How to Add a Wooden Window Sill

Remove the Old Wood Window Sill

• 1

  Put on safety glasses and make a horizontal cut along the back of the old wood window sill using a circular saw. Be sure to set the blade to cut the thickness of the old sill.

• 2

  Make two vertical cuts along the two ends of the old wood window sill with an oscillating multi-function tool. Then, make one more vertical cut in the center of the sill.

• 3

  Insert a pry bar under the vertical cut you made in the middle of the old sill and pry up, breaking the old sill out of the window.

• 4

  Pull the old sill out and dispose of it.

Add a New Wood Window Sill

• 5

  Measure the space in the window where the new sill will go, then measure, mark and cut a new wood sill to fit the space using a circular saw.

• 6

  Fit the new sill into the empty space by hand and tap it in tight against the window using a hammer and a piece of scrap lumber. Hold the scrap lumber against the new sill and strike the scrap with a hammer so you don't mar the new sill.

• 7

  Measure and cut a new sill nose to fit in front of the replacement sill with a tape measure and circular saw.

• 8

  Fit the sill nose in front of the new sill by hand, then tap it in with a hammer and scrap lumber like you did with the sill.

• 9

  Fasten the sill nose to the replacement sill with stainless steel nails and a nail gun. Then spread a bead of silicone caulk around the new sill and nose using a caulk gun.

• 10

  Apply a coat of wood sealer to the new sill and nose with wood sealers using a paintbrush. Allow the sealer to dry, then prime and paint the sill to match the window.