Building a Fireplace Mantel
It’s a good idea to start your project with a rough sketch, which is more or less to scale so you can get an idea of the proportions you think would look best. If you plan to use tile around the firebox, you can space the legs away from the firebox according to the tile width you want to use. Keep in mind that tile can be cut to width. If you are using stone it can be cut as well. Do a little research to confirm that the masonry you wish to use will work in the proportions you have planned.
Beginning at the firebox, measure and mark the width of the desired surround, plus 3/4 inch. The extra 3/4 inch allows for the thickness of the boards, which will make up the sides of the mantel legs. Measure from both sides of the firebox out, as well as from the top of the firebox up. For this mantel you will need to allow about 2 inches for the high point of the arc, which is cut out of the frieze board. For example, if you want to use 12-inch tiles, layout for 10 inches on the sides and 12 inches for the peak of the arc.
To get the mark for the top of the mantel legs, you will need to add the height of all of the exposed components up to the bottom of the mantel shelf. At each step you will lose some elevation to the area that is overlapped. For example, our crown molding overlapped the bed molding, which overlapped the frieze board.
Use a level to mark both the vertical lines and horizontal lines. Measure and cut backing cleats for the legs. Depending on the location of the framing, the cleats for the legs can be vertical or horizontal. On our fireplace we used both techniques.
Rip the side wall pieces for the mantel legs. Cut the leg face pieces to length and rip them to width. Sand the saw marks off of the edges of the leg face pieces. Use a router to ease the edges.
Nail the sides of the mantel legs to the backing. Apply carpenter’s glue to the edges of the leg side boards. Align the front faces on the mantel legs and nail them to the mantel leg sides.
Next, locate and mark the framing above the horizontal layout lines. Measure, cut and install cleats along the horizontal lines. Measure, cut and install furring strips to bring the frieze board out to the desired thickness.
After measuring and cutting the frieze board to length, find the center and clamp a triangle-shaped scrap of wood with one point at the center line and the high point of the desired arc. Bend a thin strip of wood over the triangle point and to the corners of the frieze board and trace along the edge. Cut out the arc with a jig saw.
Sand off the teeth marks left by the jig saw. Use a section of the scrap cut off from the arc as a sanding block to match the curve of the arc. Do not ease or round over the edges of the arc.