. This prevents the roof access hatch from falling shut under the weight of people, snow, etc. When leaving the roof, the bracket must first be pushed in in order to disable the security device.

Electrically operated roof access hatch

Electrically operated roof access hatches will only move if the controls are used.