The core idea
The darkest stain edge may mark where dissolved material was deposited during drying rather than the current wettest location.
How the mechanism works
As water evaporates, salts, tannins and contaminants can migrate toward the drying front and remain after moisture has moved or disappeared.
When it becomes relevant
Typical trigger: A wetting event followed by gradual drying.
Evidence that supports this interpretation
- A ring or tide mark remains after readings fall
- The edge changes position between events
- Deposits concentrate at the perimeter
Evidence that weakens it
- The entire area remains equally wet
- The discoloration is between glazing panes
- The pattern follows a manufacturing seam
What this concept cannot prove alone
A single observation, image or meter reading cannot confirm the complete failure pathway. Use this concept to form better questions, compare alternatives and decide which safe observation would be most informative next.
Safety and professional boundary
Do not use stain color alone to decide whether the source is active or the material is safe to cover.
Primary references for this topic family
These references ground the general building-science or safety context. They do not diagnose a specific house, replace local requirements or certify a repair.
