What part of a traditional wood-joisted roof tends to lose strength gradually when exposed to fire?

The part of a traditional wood-joisted roof that tends to lose strength gradually when exposed to fire is the wooden joists themselves. As the temperature increases, the structural integrity of the wood deteriorates due to charring and thermal degradation. This weakening can compromise the roof's load-bearing capacity, potentially leading to collapse if the exposure is prolonged. Additionally, the insulation materials used can also be affected, exacerbating the loss of strength.