Based on the distinct characteristics visible on the lumber in the photo this is a classic case of Brown Rot (specifically Cubical Brown Rot).
Here is exactly what gives it away:
- Cubical Cracking (Cuboidal Fracturing): Notice how the wood has checked and split into deep, blocky, square-like chunks across the grain. This happens because the wood-destroying fungi targeted and consumed the cellulose, causing the wood to shrink rapidly and crack in a grid pattern.
- The Brittle, Brown Appearance: Because the fungi break down the light-colored cellulose but leave the dark lignin behind, the wood changes to this distinct, deep brown colour and loses all its structural integrity - essentially turning it into brittle charcoal.
- White Fungal Mycelium: You can clearly see faint white, web-like streaks running along the cracks. Those are the mycelium strands (the root system of the fungus) actively spreading to find new wood to feed on.
Given that this piece came out of a humid, trapped environment, it is the textbook result of long-term dampness meeting poor ventilation. It's a good thing this structural nightmare has been pulled out and replaced with pressure-treated lumber!
