How to Install Interior Window Trim?

Have you replaced your window? After the windows replacement, the next important thing you should do is make it look more beautiful and decorative by adding a window trim to it. The main reason behind adding a window trim is that it magnifies your window’s look and fill the gaps present in it, giving it a more finished look. Window trimming makes your place look more beautiful and attractive.

Window trim is mainly for decoration. If you still doubt its usage, then you can compare the task of creating a picture frame.

How to Install a Window Trim?

Install Interior Window Trim

Installing window trim is somewhat a complicated process because of the different style trims available. But it is essential to install the window trim to give your room or hall a more decorated and finished appearance.

Here we have created a complete guide on installing the interior window trim. This guide teaches you a straightforward process of installing window trim through which any beginner can also do it.

Step 1 Fill the Wall Gaps

While installing a window casting, the walls’ damage and space create between the jamb and the trim, especially in old buildings. So it is essential to fill those gaps first. Block the holes with the wood filler strips rather than using the caulk.

Rip the casing and then hold a compass and extend to the wall to trim. Transfer that measurement to the scarp wood surface. Place the strip on the perpendicular spot. Cut the line using the jigsaw tool and stick it using the glue to fix the wall gap.

Step 2 Prepare Window Jamb

Ensure that the edges are at their place and if you find anyone coming out of its business, then make them into the level of the wall itself before installing window jambs.

Collect the jamb extensions of your choice and width and make sure they are 1/4 inch of thickness of window jambs in the 4d nails after every 8 inches or 10 inches and level them. Keep the point on the inner corners of window jambs. Take that marked internal point as a reference source and make some revealing lines on the remaining jambs.

Step 3 Cut and make a rebate in the stool

Now set the side casing on the wall making sure that it stays still with the reveal lines present on both the jambs on the side. On the outer edge of the housing, put some marks on the wall nearby it, and then move the frame and repeat the process. Calculate the gap between the marks, and then add 2 inches and trim stool to that size.

The front part of the stool should touch the window sill, and its rear ends meet the still trim. With the back edge lying on the threshold, push the tool side towards the frame. The next process of installing the trim is to draw a vertical sign that points to where your bench frame and frame meet.

Join both the marks and make a cross at that place made by lines.

Step 4 Make and Setup Stool

On the edge of the stool’s, measure 1 inch from each end and make a sign while setting it in the mouth. You must provide a notch at the point where the angle of this item joins the frame’s interior. Place the box on these signs and make “JAW LINE” on the stool.

Then, put one end of the compass behind the bench while the other must remain in the threshold trim. Keep the instrument point to the wall while tracking out the outline of trim to the frame.

Take a new line made as a guide, use a jigsaw to print notches on each stool’s edge. When done, confirm it correctly, and if not, adjust it to the chisel or sandpaper. Drop a little glue on the threshold and nails it to the bench.

Step 5 Step Side Casing

Cut the square end on the casing and put it on the stool. Ensure that you put the make on the meeting point of interior ends and reveal the line at the jamb.

Cut it out and follow this process for the other end. Align the edges with reveal lines and join the frame and casing using the 4d, not 8D finish nails.

Make the marks on a bit of casing at that pace where it meets every corner of the casing’s interior ends. Produce a square cut out suing miter saw on notches. Join the head jamb and head casing.

Step 6 Add Corner Blocks

Fit the blocks on both the corners and make sure if there are any holes present. Level their edges to give it a perfect look and put some adhesive to the jamb’s endpoints and casing block.

Use the 8D finish nails after making the pivot holes.

Step 7 The Aprons

Hole the apron stock with the front end down. And on this stock, make the places that come in the exterior of the side casing. Make sure to keep the ends smooth by gripping the scrap part.

Repeat all these steps for installing window trim, and when you complete it, cut the tracing and make the rough edges smooth.

Step 8 Final Step 

It is the final stage, and your work completes here of installing window trim. Here cover the gaps using putty and let it dry. Smoothen it using sandpaper so that it looks good and compliments your wall.

Now smoothen the other parts of window panes like stool, apron, casing, etc. Clean the debris and make the area look cleaner and beautiful. Last if you wish, you can use some window seat cushions to make it look more appealing.


Window trimming is not a very complicated process but requires a lot of time and patience. Follow each step carefully and watch some more videos for installing window trim. You will find it very easy then.

This project consists of two parts, mainly attaching the casing and installing the stool. But make sure to finish the surrounding walls before you start trimming the windows. If you still find it challenging to establish a window on your own, then take the help of some professional window siding contractor and get it done.