How to Frame an Arched Fireplace Surround

Measure the height, width and depth of the firebox, which is typically made of steel or masonry. Building codes mandate that at least 6 inches of nonflammable materials surround a masonry firebox. Check your local codes if yours is made of steel.

Sketch out the planned surround with detailed measurements. Include overall height, leg width, header height and mantel dimensions, if applicable. The header, or under-mantel, is the arched portion of the surround. The firebox must be at least 12 inches deep if any part of the surround will project 1 1/2 inches or more beyond the face of the firebox.

Measure out four lengths of 1 1/2-by-3 1/2-inch lumber for the leg framework. The vertical framing pieces should extend from the floor to the top of the header. Cut these to size with a skill saw, using standard 90-degree angle cuts.

Measure out a piece of 1 1/2-by-3 1/2-inch lumber equal to the length of the header. Cut it to size.

Lay out the header and leg pieces on the floor. Position them so the header caps the leg pieces and its outside edges are flush with the outside surfaces of the outermost leg pieces. The location of the other two leg pieces depends on the overall width of the legs. Mark the location of each leg piece on the header.

Attach each leg piece to the header with wood glue. Secure the joints by driving 10d finishing nails through the topside of the header.

Measure the distance between the vertical pieces of each leg at its base. Cut a length of 1 1/2-by-3 1/2-inch lumber to fit each leg. Position each horizontal piece so its bottom surface is flush with the bottom edges of the vertical supports. Secure them with wood glue and 10d finishing nails.

Measure out two more pieces of 1 1/2-by-3 1/2-inch lumber to create the vertical supports for the header’s arch. These supports should run from the floor to the arch’s ends. Cut them to size with the skill saw.

Position the vertical arch supports flush against the outside surface of each inside vertical leg piece. The bottom edges of the supports should be flush with the bottom of the unit. Glue and nail each piece into place.

Measure out a piece of straight-grained 1 1/2-by-3 1/2-inch lumber to create the arching portion of the framework. The exact length of the piece depends on the width of the surround’s opening and the height of the arch. Cut the piece to size with your skill saw making 90-degree angle cuts.

Mark out the kerfs on the bottom side of the arch piece. Kerfs are evenly spaced vertical cuts that leave about 1/8-inch of wood below them, allowing the wood to bend. The exact spacing of your kerfs depend on the radius of the desired bend. In general, spacing them 1/4-inch apart allows for easy bending. Kerfs should begin about 1 inch before the piece curves and end about 1 inch past the curve’s other end.

Cut the kerfs, carefully and consistently, with a miter saw. Maintain consistent downward pressure to ensure you don’t cut all the way through the wood.

Position one end of the arch piece up against the top side of the corresponding inner vertical support. The surfaces should be flush, and the kerfs should be face down.

Gently bend the arch piece until it curves into position. Bring the other end flush against the top side of the opposite inner vertical support. Check the shape of the arch to ensure it’s even before you glue and nail it into place.

Smooth out each butt joint with a belt sander before constructing the face of the surround on the framework.