Replacing wood on sides of garage flat roof

Hi Gary, I believe the rotting wood bordering the roof would be your fascia and soffit. To answer your question no you would not need to redo your roof to replace this. What is quite common is for cap-it board to be placed over the timber to hide the old wood, however although a budget friendly option for some my personal recommendation on a job where the timber is too far gone would be to fully remove the the timber (and guttering if there is guttering on this side) and any nails screws or debris without damaging the over hanging roof covering. And the the option for replacing would be replacing again with timber and protecting with and appropriate paint, or what is more common now days is to replace with Plastic fascia and soffit boards fixed with poly pins to the rafter tales. Lee LC build and renovation

This is the right any rotting timber should be changed asap before it gets any worse 👏👍