How to Build a Cornice Box and Crown Molding

RON HAZELTON: Well, talk about Americana, we drove down the eastern seaboard and pulled into Plymouth, Massachusetts, home of the Plymouth rock. You might say, our hosts, the Boderthas own a piece of the rock, or at least a piece next to the rock. Good morning, Susan!

SUSAN BODERTHA: Hi Ron! Welcome

RICH BODERTHA: Hi Ron, its great to have you here in Plymouth.

RON HAZELTON: Well thank you very much, me and my rolling workshop. What a thing to have in your front yard, what a sculpture!

RICH BODERTHA: Well, Ron that is the fore fathers monument built in the late 1800’s. And on it is the name of each individual who came over on the Mayflower.

RON HAZELTON: Wow!

RICH BODERTHA: And this house right here, is the monument house and for years the caretaker for the monument lived here.

RON HAZELTON: Right here?

RICH BODERTHA: Right, right here.

SUSAN BODERTHA: Right here, Ron.

RON HAZELTON: So, this house is 150 years old, huh?

RICH BODERTHA: That’s right.

SUSAN BODERTHA: Sure is.

RON HAZELTON: Well, can you show me the inside?

RICH BODERTHA: Sure, we’d love to have you come in.

SUSAN BODERTHA: Sure, come on in, Ron.
RON HAZELTON: Great, I’ll follow you guys. Well, would you look at this?!

RICH BODERTHA: Old house, old room, Ron. As you can tell where the wall meets the ceiling we do have a problem. We had some flat molding up there, we took it down and now we would like to get some crown molding up there.

RON HAZELTON: Okay, we can do that.

SUSAN BODERTHA: Okay, and that’s not all. We want to put a cornice box over this old pipe here. It is a steam pipe and it gets pretty hot.

RON HAZELTON: Over this pipe here? So where will that go?

SUSAN BODERTHA: It will come out from the wall and it will cover the tops of the curtains, which will be hanging underneath it.

RON HAZELTON: And it will continue over to this side?

SUSAN BODERTHA: All the way across to that side.

RON HAZELTON: And the insulation will keep the drapes from getting squashed.

SUSAN BODERTHA: That’s right.

RON HAZELTON: Okay, I think we can do both of those, let’s measure up.

SUSAN and RICH BODERTHA: That’s great!

RON HAZELTON: I’ve drawn a working line here, which is going to be the approximate end of the cornice. We’re going to make the cornice out of ¾ inch plywood. What I want to check here is to be sure that I have a space between the pipe and the inside of the cornice box. I would say that is about a ½ inch, that should be enough. So, let’s measure the distance then from the inside of the frame out to that line. That’s five inches. Susan measures five inches down on her side. Then we use the level to draw a vertical, or plum, line down from the ceiling. The cornice will be 7 and ¼ inches out from the wall. In order to clear the pipe and allow space for the curtain rod. And then we should come down far enough to cover the part of the seal, I would say about 11 ½ or 12 would do that. It would put the bottom of it about right here.

SUSAN BODERTHA: Good.

RON HAZELTON: So, let’s go cut some wood, huh?

RICH BODERTHA: Great.

RON HAZELTON: Now how about this, huh? New portable workshop, new saw, I mean, eat your heart out, right?

RICH BODERTHA: Honey, this is on the Christmas list.

RON HAZELTON: You know that this is the material we will be using for the cornice, right? It’s ¾ inch thick, plywood, nice and flat. Turns out, Susan is pretty handy and she stepped right up to the saw. Good for you. All the way in. Excellent. And this will be one of our ends, right here. Now this is how we are going to join the front of the cornice to the sides, okay, with something called a miter joint. It actually requires two double cuts on the ends of the lumber. So we are going to set this saw up, or reset this saw up for the cut. Rich, if you could loosen that up and tilt it up to 45 degrees there. Okay, good. For a really strong hold, apply glue with a brush to the beveled edges. Then use these corner clamps to hold the piece steady while we secure it with our nail gun. This is a nail gun, now we’ve got 6 penny finish nails in here and you’re left handed, right?

SUSAN BODERTHA: I am.

RON HAZELTON: I like using a combination of glue and nails. This way, we don’t have to wait for the glue to set completely and we can move on. Alright, ah, this is the top of the cornice. Now I am putting this in here really for structural strength as much as anything else and also to keep the dust from off of the top of your drapes. Now we are going to cut out this top to go over that steam pipe which was the whole reason for this project in the first place. Very nice. Good. Alright, now, we will put a little trim on this and we’re just about ready to take it inside. Okay, this is perfect timing. Let’s get the brackets up. We use two inch right angle brackets for mounting the cornice to the wall. And there you have it. So is that what you had in mind, Susan. Did that do the trick for you?

SUSAN BODERTHA: That’s just perfect.

RON HAZELTON: So we’ve got a whole bunch of crown molding to put up now, right?

RICH BODERTHA: Right, with your name on it.

RON HAZELTON: With my name on it? Well, I’m not going anywhere for a while, I can tell. Alright, let’s start on that part.

SUSAN BODERTHA: We’re ready.

RON HAZELTON: Now the situation we have here is pretty typical of a lot of old houses, walls and ceilings that are not perfectly straight. So to help us put the crown molding up, I am going to use these little corner blocks, which I have cut. We’ll put one of these every few inches, every place we can find a stud or some bracing. To attach them, I’ll first put some construction adhesive on the back, and then nail them to the wall studs we located earlier. I think we might have a problem. As I am getting ready to install the molding, I come across a little problem up on the ceiling, some sagging plaster. So I decide to perform a little surgery. I apply some quick drying patching plaster, have some lunch, and before you know it, we are back on schedule. There, that’s a much better fit. You know, even for me, cutting crown molding can be a little bit confusing. You probably have had the same experience from time to time. The key is to go slowly and to keep double-checking yourself. Couple things to keep in mind, though. When you are cutting crown, you are going to be cutting it in the same position that it is installed, except that it is bottom side up if you will. The fence back here you can think of as the wall, the table down here you can think of as the ceiling. So, we are going to hold it on the diagonal position on the ceiling the same way that it would be when it’s installed, but bottom side up. These crown molding stops here and here, will make the cutting easier. It also holds it in the correct position. Okay, so we’re all set. Let’s make our first cut now, right on the end of this piece of molding. Now we measure each piece of molding along the backside. That’s the side that touches the wall, and start cutting. Ah, finally, time to hang the molding. But first, we mark the position of the wooden block on the wall, so we know exactly where to nail. Now that all the crown molding is in place, it’s time to fill the nail holes. A finger and a dab of spackle should do the trick. We let the spackle fully dry which usually takes about 30 minutes, and then sand it lightly with some sand paper. Now, despite our careful fitting, there are still some gaps between the top of the molding and the ceiling. But these can be easily concealed by filling them with a bead of painters caulk, and then smoothing it out with the best tool for the job, once again, our finger. By carrying the molding across the new cornice, it really becomes part of the room. Well, we didn’t beat the sunset, but we did a heck of a good job, if I do say so. Are you happy?

SUSAN BODERTHA: Oh, very pleased!

RICH BODERTHA: Very impressive. It changes the entire room, Ron.

RON HAZELTON: You know, I am a restorer at heart. And I have an expression, which I think is appropriate for this room. It’s good as old.

RICH AND SUSAN BODERTHA: Alright!

RON HAZELTON: Go team!

Add Crown Molding to Disguise a Crooked Joint, but Build a Cornice Box to Hide a Steam Pipe

Take care of a 150-year old monument caretaker's house and protect the curtains as you build a birch plywood cornice box to hide an ugly steam pipe. New crown molding ties the cornice box to the room and covers unsightly seams at the ceiling. The result maintains the old-fashioned character and makes the room "as good as old."

building a cornice box to cover the pipe

Step 1

Begin to Build a Cornice Box by Determining Its Length

Mark a line at the approximate end of the cornice box, leaving space between the side of the pipe and the end of the cornice box. We'll build the cornice box from .75 inch birch plywood.

Step 2

Plumb Two Vertical Lines to Build a Cornice Box

Measure the horizontal distance from the window trim to the line--about 5 inches. Use a level to draw vertical lines from the ceiling 5 inches from the window trim to build the cornice box.

Step 3

Custom Fit Measurements to Build a Cornice Box

Build a cornice box to fit by taking good measurements: ours protrudes 7.25 inches from the wall to clear the pipe and curtain rod. It will reach 12 inches from the ceiling to cover the top of the window frame.

Step 4

Miter Corners to Build a Cornice Box

Cut panels and angle the ends to build the cornice box with professional-looking corners. Use a compound sliding miter box set to miter the joints at 45 degrees.

hold joints with corner clamps while you build a cornice box

Step 5

Glue and Nail Joints to Build a Cornice Box

Secure the corner joints with glue and nails to build a cornice box that's structurally strong. Brush on the glue, and then fit and hold the joints together with corner clamps while securing them with 6-penny nails.

cutting an opening after we build the cornice box

Step 6

Build the Cornice Box with an Opening for the Pipe

Mark dimensions and use a jig saw to cut out an opening for the steam pipe in the top after you build the cornice box. Add trim strips to dress the bottom edges, securing them with glue and nails.

build the cornice box to be hung on angle brackets

Step 7

Install Angle Brackets to Support the Cornice Box

Mount 2-inch right-angle brackets to hang the cornice box. Screw the vertical arm of the bracket to a wall stud. Add the cornice and drive screws through the horizontal piece of the bracket through the top of the cornice box.

installing wooden blocks to support crown molding

Step 8

Mount Triangular Blocks to Support Crown Molding

Glue triangular blocks at the ceiling/wall joint with construction cement everywhere a stud is available to provide support for crown molding, and then nail them to the studs. Remove faulty plaster and apply quick-drying patching plaster to even the ceiling.

positioning the crown molding on the miter box

Step 9

Miter Crown Molding Placed Bottom-side-up on the Miter Box

Position the crown molding bottom-side-up on the miter box, as if the miter box fence is the wall and its table is the ceiling. Measure the molding along the backside which touches the wall and miter the pieces to length.

nailing up crown molding to complement the cornice box

Step 10

Install Crown Molding over the Triangular Blocks

Mark wooden triangle positions and nail through the crown molding into the blocks. After spackling nail holes, allow them to dry and then sand. Caulk any gaps between the top of the crown molding and ceiling before painting.

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