10 Easy Steps To Install Window Trim & Window Casing

Trimming a window doesn't require you to have many years of experience to be an expert. With the following steps, you will know what tools you need,  how to make correct measurements,  and how to easily trim your window and achieve a top-quality result.  In totality, you will learn everything you need to know before trimming out a window.

Trimming a window is a necessity for homeowners and comprises of two central part;

Installing the Stool for the window trimming is one part

Attaching the casing.

You need to install the two to have a fully trimmed window. In this article, I will walk you through both steps plus.

Wood window trim is both a stylish and functional feature. Installing window trim is a necessity of every house to not only help maintain adequate heat or cold levels, but it also can really freshen up the look of your house.

There are two methods of installing interior window trim:

  • Traditional window trimming, (This involves you installing a sill and apron)
  • Modern window trimming. (This is a modern technique that is mitered around the entire frame.)

Things You'll be Needing

Trim a Window equipments

  • T-square (mark lines for cutting wood)
  • Measurements tape
  • Hammer
  • Glue
  • Sandpaper
  • Pencil (to make markings on wall and wood)
  • Plane (to smoother up cut out edges)
  • Geometric compass (taking measurements in irregular spaces)
  • Nail (to attach wood to the wall)
  • Saw (to cut out wood)

You will have to select the wood types you will need for your window trimming.

  • 1/4 x 1/2 inch wood piece for upper trimming
  • 5 inch wide MDF (along the top and for the sill)
  • 5 inch wide MDF (for the sides and the apron….which is below the sill)
  • 1 1/2 inch MDF molding piece for upper trimming

Before You Begin: Pick a Design

Pick a Design for window

This is is the first step before commencing work on trimming your window. You need to visualize what kind of design will look good on your window. Designs reflect traditional designs, country, and victorian. Aprons can be produced from all knot free materials; a sanding block can be produced around the base.

Install a Jamb Extension

The first step to before trimming your window is to install a Jamb extensions Steps to Trim your window

Step 1 – Draw to install the Stool

Draw to install the Stool

It's necessary to install a stool by using a T square to draw on the jamb edge.  That will not be covered with the casing. Use a pencil to mark a line as you slide the square along the window jamb.

Step 2 – Measure  and determine the window length

Measure and determine the window length

To determine the length of the header extension, you will need your window sill, measure the stool length, and take the inside measurement from one side of the rough window opening to the other. You must cut a piece of a plank and weigh to get the length of the window stool. Place the board on the edge of the stop window and mark the plaster wall outside the piece. Now, take the board to the window's other stop. Put the part against the stop window and re-mark the part outward.

Step 3 – Cut to Length

cut the Stool to the exact length

At this stage, you need to cut the Stool to the exact length. Also, mark to get the depth of each notch and then record the length.

Step 4 – Cut the Notches

Cut the Notches

The handsaw comes in handy in cutting the notches. Adjust the saw cut correctly. This is important to set it to slightly rather cut from the bottom than from the top.

Step 5 – Attach the Stool

Attach the Stool

It is essential to ease over the edges. Draw some lines where you want to ease into. Ensure you keep it parallel and close to the window edges. Irrespective of the positioning, cover all sides and dimensions.

Step 6 – Drill holes and keep it firm

Drill holes and keep it firm

At this stage, the window stool is ready to be nailed. You need to push the nail in and add some glue on the side that meets the walls. Adding glue will make your window stool to stay firm and tight. Now place the window stool exactly where you want it on your window. You need to place the Stool firmly and check for a tight fit all around it; Make adjustments if you think it's not placed correctly, and drill pilot holes for nails to firmly attach the Stool to the rough sill. Now, drive the nail to all sides enough to hold the window stool.

Step 7 – Cut Head Piece to attach the casing

Cut Head Piece to attach the casing

Cut the top piece of the head to precisely 45 degrees, and keep it in place on one side. Hold it in place to reveal and align the cut edge. Mark for cutting on one side after you have attached the head casing in place

Step 8 – Fit the Apron

Fit the Apron

The window apron is the final jigsaw to be installed.  The apron is the piece of the same casing that fits directly under the window stool.  You can actually install the apron with a few techniques.  The standard and widely used technique are to cut miter returns on the molding.  Measure the apron length from the outside edge to the opposite outside edge of the leg casings.  You can actually pre-assemble the apron by firmly gluing the miter first then nail the whole assembly in place. Typically it means

Step 9 – Drive in the Nails

Drive in the Nails for cutting

After you have marked for cutting,  mark the other end for a straight 90-degree cut at the top edge of the head casing. If possible, try to hold the casing pieces firmly together by Installing the casing with 6d finishing nails into studs and 3d finishing nails into the jamb.  Try as much as possible to prevent the wood from splitting by firstly drilling a pilot hole before driving in the 3d nail horizontally to hold firmly together.

Step 10 – Smoothing  the Stool

Smoothing the Stool

Use a block plane to round over the edges and smooth it with sandpaper. Once you're done smoothing the surface, vacuum its surface to make free from dust.  Then wipe with a damp cloth reveal its smoothness.  However, if you don't want to wait for the wood to dry, you can use an alcohol-based sanitizer on a cloth that will make the cloth damp enough to attract the dust but dries instantly.


Installing your Window trim with the right tools can be extremely satisfying, knowing you did it yourself.

A house beauty cannot be complete without the window trim because it helps to cover gaps and also add decorative appeal. However, It varies with home styles and designs, from the simple picture-frame window trimming to architecturally styles like Craftsman, Victorian, Prairie, and many others. A great way to choose interior window trim is to match it to the home's other molding to ensure a cohesive look.

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