Do you have an archway in your house you would like to add trim to? How about an archway with bullnose edges? I have 7 of them throughout our house. Here’s my step by step proven method to trim your archway.

I like the arches but I don’t like the bullnose edges. If there is any opportunity for me to square them off, I do. Adding trim to the archways is a great way to hide the bullnose and make the archway look more intentional. Trimming them to hide the bullnose has become a mission for me.

After completing four out of seven archways I think I have a pretty good method.

I had no idea there were so many different types of arches, although I think calling the flat arch an arch is kind of a stretch. Source: 30 Different Types of Architectural Arches

Some of the links I share are affiliate links, and if you purchase through them it won’t cost you any extra, however, I may be paid a small commission. That small support helps me keep the lights on and we appreciate all of it.

What type of arches do I have in our home

We have broken arches or segmental arches. I wish they were full round, beautiful arches, but they aren’t. They are kinda like 1/4 of a circle. I have heard that they are easier to trim out than rounded arches. Since I haven’t trimmed a rounded arch I will have to take their word for it.

The anatomy of our archway and the trim I use

Today, I am going to share with you how I add trim to an archway, specifically a broken archway.

Our door casings are fairly simple. It’s a 1×4 mdf board with a 1×2 wrapping it.

Side note: I trim the boards down to make them just a wee bit thinner. The 1x4 I trim to to 3 1/4" and the 1x2 to 1 1/4". This isn't necessary, it's just the reveal I like. 

I also use plinth blocks at the base of the trim. This is to solve a simple problem: our doors are 8′ tall. I can’t use an 8′ board to trim them because of the extra 3-4″ I need for my miter. I don’t want to buy 10′ boards in order to waste over 1 1/2 feet for each side of the doorway.

The plinth blocks add just enough height so that an 8′ board can still be used on an 8′ opening.

A blue circle is drawn over the archway in order to showcase how the arch is just a part of a circle
The blue circle is added so you can see how the archway is following the circumference of a circle

Simplifying an arch to understand adding trim

Let’s start first with this image above. It’s the archway leading into our bathroom. The most basic thing you can get from an arch is that it is part of a large circle. You are just tracing the outer line of that small portion of a circle.

Black lines are drawn in the superimposed blue circle over the segmented archway that we are learning how to add trim to in order to show how the measurement from the center of the circle outward to any point along the circumference is the same.
The black lines leading from the center of the circle outward represent the radius of this circle

If you remember from geometry class (or don’t that was a long time ago but follow me anyway), a line drawn from the center of the circle to the outer perimeter is called the radius. The radius is the same length no matter where you position the end point along the circumference as long as it originates from the center of the circle.

Knowing the radius is important because every single point from the center of the circle around the outer edge is exactly the same distance away. It never changes. I could put a point anywhere along the outer edge of that circle and the distance from the center to that point is the same as any other point along the outside line.

That’s what makes the center, the center.

It may seem like an illusion but every single line from the center of this circle outward is exactly the same length

Well that’s all great but how do I even find my radius of my archway?

Tools and supplies I use to trim and archway

Getting Started: How to measure your archway

You don’t have to know the equation to solve this problem. But you will need to trust a calculator. Measure the distance between the doorframe. In my case it is 39″.

Take a 4′ long piece of scrap 1x and your finish nail gun and using one nail on either side, nail it right where the top radius meets the straight leg parts of the door jam. Use a level and then tack it in on the other side.

The yellow line symbolizes the “rise” of my arch. It’s the length between the scrap 1x I nailed up and the center of the arch

Measure halfway in from the inside edge of the doorway to the center, in my case, 19 1/2″ and mark that spot on your scrap piece of wood you just nailed up.

scrap piece of board is nailed level to the point in the arch where it begins to curve.
Nailing a scrap piece of board in place helps you find the rise accurately and quickly

Using a level with a measuring tape or a framing square, measure the distance between the mark and the edge of the top center of the doorway arch. (As long as you are measuring perfectly vertically from your center mark on your scrap you will be measuring the up to the center of the arch). This will give you the rise of your arch.

I used a level and measuring tape to find the center of my arch.
See how the bubble is centered, I know that I am holding the level it straight, it is placed against my mark, now I just need to mark the top and I know it will be the center of the arch.

See the yellow line above as a reference. That is the rise line. Mine was 7″ hight.

Measuring tape is being held up against the two marks in the archway to find the rise of the arch.
The rise of my arch is 7″. I found this by measuring the distance between my two marks

Now that you have the width between either side of the doorway (39″) and the height difference (7″) we can use the arch calculator on this page to find the radius. In my case I entered 39″ into the width cell, and 7″ into the height cell (make sure you change the units to inches, the default is feet).

Don’t let the math keep you from adding trim to your arch

The radius it spit out was 2.56 feet. I convert this to inches 24″ + (.56×12″) and get 30.72″. I would like a 1/4″ reveal so I add .25″ to the 30.72″ and get 30.97″. The closest fraction that I can easily measure with a measuring tape accurately is 30 15/16″. I could go into the 1/32 range but 15/16″ is close enough for me.

Now I know the radius of my circle – the blue circle superimposed over my archway – is 30 15/16″.

I love helping you learn new things. That is the sole purpose for me sharing. If you are having a hard time understanding the math or any part of this, just send me an email or dm me on IG.

How the radius makes adding trim to an archway easier

If the width of my door is 39″ plus a 1/2″ for a 1/4″ reveal on either side, I know that the inside of my board needs to be a minimum of 39 1/2″ wide. My trim is 3 1/4″ wide, if I account for two legs of trim (3 1/4″ + 3 1/4″) the very edge of my top trim piece will need to be at least 39 1/2″ + 6 1/2″ or 45″ wide.

I not only need to account for the height difference, but also the height of the trim. 3 1/4″ + 7″ means that my top trim board, the one that will create the arch in the archway trim, needs to be 10 1/4″ tall at a minimum. The minimum is what is needed in order to cut the arch without having to splice two materials together.

I did make splices but only because I didn't have a bunch of 1x12 lying around and began this without a clue. Following this guide you won't need to have a splice, but if you do, us mye finishing techniques I share here you can't see them. 

This all means to cut the top arch trim piece I need a board that is 45″ wide by 11″ tall.

Measure and mark the center of this board along the bottom edge.

measuring tape along the bottom edge of the board with a mark at the center of where the arch will be drawn
This board is large enough for me to make my arch out of without having to splice two boards together like I did with my first couple of arches

Drawing and cutting the arch on your trim

First we will draw the inner archway for our trim

Place the board in an area with a large work space or on the ground. Using a framing square, measure from the center mark down, your radius minus your height, for me it was 30 15/16″ – 7″ or 27 15/16″. If you’re working on the ground, mark that with a pencil, if you’re working on a wood surface and you don’t mind it getting a nail hole, mark and place a nail at that location.

measuring tape showing where 27 15/16" lines up squarely to the board that is going to make up the arch for the archway trim
I don’t have a framing square long enough, but by placing my framing square against the board and using it as a straight edge for my measuring tape I was able to mark 27 15/16″ and know it was square to my center mark
The 7″ mark represents the rise of the arch. By starting here you know you are making the right size arch

Measure up from your center board the rise of your arch and mark it. Remember mine is 7″, so I made a mark there.

Cut some string that doesn’t stretch easily to 3′ or more if your arch is larger. Tie it around a pencil, then pull it tight to the mark you made (mine is at 27 15/16″) and keeping the string at the point and using your arm like a pendulum, place the pencil on the mark you made above your center mark – the rise mark. Trace from the center out to the bottom edges of the board on either side of the mark. This will give you the inner edge or bottom edge of your arch that you will cut.

Joannie is holding a pencil tied to a string line and pulling it tightly to help guide her marks when drawing the arch onto the board for cutting.
Here I am using a string line pulled tight as my radius for drawing my inner arch

Now draw the outer edge of our archway for our trim

Now, move your pencil out the width of your trim. Mine is 3 1/4″ wide. So I moved my pencil 30 15/16″ + 3 1/4″ out further from my point (the 27 15/16″).

Joannie is stretching string line from her center point to the outer edge of her circle circumference to draw the outside edge cut line for her archway trim.
Using the same string line but pulled 3 1/4″ further out, I drew the outer archway cut lines.

Repeat the same thing you did before on the inner archway mark but this will create the outer archway cut mark.

Cutting the ends

This picture below may look complicated, but it isn’t. Now that you have your arch drawn on your board. Remember that the guide you used was the 4′ scrap nailed to the doorway? Well that intersected the doorway at 180 degrees. So now, where your arch meets the edge of the board, would put the ends of your arch at 180 degrees, this won’t work because it lengthens the ends of the board. You need it to always be 3 1/2″ wide (the width of your trim).

Measure from the inner arch towards the outside arch 3 1/2″ and make a mark, then, using your speed square, draw a straight line upwards from that 3 1/2″ mark until it intersects the top of your arch. Use a straight edge and connect that intersecting point to the inner arch.

It will be a diagonal line. See the picture below.

The red circle is the intersection. This is where the trim that is on the side of the doorway will come up and join the arch. Repeat this to the other side of the arch and then cut both of those lines with your miter saw.

Cutting the Arch

pencil marks thee cut line for the outer edge of the archway ready for trim
Look closely and you can see my pencil marking the outer edge of the archway and the inner archway is already cut. You can also see that I made this arch out of two boards spliced together. This was one of the first ones I did and I wasn’t sure how tall of a board I needed to start with.

Cut the board with a jigsaw.

Sand the inner edge so that it’s smooth and ready for paint.

Test fit your board to the arch. Surprise, you did it! But don’t nail it up just yet. Go back to the shop, we need to make the outer trim piece.

Make the outer trim piece for the archway

How to make your 1×2 bendable

Grab your 1x 2 x 8 and cut it in half. Set your table saw to 1/4″ wide finish cutting width. Raise the height of your blade up to the max or 1 3/8″ high.

Rip three lengths off of the boards. You are shaving the boards – imagine like a deli slicer. You want finish pieces that are 1 1/2″ tall x 1/4″ thick and 4′ long.

The 1/4″ thickness of the mdf will allow the board to bend.

How to join your 1×2 to you your 1×4

When you have three of these cut, place glue on the outer arch edge – make sure you cover the entire edge. Center one of your 1/4″ thick 1×2’s on the arch with the bottom edges flush. The top will go past the arch, that is great, that’s your outer trim reveal happening.

I like to finish nail this board in place so I can keep going. I use 3/4″ nails. If you want you can clamp this in place and wait for it to dry before going to the next step.

Now spread glue on the outside edge of the 1 x 2 x 1/4″ strip and add the second 1/4″ strip on top of that, centering it and keeping both edges flue with the previous board. Nail or clamp in place.

1/4" mdf board has glue spread all along it ready for it to be nailed to the archway trim. the 1/4" thickness mdf is easily bendable to mold itself along the perimeter of the arch.
Nail and glue creates a quick and easy way to join the 1/4″ board to the archway trim. You can see the glue spread on the last board ready to be nailed to the final archway top trim assembly

Repeat with the last strip of 1/4″ mdf.

Matching the cut angle on your 1×2 to your 1×4 arch

Cut your bent board ends to match the ends of your arch. I just line my blade up to continue the miter, this feels wonky, because it is.

The arch is made out of 1x4 and 1x2 mdf. With glue and sanding the seams in the 1/4" 1x2 is impossible to see. Match the miter cut angle of your 1x2 to the cut on your 1x4 arch.
The cut angle on your outer trim should match the cut angle of your arch trim.

Finishing the 1×2 bent trim piece

Sand the glued seam. Be careful not to round out your edges, you are just smoothing out your seams between your 1/4″ boards.

If you see any spots that you didn't glue, add glue on top, sand with 60 grit while the glue is still wet and then gradually increase your grit over the entire edge to 220. 
Glue on the seams of the 1x2 boards makes them disappear after sanding
Sanding wet glue creates the perfect filler for the seams of your 1/4″ bendable mdf boards. If you use this method the seams are invisible.

Now, go nail that arch up where it belongs for good.

archway doorway entrance with the kitchen framed in view. The arch is nailed in place and now it is time to cut the legs of the archway trim.
This archway leads from our mudroom/drop zone into our house. The inner edges of the doorway are clad with 1/2″ mdf and bendy board.

Cutting your side pieces for your doorway trim

Take your side trim boards and place them against your arch. Mark on the inside of the trim piece where it will meet the arch and mark on the outside of your trim piece where it will meet the outside of your arch. You can repeat this on the other side too.

line the trim up to the arch and mark where it will hit the archway trim that way you can go and cut the points with your miter saw and trim your arch easily
Marking the board in place takes the guesswork out of measuring the cut angle for your miter saw

Head over to your miter saw and match the angle to your two reference points. My angle ended up being 22.5. Cut your trim, nail it in place.

the saw blade is positioned over the marked line for the cut on the archway trim
Marking your board in place makes finding your miter saw angle as easy as lining the blade up to your marks

When you cut the outer 1×2 trim just use the same angles you did on your miter saw.

archway painted and finished leading in to the bathroom
The archway leading in to our bathroom now hides the bullnose edges and looks amazing
one room challenge logo for shared girls bathroom

If you would like to read more about the challenge or see the incredible transformations being done by other guest designers, click here. If you liked this post you may like my post from the third week. It’s all my go to tips for installing baseboards.

If you enjoy building stuff for your home, check out my nightstand plans, they’re easy to follow and look amazing when completed. Or if you love a good hack I converted my standard stainless steel dishwasher into a paneled dishwasher here!

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Disclaimer: Working with power tools and DIY projects can be dangerous and post inherent risks. While we work hard to ensure the accuracy and effectiveness of the tutorials along with the information displayed on this website, Hambels Get Real cannot be held responsible for damages or losses sustained or incurred in the course of your project or in the use of the item you create.

apartment therapy the official media partner of the one room challenge

Apartment Therapy is the official media partner for the One Room Challenge and I am pretty excited about it because they do such a great job featuring small time and big time designers.

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