/diy/ - Do It Yourself

Heyyo /diy/nos. I'm peeling off my metal roofing because whoever installed it before I got the house did a shit job and left orientated strand board (osb, the shitty pulp plywood stuff) exposed to moisture so it started to rot ofc. This portion of the roof was built with osb over tongue and groove boards. My problem is as follows:

In an area facing direct sun, the osb looked fine, but I noticed the tongue and groove boards underneath were black between the gaps, so I pulled the osb off to find that the tongue and groove boards were charred. Not rotted, but charred. Only on the upaward facing side nearest the perimeter of the roof, too. Below this section of roof, there is a big hot window next to a glass door that absolutely bakes the hallway in the summer, so I'm thinking maybe that was the cause? I am absolutely baffled by it and not sure how to proceed (yes, I have the same tongue and groove boards handy - just not sure if I should toss them in there without figuring this out).

Thanks!