All of the following suggest possible rot in flat or cathedral roofs except?

Mold or mildew on an interior wall surface, while a sign of potential moisture problems, does not directly indicate rot in flat or cathedral roofs. Instead, it may suggest issues such as high humidity, insufficient ventilation, or leaks from plumbing rather than structural damage to the roofing materials themselves.

On the other hand, sagging or spongy roof surfaces are clear indicators that the structural integrity of the roof has been compromised, often due to moisture intrusion leading to rot. Sagging plaster or drywall ceilings can also signify that the roof above is deteriorating, potentially due to water damage. Lastly, the recent addition of vents to the roof could indicate attempts to alleviate moisture issues, which, if not done properly, can sometimes stem from existing damage. Therefore, the presence of mold or mildew is not a direct sign of rot in the roofing structure, making it the outlier in this context.