How to Replace Windowsills

When your windowsills start to rot, take care of the problem at the source and install replacements. This DIY project is easy with our smart steps.

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Windowsills with draping flowers and bright pops of color can be gorgeous additions to your home's exterior. However, they are also prone to rot. Since sills sit horizontally, rain water doesn't fall right off. Instead, it sits in place, which can be hazardous to the wood over time. When it's time to replace the windowsill, know that it's not too difficult and can be done by most homeowners. Follow along with our steps below to see how to do it. 

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Getting Started: How to Check for Rot

A windowsill is the part of a window that extends outside the house; what is commonly called an interior sill is technically the stool. A wood sill is one of the few exterior surfaces that presents a horizontal face to the elements, making it vulnerable to rot.

Check the extent of rot by poking with a screwdriver; where you can easily push in, the wood is rotted. If the sill is rotted near the exterio casing, the casing may be rotted as well. If the sill is rotted all the way thorugh, the house's framing or siding may be rotted as well.

If the rot is localized, use wood hardener and two-part epoxy filler to repair it. Replacing a sill is tricky but only requires basic carpentry skills. If the old sill is so rotted that you cannot use it as a template, measuring and cutting will be more difficult.

Buy special sill stock or cut the sill our of 2x6 (1 1/2 x 5 1/2 inches). If the sill is 1 inch thick, use 5/4 decking or buy special sill stock. Use pressure-treated lumber to resist rot.

What You Need

  • Hammer
  • Tape measure
  • Flat pry bar
  • Circular saw or poer mitersaw
  • Handsaw
  • Small hacksaw
  • Large slip-joint pliers
  • Reciprocating saw
  • Drill
  • Sawhorses
  • Lumber to match the sill in thickness and width (its OK if it's wider)
  • Shims
  • Galvanized nails or decking screws
  • Primer and paint

Step 1: Remove Stool

Start by removing the stool. You may need to remove the interior casing molding first. Some stools are rabbeted and overlap the sill. Others are straight and simply butt up against the sill. To remove a rabbeted stool, pry it up from the outside. Pry out a straight stool from the inside.

Step 2: Cut Sill

If the sill is so rotted you will not be able to use it as a template (step 4), measure its thickness, width, inside length, and outside length or make a cardboard template. Use a handsaw or reciprocating saw to cut through the sill in two places (to produce three pieces).

Step 3: Remove Sill Pieces

Use a flat pry bar to pry out the middle piece and then the side pieces. You may need to use a reciprocating saw or small hacksaw to cut through the nails that poke through the jambs on each side. Or use slip-joint pliers to pull them out.

Step 4: Mark New Lumber

Place the three pieces on top of the new lumber and mark the new piece for cutting. The sill is likely beveled at the outside edge. If you buy sill stock, this bevel will already be cut. Otherwise cut it with a circular saw or tablesaw.

Step 5: Cut Sill

Cut the sill to length and start the notches with a circular saw or power mitersaw. Finish the notch cuts with a handsaw or reciprocating saw.

Step 6: Place New Sill

Push the new sill into the jamb grooves on each side. If you need to tap it into place, use a piece of scrap lumber to protect the sill. If needed, insert shims under the sill so it rests firmly on framing. If the stool is straight, you will need to shim it as well.

Step 7: Secure Sill

Attach the sill by driving screws up and into the jambs. Push in shims as needed to make the sill firm at all points.

Step 8: Nail From Top

Angle-drive nails from the top and into framing below. If possible do this only where the nails will be covered by the molding. Replace the apron and the casing.

Step 9: Caulk Joints

After all the trim is installed, apply exterior caulking to the joints. Prime and paint the sill.

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