How to fix a rotten fascia board?

To fix a rotten fascia board that caps rafter ends and supports gutters, you need to assess extent of rot and choose repair or replacement to prevent gutter failure and water damage to home. For minor surface rot where wood is soft but structure is solid and rot is less than 10 percent of board, you can repair by scraping out rotted wood with chisel, applying wood hardener that consolidates soft wood and prevents further decay, filling with exterior wood filler or two-part epoxy that is sandable and paintable, sanding smooth, priming with exterior primer, and painting with exterior paint to seal against moisture, costing 20 to 50 dollars DIY. For moderate rot where fascia is soft and crumbling but rafters are solid, you may need to cut out rotted section using circular saw and splice in new piece of matching wood using scarf joint and exterior wood glue and screws, ensuring joint is sealed with caulk. For extensive rot where fascia is severely deteriorated, sagging, or causing gutters to pull away and fall, full replacement is needed: remove gutters by unscrewing brackets, pry off old fascia, inspect rafter ends for rot and repair by sistering if needed, install new rot-resistant fascia board such as PVC, cedar, pressure-treated pine, or aluminum-wrapped wood, securing to rafters with nails, priming and painting, and reinstalling gutters with proper slope of 1/4 inch per 10 feet toward downspouts. Address cause of rot.