How to Install an Interior Window Sill

By Robert Robillard on Tool and Product Reviews

Finished look

Finished look

How to Install a Window Sill

People do not often think about window sills but they are an integral part of interior time and window trim. A well-executed window sill installation can create a finished, classy look.

A window sill is made up of two parts, the shelf is the base or bottom of the wind w and the apron. Or “stool” is the decorative window trim underneath the shelf.

Whether you are changing out trim, remodeling or simply adding windowsills to plastered openings, the result can create a customized, finished look.

Finished look

Finished look

Steps on How to Install an Interior Window Sill

  1. Mark 3/16” or a ¼” reveal mark on your window jambs.
  2. Hold a scrap piece of your window trim casing against each window side, and at your reveal mark.
  3. Mark a reference line on the wall, on each side of the window.
  4. Determine your window sill “horn” overhang. I typically use ¾” for an overhang.
  5. Measure the distance between the two wall marks, then add 1-1/2 inches to factor in your ¾” overhang on each side. This measurement is your window sill length.
  6. Cut the window sill stool to length.
  7. If desired, use a router to create a ½ round over on the front edge and sides
  8. Sand out the router bit mill marks
  9. Hold the window sill against the window and on your layout lines. Use a compass scribe to mark where it meets the window sash and wall. Use a combination or speed squared to complete your lines. I often put an “X” in the areas I will be cutting out, as a reminder.
  10. Trim the stool along the scribed lines using a jigsaw.
  11. Set the stool in place, and make necessary adjustments to get the window sill tight to the window and flanking walls.
  12. Attach the stool to the windowsill using 6d or 8d finishing nails.
  13. Install the side window trim
  14. Nail window trim casings to the window’s side jambs using 6d finishing nails. Nail up through the window sill and into the vertical casing for a tight joint.
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About the author

Robert Robillard

Carpenter / Remodeler / Editor

Rob Robillard is “The Concord Carpenter” Rob is a builder, general contractor, carpenter, woodworker, and editor of Concord Carpenter and ToolBoxBuzzr As a General Contractor and carpenter, Rob owns and operates Concord Carpenter LLC. A full-service remodeling and construction company. Rob is a recognized leader in home building best practices and a source for how-to information for building professionals. On this website, Rob covers all aspects of home construction, building science, home improvement, woodworking, remodeling, and some of the best product and tool reviews. Rob is in charge of our Tool and Product Review series - Concord Carpenter Videos where we post all of our tool reviews and video tutorials. Rob approaches remodeling and building construction with a pragmatic and problem-solving approach. He enjoys using his knowledge and experience to help and educate building professionals as well as DIYers on best practices in the construction and remodeling industry. He's a strong advocate for "raising-the-bar" in the construction trades and promoting the trades to youth. #BeAMentor #Green2Great Craftsmanship, quality, and pride guide his journey on this channel The Concord Carpenter's motto: "Well done is better than well said!" : Read more about Rob

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