How to Trim a Basement Window

Written by Sage C.
on Dec 02, 2010

To ensure our content is always up-to-date with current information, best practices, and professional advice, articles are routinely reviewed by industry experts with years of hands-on experience.

Reviewed by
Charles Ramos, Jr.
on Jun 13, 2021

When you are considering trimming a basement window, there are a few things that you need to keep in mind. No matter how old or new your home is, it is important that you take the time to properly trim the windows in your basement just like you do in the rest of your home.

Check the Jambs

Hold a straight edge against your window to make sure that the jambs are flush with the wall. If any of them stick out, then you need to plain them down to your wall. Measure the biggest gap for the ones that are too short and get jamb extensions. From there, drive your pneumatic nails every 9-inches.

Draw Edges

Spread the legs of the compass .25-inches, then allow the point to rest on the lower inside of the jamb’s corners. It is important to use the inside face of your jamb as the guide that will help you to correctly draw your reveal line. Do so along the edges of all of your jambs.

Side Casing

Place the side casing against the wall and align it properly with your reveal line on your side jamb. Mark the wall along the outside edge of your casings for both jambs. Measure carefully between your marks and add 2-inches to cut the stool properly.

Mark Stool

Mark the interior edge of the stool 1-inch from each end. Place it across the opening. Mark the area where the edge of the stool meets with the inner edge of your jambs.

Align your jambs with the inner edge of the jambs. Your compass will then need to be set to the length of the back edge of the stool to the trim’s sill. Hold the compass point against the wall, then record the lines from either end of your stool to the jamb lines.

Finish Up

Cut your square end on the piece of the casing, then apply glue to the casing where the side jamb overlaps the end on the stool. The head casing is now ready to be added. Hold your casing length atop the window and make a mark where the inside edge meets the side. Add your corner blocks, then cut and install the apron.