How to Repair Water Damage to Window Sills

Repair rotted windowsills to prevent other damage.

Water damage causes wood windowsills to rot. The windowsill lies horizontally across the bottom of the window, so it's where rain water lands when the window is open. It provides protection against water seeping into the space under the sill and between the walls. Water damage occurs on windowsills due to improper sealing and protection. Depending on the extent of the rot, windowsill repair is a possible solution. If rot penetrates through the entire windowsill, replacement is necessary.

Things You'll Need

• Framing chisel or mortise chisel
• Mallet
• Epoxy-based penetrating wood sealer
• Epoxy-based repair putty
• 60-grit sandpaper
• 120-grit sandpaper
• Broom
• Epoxy-based primer
• Paintbrush
• Epoxy-based paint

Instructions

1. Chip out the soft, springy or rotted areas of wood. Use a framing chisel or a mortise chisel and a mallet to dig out rotten wood and expose good wood.

2. Paint on a coat of an epoxy-based penetrating wood sealer to the entire windowsill. Let the penetrating sealer dry completely, generally three to four hours or when it is no longer tacky to the touch.

3. Fill in chipped out areas with an epoxy-based repair putty. Firmly push the filler deep into the recesses of the windowsill. Shape carved-out areas to match the original windowsill. Work with one larger piece of repair putty, rather than layering it on, to form a stronger one-piece repair.

4. Allow the repair putty to dry fully. Drying times vary based on the thickness of the putty. The repair putty is dry when it is hard and solid to the touch.

5. Sand the repair putty surface with 60-grit sandpaper to make it level with the windowsill surface. Follow by sanding with 120-grit sandpaper to smooth the surface of the windowsill. Sweep away sanding dust with a small broom.

6. Apply a coat of epoxy-based primer to the entire windowsill. Allow the primer to dry for two to four hours. Apply two coats of an epoxy-based paint to protect the windowsill from water and weather damage.

Tips & Warnings

• Repair windowsills as soon as you see damage, to prevent water damage between walls and under the windowsill.

• Wear safety glasses and gloves when repairing windowsills.

References

• "Home Repair and Improvement"; Creative Homeowner Editors; 2006
• "Windows and Doors"; Time Life Editors; 1997