How to Replace a Rotted Window Sill Exterior

Updated February 21, 2017

Wooden windows have a window sill, which is located at the very bottom of the window frame. This bottom window sill keeps out insects, small rodents and weather elements from entering the home or building. When these window sills get exposed to excess moisture, the wood material begins to rot. This rotting wood will attract termites and insects. Replacing this rotted window sill takes a minimal amount of tools and time.

Insert a utility knife into the seams between the wood trim and the window frame. Cut caulk on the outside of the window frame with the utility knife. Pry the wood trim away from the window frame with a pry bar. Use needle-nose pliers to remove all nails in the wood trim. Do not damage the wood trim with the utility knife or pry bar, since it will be replaced later.

Secure the window in the raised and open position. Use the pry bar to pry the rotting window sill off. Remove the rotted window sill in one complete piece to use as a template for the replacement sill. Cut off any protruding nails or screws coming out of the window frame with a hacksaw.

Select a new wooden window sill that has the same thickness as the rotted sill. Lay the new sill on a flat surface, and place the rotted window sill piece on top of the new wood. Use a pencil to trace the outline of the old window sill onto the new piece of wood.

Cut out the traced outline with a jigsaw. Round off the sharp edges of the new window sill with sandpaper. Sand the new window sill until surface is smooth and free from any wood protrusions.

Apply a coat of wood sealer with a paintbrush. Refer to manufacturer's instructions on drying time, and recommended number of coats. This extra coat of protection will help the new window sill from becoming rotted from water.

Set the new window sill in its designated place. Hammer galvanised finishing nails into the new window sill, securing it to the window frame.

Countersink all finishing nail heads further within the window sill with a nail punch and hammer. Use the hammer to tap the nail punch on top of the finishing nails until the nail head sinks below the surface of the wood.

Push a small amount of wood putty, in a matching colour, into the finishing nail holes. Refer to wood putty instructions on amount of drying time needed before sanding. Sand each spot of wood putty with a piece of sandpaper.

Fill any surrounding gaps within the window sill and frame with exterior caulk. Allow the exterior caulk to dry completely.

Things You'll Need

  • Utility knife
  • Pry bar
  • Caulk
  • Needle-nose pliers
  • Hacksaw
  • Replacement wood
  • Pencil
  • Jigsaw
  • Sandpaper
  • Wood sealer
  • Paintbrush
  • Hammer
  • Galvanised finishing nails
  • Nail punch
  • Wood putty
  • Exterior primer
  • Exterior paint
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About the Author

Rachel Turner has been writing professionally since 2000, focusing on gardening and home improvement topics. Her articles have appeared online at SlowTravel and in publications such as the "Arkansas Gardeners," "One Step Ahead" and "Writers Now." Turner holds a Bachelor of Arts in English from Arkansas State University.