How to Build a Wood Fireplace Mantel

Determine Its Size

The first step, and perhaps the most important, is determining the size of the mantel. There’s no one-size-fits-all rule or formula, though a few components of your fireplace can guide your decision.

Choosing crown molding played a major role in determining the height of our mantel. I designed it with readily available bed molding available at most home centers. To accommodate larger crown molding, simply adjust the top rail height and mantel top to suit.

For width, often you’ll see mantels one to two feet longer on each side than the firebox or gas fireplace insert. It’s also common to make it the same width as the fireplace’s hearth.

I suggest cutting a few strips of cardboard 9-1/2-in. wide (the height of our mantel) and asking a helper to hold the cardboard above the fireplace, cutting off or bending over pieces until you find the right size for your eye. In my case, I settled on 71 inches, which is 20-1/2-in. larger than my firebox on each side and the exact width of my hearth.

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Build the Front Panel

To make the front face of the mantel, grab pieces A-D and panels F-G from the cutting list. I constructed the front panel in the same manner as a shaker cabinet door, with a 1/4-in. flat panel dadoed into 3/4-in. stock.

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• Start by running a 1/4-in. wide by 1/2-in. deep dado on the inside edge of the stiles (A) and bottom (B) and top (C) rails that will receive one of the panels. This is best done on a table saw with a dado set, or with the “groove” bit of a tongue and groove router bit set installed in a router table. The mullions (D) will require a dado on both their edges.
• With the grooves cut, cut 1/2-in. tongues or tenons on each end of the top rail (C), bottom rail (B) and both mullions (D). Do this on the table saw with the dado set, or by switching to the “tongue bit” on the router table. Dial this in using scrap wood to ensure the tongue and groove are in exact alignment.
• Glue the panel pieces together and clamp as shown below. Be meticulous and ensure the panel comes out square. If you’re inexperienced at gluing panels like this together, use extended set wood glue to give yourself more time.

Note: Aesthetically, I like the look of a larger center panel and smaller outside panels. However, this is up to the builder. If you’d like an even larger center panel, or three equal panels, adjust panels E and F accordingly.

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Build the Box

When the glue holding the front panel together has cured, build the mantel box.

• Start by drilling pocket holes along one edge of the sides (J) and box top/bottom (H).
• Drill another couple of pocket holes in each end of the box top/bottom (H) pieces.
• Glue and pocket hole screw the sides (J) and box top/bottom (H) together so the edges and faces are flush to form a four-sided box.
• Glue and pocket hole screw the four-sided box, composed of sides (J) and top/bottom (H), to the back of the finished front panel.

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Add the Cleat

We’re hanging this mantel with a French cleat. Here’s how to rip and install the cleat on the mantel side.

• Set up your table saw to rip a 45-degree bevel on the mantle (L) and wall sides (K) you’ll use at installation. Use feather boards to ensure the workpiece stays flat, to ensure both sides of the cleat come out as straight as possible.
• Attach the mantle side of the cleat (L) to the top of the box (H) with wood glue. Countersunk 1-1/2-in. screws so the back is flush to the back edge of the sides and box top.

Pro tip: Before you attach the cleat to the mantle, take something straight, like a four- or six-foot level, and place it against the wall where you’re installing the mantle. If the level sits flat against the wall, proceed as described above. If you find significant gaps between the level and the wall, inset the cleat inside the mantle box as deep as the gap between the wall and the level. Then scribe it for the perfect fit.

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Attach the Mantel Top

To prevent the top (E) from warping, take two four-inch-wide boards and edge-glue them together, rather than going with one eight-inch-wide board. Hence, the two top pieces (E) on the cutting list.

After they’re edge-glued and sanded, attach the mantel top (E) to the top of the mantel box (H). Be sure it’s flush with the back of the box, as well as a 1-1/2 in. overhang on both ends and along the front of the mantel. Secure it with wood glue and 1-1/4-in. screws from inside the mantel box.

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Install the Crown Molding

Fit and install the crown molding along the sides and front of the mantel. Attach it with a pneumatic nailer and 18-gauge finish nails.

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Paint or Finish the Mantel

If you intend to paint the wall behind the fireplace, install the mantel first, then paint or finish it. Together with some caulk or filler, this will create the illusion it’s been there forever.

Install the Mantel

• Hang the wall side French cleat (K) 2-3/4-in. below the finished height of the mantel, centered on where you’d like to place the mantel. Screw it into the wall studs.
  Note: Depending on your state and municipality, there may be codes regarding mantel heights above fireplaces. Be sure to follow these guidelines.
• Slide the mantel onto the wall French cleat so the cleats marry together as pictured in the cross-section below.
• You’ll be able to slide the mantel two inches side to side because the wall cleat (K) is shorter than the mantel cleat (J). Once you’ve found the desired position, mark it on the wall and remove the mantel.
• Squeeze a bead of 100% silicone or construction adhesive onto the top side of the wall cleat (K) and set the mantel onto the cleat. I prefer silicone, since it’s more flexible and moves as the mantel expands and contracts with the fireplace heat.
• Place heavy objects like books or dumbbells on top of the mantel for 24 hours, or until the silicone or construction adhesive dries.

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