How to Trim a Basement Window

When you are looking to trim a basement window there are a few things that you will 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.

Step 1 – Check the Jambs

The first thing that you are going to have to do is get a straight edge and hold it up against your window. You will do this to make sure that the jambs are flush with you wall. If any of them happen to stick out, then you will need to plain them down to your wall. Measure the biggest gap for the ones that are too short and get jamb extensions that are as big as you needed and about ¼” thinner than the thickness of your jamb. From there, drive your pneumatic nails that are 1 1/2” and 18 gauge every 9 inches.

Step 2 – Draw Edges

You will then need to get the legs of the compass spread about ¼” and 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. This will need to be done all along the edges of all of your jambs.

Step 3 – Side Casing

You will then need to get your side casing placed up against your wall and align it properly with your reveal line. This will be done on your side jamb. Mark our wall along the outside edge of your casings. You will do this for both jambs. Measure carefully between your marks and add another 2 inches to cut the stool properly.

Step 4 – Mark Stool

The inside edge of the stool needs to be marked about 1” from each of the ends. You will then place it lying across your opening. Make sure that you take the time to mark the area where the edge of the stool meets with the inner edge of your jambs. Align the lines of 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 out your compass point up against the wall. You will then scribe the lines from either end of your stool to the jamb lines.

Step 5 – Finish Up

You are now ready to cut your square end on the piece of the casing and 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 is met with the side. Add your corner blocks and then you can get the apron cut and installed.