The answer to 'when to replace fascia' is that you should replace your home's fascia boards when they show signs of significant damage, deterioration, or rot. Fascia boards are the horizontal boards that run along the roofline and support the gutters. Over time, exposure to the elements can cause them to warp, crack, or become infested with insects, requiring replacement.
As a general guideline, you should inspect your fascia boards every few years and replace them if they are more than 25% damaged. Signs it's time to replace fascia include visible cracks, holes, warping, or significant discoloration and rotting. Replacing damaged fascia is important to maintain the structural integrity of your roofing system and prevent water damage to your home.