As we’ve been updating our builder-grade fireplace, we decided to build a new mantel to complete the look. This DIY mantel is a simple floating beam design, very similar to the floating shelf we built over my mom’s washer/dryer. The main difference between that floating shelf and this mantel is that the mantel has sides as well.
Since we added the stone to the fireplace, we left an area without stone to install the mantel to. Because of that, we did not have to install our DIY mantel to stone, so we will not be going into the nuances of how to install it over stone.
Alright, let’s start DIYing and build a fireplace mantel!
This post contains affiliate links for your convenience (which means if you make a purchase after clicking a link, I earn a teeny-tiny commission, but it won’t cost you a penny more)! Click here to read our full disclosure.
How to Make a DIY Mantel
Recommended Tools:
- Circular Saw or Table Saw
- Kreg Jig
- Drill
- Clamps
- Measuring Tape (new to DIY? Be sure to check out this post with measuring tips)
- Brad Nailer
- Safety Equipment
- Stud Finder
Shopping List:
- ¾” to 1” thick wood for your shelf (we used 4/4 reclaimed walnut for ours. In step 1 below, we’ll show you how to figure out what you need)
- 2x4x8 (quantity may vary depending on shelf size)
- Wood glue (this is our favorite type)
- Sandpaper (120, 180, and 220 grit)
- 1.25” Kreg screws
- 2.5” softwood Kreg screws
- 1.5” nails
- 3” screws
- Painter’s tape
For our finish, we applied 4 coats of Shellac.
Dimensions
The length of this shelf is completely customizable! Our final mantle is 59 ¼ L x 6 ¼” H x 7 ½” D. It’s 2.5″ shorter than each side of the fireplace.
If you are installing yours into stone like we did, be sure to make your shelf slightly deeper than you actually want it to be to account for the thickness of the stone that will be covering part of the top and bottom.
STEP 1: CUT THE WOOD FOR YOUR BEAM MANTEL
This is arguably the most difficult step of the project because it can be a challenge to cut long, straight cuts at an angle.
For this project we used a table saw because we were attempting to get to an accurate 44.9-degree angle. Unfortunately even with a digital angle gauge, we weren’t able to get it as precise as we wanted to.
We had comparable results using the table saw to cut the angles as we did using the circular saw. For more tips for using the circular saw to make your long angled cuts, check out the DIY floating shelf tutorial.
If you are newer to DIY or aren’t sure you want to tackle the long angled cuts, you could also assemble your mantel the same way we assembled these floating shelves.
Anywho, the size of the boards you need will vary depending on how big you want your mantel to be. Below are the formulas to figure out what size pieces you need. For angled pieces, the dimensions reflect the longer side of the board. Once you have that information, you can also determine how much/what sizes of wood you need.
In the formulas, D = desired final depth, H = desired final height, and W = desired final width. T = thickness of the wood you’re using for the beam.
- (1) front piece = W x H (all four sides are cut at 45-degree bevels)
- (2) top/bottom pieces = W x D (both depth sides and one width side are cut at 45-degree bevels)
- (2) side pieces = D x H (both depth sides and one height side are cut at 45-degree bevels)
- (2-4) supports = (D – T – 3*”) x (H – 2T)
*this doesn’t need to be exact. These supports are really to help hold together the beam with some nails while the glue dries, so they don’t need to go all the way to the end of the shelf. We just need to account for the support bracket which is 1.5” thick.
STEP 2: CUT YOUR SUPPORT STRUCTURE
Once you have your plywood cut down, make the cuts for your support structure out of the 2×4. If you are making a narrower shelf, you might need to use a 2×3 instead. The support structure does not have to be the same width as the inside of the mantel – it just needs to be smaller than it so that it can slide inside of the mantel.
Fair warning: our picture isn’t going to look like a 2×4 because it isn’t. We had some scrap plywood that was glued together already and basically unusable for anything that you see, so we just cut it down to use for this.
With this 2×4, we will need a back piece and support boards. The number of supports you’ll need will depend on the length of your mantel. We cut 3 supports for our 59” mantel.
The supports don’t need to be an exact length. We recommend cutting them 3-4” shorter than the total depth of your mantle. This will account for the front of the shelf and the thickness of the support board behind them.
For the back piece, again, this doesn’t need to be precise. You can cut it to the exact length of the inside of your mantel for a snug fit, but we recommend cutting it 1” shorter than the inside of the mantel to give yourself some more wiggle room.
Another way to think of it is L = W – 2T – 1”.
STEP 3: DRILL POCKET HOLES
Using the 1 ½” settings, drill (2) pocket holes on one side of the supports that will be used to attach to the wall support.
Using the ¾” settings, drill (2) pocket holes on one side of the supports that will be used to hold the beam together.
STEP 4: INSTALL THE SUPPORTS
First, lay out the front piece with the shorter side facing up and then place the back 2×4 support board next to it. Place the supports onto each board, making sure that the supports on the beam aren’t lined up with the supports on the 2×4.
You’ll secure the ones on the inside of the front mantel piece with glue and 1 ¼” Kreg screws. You’ll use glue and 2.5” Kreg screws for the ones on the 2×4.
STEP 5: ASSEMBLE THE MANTEL
Lay out the boards with the wider sides facing up. Place the front piece in the middle and the top and bottom pieces on either side of it, lining up the long edges. Add painter’s tape across the boards – this will help “clamp” the boards and make them easier to fold.
We don’t have a picture of this step from our mantel project, so the following photo is from the floating beam project.
Once your pieces are taped, carefully flip everything over.
Add a generous amount of wood glue to the long angled edges. Fold the top and bottom pieces so that they touch the supports.
In the photo you will see that we originally had the side pieces in place to fold and clamp up, but decided later that it will be easier to install them after gluing up the main section.
Place your mantel so that you can see the front. We placed a piece of parchment paper underneath the mantel so that it wouldn’t stick to the wood or table underneath it as the glue dried.
Add clamps to hold everything together, really focusing on how the seams look in the front. Our reclaimed walnut was pretty warped, so it required a lot of clamps to keep everything lined up.
Add (2) 1.5” nails through the top/bottom pieces and instead to supports, alternating the angle on your nail gun for a stronger hold.
Now it’s time to add the sides. Add some painter’s tape either to the side pieces or to the front, top, and bottom pieces of the mantel. You want it ready to go so that you can use the tape to clamp the sides in place when you put them on.
Add glue to the 3 angled sides of the side pieces and then place them on your mantel. Position the tape to “clamp” them in place. You can also add real clamps or a few nails to help keep the side pieces in position while the glue dries.
Wipe off any excess glue that might have seeped out.
STEP 6: FINISH THE MANTEL
After letting the glue dry for 12-24 hours, it’s time to finish this DIY mantel! If you have some small gaps between your mitered corners, you can use the “screwdriver trick” to make them disappear.
Wet the corners with a damp paper towel and run a screwdriver (or the Kreg square head driver) over the edges of the boards to bend the wood fibers. This will help minimize or eliminate the appearance of any gaps and soften the corners so they aren’t so sharp.
If you still have gaps after this, apply some stainable wood filler to the seams. Since we were planning to just use Shellac to finish the mantel, we mixed together sawdust from our project and some Shellac to make stainable wood filler.
Once it’s dry, sand off any excess.
Sand the entire mantel with 220 grit sandpaper to prep it for stain or paint or Shellac. We applied 4 coats of Shellac, following the instructions on the can.
STEP 7: INSTALL THE WALL SUPPORT BOARD
To install the mantel, we’ll first secure the back 2×4 support into the studs using 3” screws. Since we wanted to bump ours out a bit more, we already had a 2×4 secured to the wall, so we placed our support board right on top of it and didn’t need to locate any studs.
To determine the height of your wall support board, take the final desired height minus the thickness of the top piece of your mantel. In our case, the mantel wood was 1”, so we placed the top of our support board 55” from the ground, making our finished mantel sit 56” from the ground.
As you’re installing, make sure that your support is straight and level.
STEP 8: INSTALL THE FLOATING MANTEL
The final step is to slide the mantel onto the wall support board. The top of the mantel should sit on top of the wall support. To prevent it from sliding off of the support boards, add a few 1.5” nails through the top of the shelf to secure it to the support boards.
We left the nail holes on ours since it’s rare that anyone is looking over the top of your mantel like that, but if you want to fill them, grab a wood putty that matches your mantel color.
There you have it! Now you know how to build your own DIY mantel!
I will say, in theory this sounds like a pretty simple project. It’s not too many pieces and doesn’t require complicated joinery techniques. That said, the long angles definitely make this project a little more challenging. I just wanted to give you a heads up so that you didn’t trick yourself into thinking this was going to be the easiest project ever!
