Everything You Need to Know About How to Trim Out a Window

No matter if you are renovating your old house or building a new one. If you plan to trim out a window yourself, you are in the right place. Everything you need to know before trimming out a window is mentioned in this post.

How to trim out a window

You need to use hard, solid wood to trim out a window. Something like primed pine will work perfectly. You ideally need a width of about 3 – 3.5”. The thickness of three-quarters of an inch would suffice. That’s what we call a ¼ in lumber nomenclature.

Here’s a list of other items you will need to trim out a window

  • Saw (to cut out wood)
  • Inch tape (for taking measurements)
  • Geometric square (mark lines for cutting wood)
  • Geometric compass (taking measurements in irregular spaces)
  • Nail (to attach wood to the wall)
  • Hammer (to send nail against the wall)
  • Glue (to glue up wood)
  • Pencil (to make markings on wall and wood)
  • Plane (to smoother up cut out edges)
  • Sandpaper ( to round over the edges)

Parts of the window

Before you trim out a window, you need to know its parts. This nomenclature is widely followed around the world.

  • On the exterior, the bottom is called the window silt
  • On the interior, the part that goes in the bottom is called the window stool
  • Underneath the window stool is the apron
  • On the sides, it’s called casing and on the top, you have the header


Cut your window stool for length

Cut a scrap piece of the cut-out wood. We will use it to get the length of the window stool. Position the scrap piece against the edge of the window stop. Mark the outside of the piece on the plaster wall. Take the scrap piece to the other stop of the window. Place the piece against the window stop and mark the outside of the piece again. Between the two markings, you get the length of the window stool.

Now, mark the length on the primed pine wood. Use the square tool to precisely cut the length. Use a saw to cut the wood cleanly. Position the cut-out length of wood on the wall. Place it against the bottom of the window. Adjust it within the inside of the markings.

Now we got to decide on the width of the window stool. Place the plank against the wall. Keep it at the bottom of the window casing. On top of the plank keep your scrap piece on either the left or right side of the window. Take the measurement from the moving part of the window (out past the casing) to the outer end of the scrap piece. Such measurement mostly stays in a range of 2-3 inches in standard sized windows.

Cut your window stool for width

Cut out the measured with on your window stool.  When you are ripping, keep your shoulder over the saw. If you tit on the sides, it won’t result in a clean cut. Try achieving the perfect square cut with utmost precision. You need to smooth up the edge after cutting the plank. Smooth it up with plane and sandpaper. Try using all of your hand tools in the process.

To make sure the window stool fits tight you need to notch around the window stop and the wall. Hold the plank tight against the window stop and measure the width of the window casing. Use a compass for measuring it. Mark it on the portions of the plank that is outside the window.

Mark the width on the side of the plank. Also, mark the length from one end of the plank until the start of the window. Repeat the process on the other side of the plank. Make two L shaped cuts on either side of the plank. Make sure your window stool fits perfectly, by placing it on the bottom side of the window.

Smoothing and attaching the window stool

Now, we have to ease over the edges. Draw some rough lines where you want to ease into. Keep it parallel and close to the edges. Cover all dimensions, regardless of where it will be positioned. Use a block plane to round over the edges. Fine tune it with some sandpaper.

Now your window stool is almost ready. It has to be nailed to the window. You need to nail it straight through the nose. Push the nail to the position of almost coming through the other side.  Add some glue on the side that meets the walls. It makes your window stool to stay longer. Now place the window stool exactly where you want it on the window and drive the nail in. Usually, 3 nails – both ends and center – are enough to hold the window stool.

On to the side casing

You can get the length of the side casing you can either measure it or just mark it. For the latter, you need to hold the ¼ perpendicular to window stool and along the sides of the window. Mark the lengths and cut it out accordingly.

Since we are working against gravity here, you have to add glue along the edge. The glue goes right on the wood, across the stool and definitely not on the wall. Now just hold the side casing and nail it in place. Add a nail to go from the underside of the stool to reach the side casing. Rinse and repeat again to complete both sides.

On to the head casing

Hold the piece of wood on top of the side casings and against the wall. Mark the length of your head casing. Cut it out using a saw.  You don’t want any part of the wood hanging outside the side casing.  Add glue to the top of the window before placing the head casing. Just place it and nail it against the wall.

Once it’s all done on the interior, you need to work on the apron. The apron should be marked using the end of the side casing as reference. Run a bead of glue across the top edge of the apron. Place it just down the window stool and let the glue do its work. Nail it to the wall too. Now all that’s left is to put a coat on the window trim.

How to trim out a window