Easy DIY Fireplace Mantel Tutorial

Build a quick and easy DIY fireplace mantel with this simple tutorial.

Easy DIY Mantel Tutorial: wood box beam mantel on a workbench and finished fireplace with DIY mantle in place

The girls and I just moved into our new (to us) house. We’ve been busy unpacking and organizing and repainting the whole interior.

Here’s the fireplace before…

empty living room with old tile fireplace

But I finally got to tackle one of the first fun projects I’ve been dreaming up: The fireplace makeover! And just in time to decorate for Christmas.

Fireplace Makeover

I began by painting and stenciling the tile fireplace surround. You can see the tutorial for that project here.

hand using a grout pen to color the grout between fireplace tiles dark grey

Next, I gave the old gas insert a refresh with high-heat paint. Here’s the tutorial for that:

DIY Fireplace Makeover With High Heat Paint

Once everything was painted, the fireplace was just begging for a mantel. So my dad and I built this quick and easy box beam mantel last week. Then I sanded, stained, and varnished it, and now it’s done!

We built a similar box mantel at our old house, but with pine instead of alder.

I opted for harder and more expensive wood this time because I didn’t want as rustic of a finish for this space. But you could use pine and save a lot of money.

How to Build A DIY Fireplace Mantel

Box beam mantels are pretty simple, and you’ll see different methods for constructing them. I wanted to keep all the seams off the front of the mantel. So the top, bottom, and side boards are all behind the front face board.

Materials

3 1″x7″ hardwood boards (I’m using clear alder)

1 2×4″

4 3″ wood screws

Tools

miter saw

pencil

tape measure

speed square

orbital sander & 220 grit sanding disc

wood glue

pin nailer

drill/impact driver

Directions

1. Choose Sides

We began by choosing the best (prettiest) sides of the wood, and designating which piece would go on the front, top, and bottom. We marked them with painter’s tape to help us keep track.

It’s easy to mix up the boards once you get going, since they’re all going to be cut to the exact same size.

close-up of a board with a piece of painters tape labeled "front"

2. Cut Boards

Measure and cut the board to the precise length. Mine are 57″ long because I wanted the mantel to extend just two inches past the tile surround on each side.

board being measured with a speed square

A miter saw works best for this.

3. Glue Front & Bottom Together

It’s helpful to have two sets of hands for the next few steps. And if you don’t have a second person to help, you’ll probably need to use some wood clamps.

Line up the front and bottom boards, and apply a thin line of wood glue along the top edge of the bottom board.

wood glue being applied to boards before attaching them together to make a mantel

Then secure the boards together, making sure both ends and sides are flush.

4. Nail

Use a pin-nailer to attach the front and bottom boards together. The tiny nail holes will hardly be visible on the front once this is finished.

5. Glue Front & Top Together

Next use the wood glue again to glue the front and top boards together.

6. Nail

And pin-nail these two board together as well.

pin nailer attaching the boards together to make a DIY fireplace mantel

7. Measure Inside

Once the three long boards are assembled, measure the inside opening on each end of the mantel. This needs to be very precise, so measure both ends.

end of a partially assembled box beam mantel with a tape measure measuring the inside edge

8. Cut End Pieces

Mark the end pieces with a speed square and cut them to size with the miter saw.

man using a pencil and speed square to mark the end of a board to build a mantel

9. Glue End Pieces

Run a line of wood glue along the inside of the mantel end, then smear is with your finger.

finger applying wood glue to the inside of wood mantel before attaching the end piece

Set the end piece in place, wiping away any excess glue that squeezes out on the outside end.

As you can see, we in-set the ends a little bit just because. You can do this or make the end pieces flush with the ends of the other boards.

inside end of a DIY box beam mantel being assembled

10. Nail End Pieces In Place

Nail the end piece in place with the pin-nailer. We did two nails on the bottom and two on the top.

man using a pin nailer to attach pieces of a DIY wood mantel together

11. Sand

Use an orbital sander with a 220 grit (very very fine) sanding disc to go over the whole mantel and smooth out any rough edges. Be sure to wipe it down with a damp rag afterward so you don’t get any sawdust in the finish.

woman using an orbital sander to sand a DIY box beam mantel

12. Apply Stain

Choosing wood stain is difficult. I highly recommend doing a test on a scrap of the same wood you used for your project to be sure you like the final color.

can of special walnut colored wood stain and a paintbrush

My go-to wood stain color is “special walnut”, so that’s what I’m using for this project too.

hand holding a wood stain sample up to the fireplace tile

You’ll want to be sure to read the instructions on the can.

Apply the wood stain with a paintbrush or chip brush (disposable paintbrush). Let it sit for 10 minutes or so, but no more.

close-up of a paintbrush applying dark brown wood stain to a DIY fireplace mantel

DIY fireplace mantel with stain soaking into it

Then use a rag to wipe it off, following the direction of the wood grain.

close-up of a hand using a paper towel to wipe wood stain off of a DIY mantel

Let this dry for at least two hours before applying a second coat (optional). It’s important to note that the cut ends of the wood will end up much darker than the broad sides of the boards. I find it’s easiest if you can just embrace this.

end of a handmade wood mantel with walnut colored wood stain

13. Apply Polyurethane

Next apply polyurethane to protect the wood. I’m using a matte finish because I don’t want my mantel to be shiny. But you can choose whichever sheen you like.

can of matte polyurethane with a paintbrush on top of it

The polyurethane looks a bit milky in the can, but it will dry clear. Let it dry and then apply a second coat.

close-up of a paintbrush applying polyurethane to a DIY wood mantel

14. Mount 2″x4″

We used the 2″x4″ method to mount the mantel on the wall. This is pretty simple, but it’s important to be sure the screws are going into studs on both ends. No one wants this thing to come crashing down!

We first used a stud finder to locate the studs, and marked their positions on the wall.