Which visual cue is a strong indicator that a roof may fail?

A strong visual cue that indicates a potential roof failure is the appearance of excessive snow accumulation. When a roof has too much snow, it can lead to increased weight on the structure, putting undue stress on the rafters and trusses. Firefighters need to be aware of this because roofs have specific weight limits, and exceeding these limits can compromise the integrity of the roof, leading to a sudden failure.

In contrast to the other options, excessive snow accumulation poses a significant risk since it can be difficult for the fire personnel to assess the condition of the roof beneath. Therefore, being vigilant about this particular visual cue is crucial for safety during firefighting operations and when assessing the structural stability of a building.