How to Repair a Rotted Window Sill

Window sills are the frontline defenders against the elements, constantly pelted by rain, snow, and UV rays. When the paint chips and water finds a path into the wood fibers, rot starts its slow, silent work. Ignoring it turns a localized surface problem into a structural threat that can compromise your window frame and allow moisture to seep into your wall cavity. Done well, a sill repair is not a patch-job but a permanent restoration. You are aiming to excise the decay completely and seal the area so effectively that it outlasts the original construction. This isn't about hiding the damage; it is about rebuilding the integrity of the sill so that water sheds away from the house rather than pooling where it shouldn't.

  1. Dig out all decay. Use a sharp chisel and a hammer to scrape away all soft, decayed wood until you reach solid, dry timber. If the wood is still spongy, keep digging, as failing to remove all rot will cause the repair to fail from the inside out.
  2. Dry it completely. Remove all sawdust and wood chips using a stiff brush or a vacuum. If the wood is damp, use a heat gun on a low setting or let it air dry completely for several hours to ensure the epoxy adheres properly.
  3. Harden the substrate. Brush a liquid wood hardener into the exposed wood fibers according to the manufacturer's directions. This substance saturates the wood cells and cures into a rigid plastic, providing a solid foundation for your filler.
  4. Pack the epoxy in. Mix two-part wood epoxy putty until it is a uniform color and press it firmly into the void. Use a putty knife to overfill the space slightly, ensuring you have enough material to shape later.
  5. Match the original profile. Once the epoxy is fully cured, use a sanding block with 80-grit sandpaper to shape the filler to match the original profile of the sill. Follow up with 150-grit paper to smooth the surface until it is flush with the surrounding wood.
  6. Seal and paint forever. Prime the entire repair area with an exterior-grade oil-based primer to lock out moisture. Once dry, apply two coats of high-quality exterior acrylic paint, ensuring you seal the joint where the sill meets the window frame.