How to Build a Wood Fireplace Mantel

Place a measuring tape across top of your firebox opening from edge to edge and take an accurate firebox width measurement. Hold the measuring tape at one of the upper outside corners of your firebox and extend it straight to the bottom of the firebox opening to measure the firebox height. These measurements determine the minimum internal height and width of the fireplace mantel and surround. Measure from each outside corner of the fireplace box opening to the next corner or doorway in the room to determine the maximum available wall space surrounding the firebox. Make a note of these measurements on a piece of paper with a pencil.

Sketch your wooden fireplace mantel design to scale. Include depth and width dimensions for your mantel as well as the horizontal and vertical sections of your surround. Include the dimensions for the plinth blocks (the decorative wooden blocks that are placed directly under your mantel on each side of the fireplace mantel), and any columns, raised panels and decorative moldings that you want on the front face of the mantel, on your scale drawing. Keep the overall size of the room in mind.

Contact your local building inspector and obtain the building codes regarding the placement of wood to the left, right and above the firebox opening. Compare your scale design with the local fire-safety codes and make any adjustments to the exact measurements, if needed.

Clamp your choice of standard dimensional lumber level on a table saw table. Cut the vertical and horizontal mantel and surround components to the measurements on your scale drawing, one at a time, with a table saw.

Install an edging router bit in your router collet (the opening in your router in which the router bit is inserted). Clamp one of the pieces of surround face or decorative moldings that requires profile edging flat on your work table, with the edge you are routing extended beyond the work table edge, with wood clamps. Hold the cutting tip of the router bit firmly against the length of wood and slowly move the router along that edge. Repeat the clamping and routing steps for all of the fireplace mantel pieces that will have a sculpted edge.

Install a profile shaping cutter bit in your router collet. Clamp one of the plinth blocks on your work table, with the profile you are routing extended beyond the work table edge, with wood clamps. Hold the cutting tip of the router bit firmly against the block of wood and slowly move the router along that edge. Repeat the clamping and routing steps for the other plinth block.

Install a crown molding shaper bit in your router collet. Clamp one of the columns or half-columns or raised panels on your work table, with the profile you are routing extended beyond the work table edge, with wood clamps. Hold the cutting tip of the router bit firmly against the piece of wood and slowly move the router along that edge. Repeat the clamping and routing steps for the other decorative columns or raised panels.

Adjust a miter saw table to 45 degrees by turning the lower adjusting knob. Hold the first piece of decorative molding that requires a miter cut firmly against the miter saw guide (also known as the fence) and lower the miter saw blade onto the molding. Repeat this for all of the lengths of molding that you routed.

Assemble the horizontal and vertical mantel base surround components by applying fire resistance rated construction adhesive to the joint edges, using the adhesive applicator tip, and nailing the components together with a nail gun. Building the mantel surround horizontal piece and side pieces with two corresponding edging pieces flush within the one flat face piece creates the needed depth.

Locate the wall studs above the planned top horizontal mantel surround position with a stud finder, and mark their location on the wall. Hold the horizontal mantel surround against the wall in its planned location. Place a bar level along the top edge of the mantel, view the horizontal level tube to verify that the air bubble is in the center of the tube between the two lines and then place the level along one of the vertical sides and view the vertical level tube to verify that it is plumb. Nail the horizontal mantel surround structure to the studs, above the firebox opening, with a nail gun.

Apply fire resistance rated construction adhesive to the inside edges of the vertical surround base components, using the adhesive tube applicator tip. Plumb the vertical surround base components with a bar level and visually verify that they are resting squarely on the floor before attaching them to the wall with a nail gun.

Apply fire resistance rated construction adhesive to the backs of any fluted pilasters, columns or half-columns, raised panels, ornamental carvings and moldings, with the adhesive tube applicator tip, and press them to the surround face. Reinforce their placement by placing evenly spaced brads along each joint with a brad nailer. Do not add any plinth block moldings at this time; they will be added after the mantel is installed on the surround.

Apply a wide bead of construction adhesive to the upper surface of the horizontal and side components of the surround, where the mantel board will rest, with the adhesive tube applicator tip. Position the mantel centered on the completed mantel components, level it with the bar level, and secure it to the surround with a nail gun.

Butt the top edges of the plinth blocks level against the bottom of the mantel and nail them in place with a nail gun. Add the decorative plinth block moldings with construction adhesive and the brad nailer.

Fill the nail holes with interior-grade spackling compound, by dipping a putty knife into the compound and then wiping the edge of the knife over the nail holes. Allow the putty to dry for thirty minutes. Sand the mantel with 220-grit sandpaper. Wipe the mantel with a tack cloth to remove the sanding dust.

Apply a consistent coat of wood stain to the mantel with a cloth, wait five minutes and wipe off the excess stain with a clean cloth. Allow the stain to dry for two hours before adding an additional layer of stain to darken it, or continue applying a fire resistance rated sealer with a detail paintbrush.