There are two types of household fireplaces: masonry and prefabricated. A masonry fireplace consists of a masonry firebox installed with a brick, stone, or tile facing, and a masonry chimney system, all of which are installed by a mason during the construction of the house. A prefabricated fireplace is made in a factory and then installed in a house. The shell is often made of metal although the firebox may contain a brick lining; the chimney system is usually made of metal. You can create a beautiful stone fireplace frame for a prefabricated fireplace by framing a hearth around a ready-made fireplace insert, boxing it in, and lining it with stone.
Nail standard 2x4 lumber around the fireplace insert and chimney chase to create a sheath or frame. You will also need to nail a board over the top of the fireplace opening to support the frame you have created.
Sheath your frame with ½-inch OSB (oriented strand board) or plywood. OSB is generally less expensive than plywood but also less water-resistant. Sheathing the frame gives it structural support and provides a smooth surface for further work.
Staple #15 builder's felt onto the sheathing using a staple gun. Nail expanded wire lath over the felt using 1-inch galvanised roofing nails. You can also use screws but be careful that the screw heads do not push through the mesh openings of the wire lath.
Apply a light coat of concrete over the wire lath using a trowel. Do not smooth the concrete but leave it rough; the rough texture helps the stone to adhere.
Install thin stone facing according to manufacturer's instructions, covering the entire fireplace. Fill in the grout lines using a grout bag to squirt mortar in between the crevices; smooth with a trowel or your hands.
Consult local building codes before installing the mantelpiece to determine the required clearance. Generally, the deeper the mantelpiece, the more clearance you will need. It is a good idea to know the type of mantelpiece that you want to install before you start building the frame. Decide on what modifications, if any, you need to make to accommodate your choice of mantelpiece. You can begin installing a log slab mantelpiece during the sheathing process by bolting it onto the back of the framing, leaving off one panel of plywood or OSB to allow access for the mantelpiece, then installing the last panel after installing the mantelpiece.