When Jo recently renovated her bedroom, one of the key elements to the design was the unique accent wall we built behind the bed. Adding box trim molding (a.k.a. picture frame molding) is a great way to add dimension to an ordinary wall to create an accent wall with character.

Unique box trim moulding design

Jo wanted something more than just ordinary, rectangular box trim moulding. She wanted something unique and special, with a touch of drama. She came up with a cool double-box design with rectangles on the outside of each panel, but faceted gem-like shapes on the inner trim piece for a truly one-of-a-kind look.

Installing the box trim to create an accent wall

We will walk you through the process of installing this unique picture frame moulding design on your wall, including the planning, calculations, and installation process. Buckle in, this is going to be quite a ride, but so worth the effort!

If you would like to see a video version of the installation process, check out our Box Trim Moulding Step-by-Step Installation Video on YouTube.

First, some assumptions

You will need to acquire trim moulding that's flat on one side. For all the calculations below, we are going to assume that your trim is 1-1/4" wide. If your trim is a different size, you will need to adjust all measurements below accordingly.

We are assuming that there will be 3" of space between each panel and 3" between the outer (rectangle) and inner (faceted) shapes. If you want different spacing, you will need to adjust all measurements below accordingly.

Step 1: Determine your panel width

The first step to creating your paneled accent wall is to determine how many panels you will need to cover the wall.

Start, of course, by measuring the wall to get the total width in inches. (Ex: Ours was 200 inches wide)

Then divide that number by the number of panels you want (aim for something in 26-30" range (Ex: We wanted 7 panels, which made each one 28.5" wide)

If you have an odd fraction of an inch left, split it between the blank wall space on the sides, in the corners, so it's less noticeable. (Ex: We actually had a half inch extra. 28.5 x 7 = 199.5, but the wall is 200, so we put an extra .25" of space on each end of the wall)

Whatever width you come up with, measure and mark each panel's sides on the wall to be sure they all fit and your spacing is accurate.

Step 2: Determine your panel height

This one is totally up to you! You can do a half wall, a full wall, or anywhere in between. Since our accent wall was going behind a bed, we chose a height that looked good peeking out from behind the headboard.

If your accent wall doesn't cover the entire wall, you may also want to consider adding a flat, wide board across the top to define the space. Be sure to take this into account when determining the height of your rectangular panels.

Step 3: Calculate your final measurements

Most of the measurements for your panel are going to be set, given the assumptions we stated above that you are using 1.25" wide trim with 3" spacing.

The one measurement that will change is the width of the very middle space, which will vary depending on your overall panel width.

The measurements for each panel pieces, left to right, will consist of:

• 1.5" of blank wall space
• 1.25" of trim (outer rectangle, left side)
• 3" of blank wall space
• 1.25" of trim (inner shape, left side)
• TO BE DETERMINED" of blank wall space
• 1.25" of trim (inner shape, right side)
• 3" of blank wall space
• 1.25" of trim (outer rectangle, right side)
• 1.5" of space

To figure out that "TO BE DETERMINED" measurement:

• Start with your desired total panel width (Ex: ours was 28.5")
• Subtract 14" (the sum of all the known measurements)
• The number you have left is your blank space measurement for the inner shape. (Ex: ours was 14.5")

Step 4: Mark the first panel with tape to test placement

Before you start cutting any trim, let's mark the first panel on the wall with tape to be sure the placement is correct and you like the way your panel size looks.

Start by marking your outer rectangle:

• Starting in the center of wall, add tape to indicate where the rectangle will be.
• If you have an odd number of panels, center this one in the center of the wall.
• If you have an even number of panels, line up one side of the panel with the center point on the wall (don't forget to leave that 1.5" of blank space!)

Then mark the inner shape:

• Come in 3" from either side of rectangle and add vertical strip of tape
• Come down 2-1/4" from top of the rectangle and create a point of tape, with the two pieces of tape forming a 90 degree angle
• Connect top angled tape to the side vertical tape to create final faceted shape.

Stand back and look to be sure you like it and double check all your measurements. Adjust if necessary (you might need more or fewer panels)

Step 5: Calculate the length of your inner angled pieces

The only tricky thing left to calculate is the length of the angled pieces that form the facets on the inner shape. For that, we'll reach back into what we learned in our high school geometry classes.

Math alert! Since we made that top point a 90 degree angle, a right triangle is formed and we can use the Pythagorean Theorem to determine the length of the sides of that triangle. Do you happen to remember a² + b² = c²? Let's use it!

To calculate the length of the angled pieces:

• Take half of whatever the width of your inner space is (Ex: 14.5 / 2 = 7.25)
• Multiple that number by itself (Ex: 7.25 x 7.25 = 52.5625)
• Double that (Ex: 52.5625 x 2 = 105.125)
• Take the square room of that (Ex: square root of 105.125 = 10.253)

The result is the inside length of your angled pieces. (Ex: ours is 10.25"). We'll call this measurement "A" from now on.

Step 6: Create spacers

To make some of the spacing and measuring easier going forward, and to keep all of our panels consistent and straight across the wall, you'll need to make a few spacers. We made four, but you can create a couple more if you'd like to have extras.

Using any scrap wood you have on hand, cut your spacers to be 3" x 2.25". You'll need both measurements at various times, and can turn your spacer as needed for whichever measurement you need at the time.

Note that this measurement goes with the assumption we made above that you'll have 3" spacing between panels. If you want to change this, you will need to adjust your spacer size accordingly.

Step 7: Create the first inner shape

Now that we have all of our measurements and have confirmed that the tape mockup looks good, we can start to create the first panel.

To create the first inner shape:

• Pieces #1 and #2 - Cut two pieces of trim with a 22.5 degree cut on the left, 45 degree cut on the right, and inside length of A (Ex: For us, A = 10.25")
• Pieces #3 and #4 - Cut two pieces of trim with a 45 degree cut on the left, 22.5 degree cut on the right, and inside length of A
• Form the top of the shape by taping pieces #1 and #3 to the wall, 2.25" down (use your spacer!) from the top rectangle tape, with the 45 degree cuts meeting at the top. They should form a 90 degree angle
• Form the bottom of the shape by taping pieces #2 and #4 to the wall, 2.25" up (use your spacer!) from the bottom of your rectangle tape, with the 45" cuts meeting at the bottom. They should form a 90 degree angle.
• Measure the distance from the left end of piece #1 to the left end of piece #3
• Pieces #5 and #6 - Cut two piece of trim to this length, with 22.5 degree cuts at both ends
• Tape pieces #5 and #6 to the wall to be sure they fit and adjust as needed

Step 8: Create the first outer rectangle

If the inner shape looks good, let's move on to the outer rectangle:

• Place two 3" spacers to the left of the inner shape and two to the right
• Place a 2.25" spacer above the top point of your inner shape
• Place a 2.25" spacer below the bottom point of your inner shape
• Place a piece of trim against the spacer on the left side and be sure it lines up with your original tape marks indicating your desired overall panel width. If not, revisit all measurements to see where the mistake was made before continuing.
• Add straight trim pieces along the top and sides, pressed up against the spacers, with 45 degree cuts at both ends, to form a rectangle.
• Tape the outer trim pieces to the wall to be sure the overall width and size is as desired before continuing.

Step 9: Double check everything

Wow, that was a lot of work, and we want to make sure it's all done correctly.

Double check everything, including:

• Stand back and be sure you like the overall look and shape of your panels
• Double check the total width of the panel to be sure it's what you had planned. (Remember there will be 3" of space between each panel, which means an extra 1.5" of space on the left and right of each panel.)
• Make sure that multiplying this panel width out across your wall will fill the wall as you want it to.
• Make sure all your pieces are square, plumb, and level and that the miters meet.

Step 10: Record all your trim piece measurements

Now that you've done all the work to get these trim pieces exactly right, you'll want to record the exact final measurements of each one. Make note of the length of your angled pieces, and your outer rectangular trim pieces.

Step 11: Consider cutting all your trim pieces now

If you want to be extra sure that all your trim pieces are consistently sized going forward, you might want to cut them all now, using your first panel's pieces as a template.

For example, if you have 7 panels total and one is already done, cut 6 more copies of each piece you made for your first panel. Hold each piece up to the original to be sure the length matches as exactly as you can.

We marked our originals with a piece of blue tape so we could always check our new cuts against that original for consistency and accuracy.

If you're not quite ready to commit to cutting all of your trim pieces yet, at least cut one more panel's worth now before you attach these first pieces to the wall permanently.

Step 12: Permanently attach the first panel

Everything has been double-triple checked so let's attach the pieces permanently to the wall!

Starting with the inner shape, remove the tape from the pieces one at a time and attach to the wall with construction adhesive (such as Liquid Nails) and finish nails.

The finish nails can go into drywall (doesn't have to be a stud) because they are just there to hold the trim in place until the adhesive dries.

Be sure to wipe away any construction adhesive that seeps out the edges now, because it will be nearly impossible to remove after it hardens.

Step 13: Add top trim board (optional)

If you're adding a wide, horizontal trim board above the entire accent area, now's the time to add it.

Use the 3" spacer (or whatever size you like, actually) to space the distance from the top of the rectangle to where your horizontal trim board will go. Add the horizontal board, making sure it's level, and secure to the studs with nails or screws.

Continue across the wall with more boards until the trim is complete

Step 14: Add the second panel

When you have the first panel done, it's time to move on to the rest!

For the first panel, we installed the inner trim first, because centering it was critical. However, for the remaining panels, we will do the outer rectangle first, and then the inner one after. This is to ensure that the panels are evenly spaced (3" apart) from each other along the wall.

To correctly space and install the next panel:

• Place one of your 3" spacers up against the side of the first panel, and use that to line up the side of your next panel's rectangle
• If you are using a horizontal trim board, place the spacers against that as well, to mark the top of your next panel.
• Using the same measurements as your first panel, cut and add the four pieces of trim required to create your outer rectangle shape.

• Move the 3" spacers inside the rectangle to mark the left and right spacing for the sides of your inner shape.
• Move the 2.25" spacers inside the rectangle at the top and bottom to mark the spacing for the points of your inner shape.

• Using the same measurements as your first panel, cut and add the six pieces of trim required to create your inner shape.
• As you install each piece of the inner shape, you should be able to push it up against the spacers and against each other to fit perfectly within the space.

Step 15: Add the remaining panels

Repeat Step 14 until all of your panels are created. You'll find that they go faster with each one you do, as you get the hang of it.

When you have your panels done, be sure to stand back and admire that amazing feat you just accomplished. What a beautiful accent wall this is going to be!

Step 16: Fill, sand, and caulk the trim

Unfortunately, there's still a lot more work to do to finish off the trim, but at least this part doesn't require much planning, just lots of time.

Fill all the nail holes with wood filler, then sand after it's dry.

Add caulk around every piece of trim, to seal the gap between the trim and the wall, After you run a bead of caulk down the edge, use your finger and/or a slightly damp paper towel to wipe it smooth and remove any excess.

Read the caulk tube carefully and make sure you use paintable caulk. If you get 100% silicone caulk by mistake, paint will never ever stick to it.

Step 16: Paint your accent wall

When all the caulk is dry, get out your brush and add your beautiful paint color to the wall. You'll end up needing a brush to get in most of the nooks and crannies, but you might be able to use a roller for the center panels if you want an extra smooth finish. We used the color Bedford Blue 1679 by Benjamin Moore.

That's it, you're done!

We couldn't be more pleased with how this wall came out. The pretty blue color makes the texture of the trim stand out and the whole wall is just gorgeous.

And once Jo brought all the rest of her bedroom furniture in, her bedroom makeover was complete. Lovely!