How to Fix a Window Sill

Although windowsills are installed to last 10 or 20 years, sometimes, because of rain or termites, for example, they become damaged and unsightly. The longer your windowsill remains in disrepair, the more damage will occur beyond the windowsill---wall framing and sheathing will suffer as well. Fixing your windowsill is a moderately difficult task; however, it is a task that can be completed by the average homeowner in less than a day. These instructions detail removing and replacing the outside windowsill.

Things You'll Need

• Jigsaw
• Circular saw
• Combination square
• Tape measure
• Caulk
• Wood filler
• 10 penny finish nails
• Windowsill wood (available at hardware store)
• Hammer
• Wood chisel
• Wire cutters
• Nail set
• Exterior primer

Instructions

1. Check the condition of the windowsill. If the sill is wet and rotten, damaged by termites, or falling apart, then you should replace it. Try poking a wood chisel into the windowsill. If the chisel easily enters the wood, then you may also have dry rot or termite damage. You should replace to prevent any further damage to the wall.

2. Raise the window and secure it into place. Using a wood chisel, gently pry the old sill from underneath the window jambs. If it will not move, cut the sill in half with the chisel. Slowly chisel away at the sill without damaging the window sides or the interior sill. After the sill is cut in half, continue to wiggle it until both pieces break free from the sides. (Remember that the window sides will rest on top of the sill.) Cut with wire cutters any nails that remain.

3. Clean out the opening and make sure there are no bugs or termites. If you find termites, you can purchase a special termite poison that can be applied with a garden sprayer. Never leave termites in your wood.

4. 

   Measure the opening from one side to the other. Make sure your tape measure goes under the window sides when measuring all the way from frame to frame. Add 2 inches to take into account the amount that the sill will extend past both sides of the window. Measure 1 inch off the ends and make a square mark across the entire cut. Now, measure from where the inside sill and the outside sill meet and out to the outside of the window jamb or window sides. Make a mark from the back side of the cut and out toward the front, using that measurement. You will have a section of both rear corners that will need to be removed. Cut them out with the jigsaw. Please see the attached drawing for a cut diagram.

5. Replace the windowsill underneath the window sides by gently tapping the sill into place. Nail the sill, on each side and in the middle, down into the framing beneath. Sink the nails with the nail set, fill with wood filler, caulk any exposed cracks and apply two coats of exterior primer.

Tips & Warnings

• Always wear eye protection when using the circular saw.

• Power tools can cause serious damage to your body; use caution.

Resources

• Readers Digest