Installing interior window trim (also known as window casing) over drywall does not involve any complex carpentry. This makes it an excellent DIY job. However, extra care should be taken when prepping the wall. Properly preparing your drywall ensures your trim lies flush with the wall. This prevents unsightly gaps between the drywall and trim. There are several tactics for ensuring a snug fit for your window trim. These include removing excess drywall, clamping the casing to the wall, and planing the window jamb if necessary.
What Tools Do You Need to Install Window Trim?
Because installing window trim requires precise cuts, you will need to use some professional-grade carpentry tools to complete the job. However, you can do this on a DIY budget. Below, we’ll walk through the tools and materials required at each step. For reference, here’s a list of tools before we begin:
- Utility Knife
- Low Angle Block Plane (optional)
- Tape Measure
- Miter Saw
- Hot Melt Adhesive Applicator
- Hot Melt Adhesive
- Parallel Clamp
- Finish Nails
- Center Punch
- Finish Nail Gun
Many of these tools can be purchased or rented inexpensively. We’ll provide links to our top choices for the job as we go.
13 Steps to Install Window Trim Casing Over Drywall
Professional wood trim can easily be installed on top of drywall to frame your window. This guide will cover how to install window casing, which is the framing around a window, similar to a picture frame. Now, let’s get started.
Inspect Your Window
Before marking and cutting wood, look closely at the window jamb and the drywall around the window. If you’re lucky, the drywall will be flush with the window frame. However, it’s much more common for the drywall to stick out further than the jamb (known as proud drywall). If your drywall is proud by more than 1/16 of an inch, you will have to correct this to allow your trim to lie flush with the wall.
- Inspect the perimeter of the window to see if the drywall is flush with the jamb.
- If the drywall sticks out further than the window jamb by more than 1/16th of an inch, this “proud drywall” must be corrected prior to installing trim.
- If the window jamb sticks out further than the drywall by more than 1/16th of an inch, the jamb must be corrected before trimming the window.
- Mark any areas of proud drywall or proud jamb for correction.
Another possibility is that the window jamb itself will stick out further than your drywall. This is known as a proud jamb. Although it is much less common than proud drywall, a jamb that is proud by more than 1/16 inch requires correcting. Check carefully all around the window. Some parts of the drywall or jamb may be flush, while others are proud. Mark any problem areas.
Correct Proud Drywall
Proud drywall is common in many window installations. If left uncorrected, your trim will not lie flush with the wall. This leads to tilted trim, gaps between the wall and trim, and loose trim. The best methods to correct proud drywall are:
- Use a utility knife to cut away the top layer of drywall until the area that will be covered by trim is flush with the jamb.
- Strike the drywall near the jamb with a hammer to remove the top layer of drywall.
If you are a beginner, using a utility knife to cut away drywall is a more controlled method of removing drywall. However, removing large sections of drywall with a hammer is much faster. Strike with the hammer head at an angle to break through the paper and remove only the top layer of drywall. Work carefully when using a hammer to make sure you do not remove too much drywall. The goal is to remove just enough so that the trim can lie flat with both the drywall and jamb.
Correct a Proud Jamb
If your jamb is proud of the drywall by more than 1/16th of an inch, the best way to correct it is to remove some of the wood of the jamb. Use a low-angle block plane to peel off the upper layer of the wood until it is flush with the drywall.
- If the jamb extends more than 1/16th of an inch beyond the drywall, use this block plane to shave off the jamb until it is flush with the drywall.
- Work the plane in long, slow strokes to shave the wood until it is flush with the drywall all the way around your window.
The key to using a block plane is to apply gentle pressure. It’s best to guide the plane over the high point in the jamb. Allow the blade of the plane to shave off the wood as it goes smoothly. In just a few strokes, your jamb will be flush with the drywall.
Cut Your Header to Size
Select a length of window trim material and use your miter saw to cut one end at a 45-degree angle. Then, hold the piece of trim in position over your window so that the cut end lines up with one corner. If necessary, tack it lightly in place with a finish nail.
- Use this miter saw to cut one end of a trim board at 45-degree angle.
- Hold the board over the window so the cut end lines up with one corner of the window.
- Mark the position of the opposite corner on your trim board. This is where you will make your second 45-degree cut.
- Use your miter saw to make your second cut to the header, this time angling the blade 45 degrees in the opposite direction of the first cut.
With the header positioned and tacked into place, use a pencil to mark the trim at the opposite corner of the window. This is where you must make the second 45-degree cut. Remove the header and make the second cut. You have now made a perfect header without using a measuring tape.
Tack Your Header into Place
Now that your header is cut to size, put it in position over the window. Make sure to line up the shallow corners of the 45-degree cuts exactly with the corners of the window jamb. Then, tack the header into place with finish nails and a hammer.
- Position your cut header so that it lines up with the top corners of the window jamb.
- Make sure your header completely covers the top of the jamb.
- Tack the header in place with 2–4 finish nails.
Now, you can cut the remaining 3 sides of your window casing similarly. By holding the board up to the window and marking it, you ensure precise cuts without a measuring tape. This results in greater accuracy and fewer repeat cuts.
Cut the Sides of Your Casing
With the header in place, follow a similar process for the sides of the casing. Cut a trim board with a 45-degree angle at one end for the first side. Then, hold it up to the window, so it forms a 90-degree corner with the header. Tack it in place if necessary. Then, mark the position of the lower corner of the window. This is where you must make the second 45-degree cut.
- Cut a 45-degree angle in the end of another trim board.
- Hold the trim board so that it forms a 90-degree corner with the header.
- Mark the position of the window’s bottom corner on the trim board.
- Cut a 45-degree angle at this mark.
- Repeat for the opposite side of the window.
Once you have completed this process for one side, repeat for the second side. Just remember, the 45-degree cut at the top of the second board must be a mirror image of the first. In other words, if you cut the first board with a right-facing 45-degree angle on your miter saw, you must cut the second with a left-facing 45-degree angle.
Glue the Sides of the Casing to the Header During Installation
Before tacking the sides of the casing into place, use a hot melt adhesive applicator to apply a bead of adhesive to the cut ends of the header. Then, install the sides of the casing by pressing them firmly to the header and tacking them into place with 4–6 finish nails. Remove any excess glue before it dries by wiping it with a rag.
- Use this adhesive applicator to apply this adhesive to the cut end of the header.
- Press the first side of the header into place, ensuring a strong adhesive bond between side and header.
- Tack the side of the casing into place with 4–6 finish nails.
- Wipe away excess adhesive before it hardens.
- Repeat for the other side.
The adhesive bond between the sides of the casing will ensure that the pieces lay evenly, even if the house settles. A strong adhesive will preserve the appearance of your window casing for years to come.
Cut the Sill
With 3 sides of the casing in place, the last piece is the sill. The installed pieces of the casing will prevent you from laying the board in position and accurately marking the corners. Instead, use a measuring tape to find the distance from corner to corner of the jamb. Then, mark this distance on the board and make your cuts.
- Find the distance between the two bottom corners of the jamb.
- Cut the sill to length, using the miter saw to cut a 45 degree angle at each end.
- If you are concerned about the accuracy of your measurement, cut the board a little longer, check it, then recut.
When cutting pieces that were marked based on measurements, it’s best to err on the side of caution. Cut the piece slightly longer than you think it needs to be. It’s simple to cut off more, but cutting a board too short results in wasted material.
Glue and Tack the Sill Into Place
With the sill cut to length, apply adhesive to the cut ends of the board. Then, slot it into place so that it spans the bottom. It should cover the jamb and sit flush with the sides of the casing. Use 2–4 nails to tack the sill into position.
- Apply adhesive to the cut ends of the sill.
- Position the sill board in place so it spans the two sides of the casing.
- Tack the sill into place with 2–4 finish nails.
After completing this step, your window will be framed on all four sides. The following steps ensure the casing sits flush for a pristine, gap-free finish.
Clamp the Casing Flush with the Jamb
Inspect the gap between the casing and drywall all around the window. Any place where there is more than a 1/6th inch gap will need to be corrected before the final installation of the casing. The first method for reducing the gap is to open the window and position a parallel clamp to span the inside and outside of the window. Tighten the clamp to squeeze the casing flush to the jamb.
- Inspect the perimeter of the casing to search for any places where the casing does not sit flush with the jamb and drywall.
- Press the casing flush to the jamb using this plastic-faced parallel clamp.
- To use a parallel clamp to press the casing flush to the jamb, open the window and position the clamp so that it spans the interior and exterior of the window near a corner. Tighten the clamp to press the casing to the jamb.
- Once the casing is flush with the jamb, use a finish nail gun to drive several nails into the casing, securing the casing to the jamb.
- Repeat for any other corners that do not lie flush.
It’s essential to use a plastic-faced parallel clamp to avoid damaging the exterior trim of the window during tightening. This process works extremely well for double-hung windows where both the top and bottom can be opened. This allows you to fix one corner, then move to the next until the entire casing is perfectly flush with the drywall.
Fix the Casing Flush to the Jamb with Finish Nails
If your window doesn’t open, or a parallel clamp can’t be used for another reason, it’s time for another method. Use large finish nails and a hammer to nail the casing flush with the wall. Drive a few 3.5-inch finish nails through the casing to the point where it is not flush with the wall. This will pin the casing into place.
- If a window casing cannot be pressed flush with a parallel clamp, you will have to hand drive finish nails to bring the trim flush with the wall.
- Use 3.5-inch finish nails to nail through the trim and into the window frame.
- Pre-drill nail holes prior to hammering. This will prevent the trim from splitting.
- Countersink hand-driven finish nails with this center punch.
To prevent the casing from splitting during the nailing process, it’s a good idea to pre-drill the holes with a small drill bit. Then, hammer the nails into place and countersink them with a center punch.
Nail the Casing into Place
Once the casing is flush to the drywall, or there are no gaps greater than 1/16th inch, it’s time for the final installation. Either drive finish nails by hand or use a finish nailer to drive nails through the trim. Your nails should be spaced no more than 6 inches apart.
- Use this finish nail gun to nail your casing securely into place.
- Drive in 1 nail every 6 inches.
- Drive nails at alternating upward and downward angles for greater holding power.
To increase the holding power of your finish nails, drive them in at a slight angle. An angle of 20–30 degrees is fine. Alternate between upward and downward angles. This system helps prevent nails from pulling out, ensuring your casing remains flush with the wall for a long time to come.
Caulk and Paint Your Casing
With your window casing installed and no gaps greater than 1/16th inch between the casing and drywall, you’re ready to move on to caulking and painting your window. Remember to caulk all the seams between the window casing and drywall and the seams between the window casing and jamb. This will ensure a weatherproof seal.
How Do You Install Trim Around a Window?
To install interior window trim, also known as window casing, over drywall, it’s essential to follow a process designed to ensure the trim sits flush with the drywall panel wall. Here’s how:
- Inspect the window to determine if the drywall and window jamb are flush.
- If the drywall sticks out beyond the window jamb, remove excess drywall with a utility knife or hammer.
- If the jamb sticks out beyond the drywall, shave the jamb down with a low angle box plane.
- Cut your header to size and tack it into place with finish nails.
- Cut the sides and bottom of the casing.
- Apply adhesive where the pieces of trim meet.
- Tack the pieces of trim into place.
- Reduce any gaps between the trim and wall by using a parallel clamp or finish nails.
- Use a finish nail gun to secure the trim to the wall with 1 nail every 6 inches.
- Caulk and paint your window trim.
With this process, you’ll achieve professional window trim every time. It’s essential to prep the area to ensure the trim lies flat. This ensures a pristine look and durable window trim.