How to Build a Flagstone Hearth

Flagstone hearths offer a warmth and richness that only natural materials can provide to your home. While you could purchase a flagstone hearth, you can build your own instead. By building your own you will likely save money and have the advantage of a custom built hearth to suit your personal aesthetic preferences. A hearth is important because it protects your floor from excessive heat as well as stray ashes and embers that may otherwise cause a fire.

Things You'll Need

• Plywood panel, 3/4-inch, 36-by-36-inch
• Cement underlayment board, 36-by-36-inch
• Box, coated deck screws
• Screw gun
• Thinset cement
• Notched trowel
• Flagstone
• Grout
• Grout float
• Sponge
• Bucket of water
• 2 Trim boards, 1/4-by-1-inch, 96 inches long
• Tape measure
• Miter saw
• Hammer
• Box, finish nails
• Nail set
• Wood putty
• Putty knife
• Fine-grit sanding block
• Wood finish
• Paintbrush

Instructions

1. Place the plywood panel flat on the floor in the desired location for the hearth. Lay the cement underlayment board on top of the plywood panel. Align the edges so that they are flush. Drive coated deck screws every 6 inches in rows through the cement underlayment board into the plywood with a screw gun.

2. Spread an even layer of thinset cement over the top of the cement underlayment board using a notched trowel.

3. Lay the flagstones on top of the thinset cement and press them into place as you work from one side to the other. Leave a small 1/8-to-1/4-inch gap between the flagstones to create space for a grout line. Allow the thinset cement to dry for 24 hours.

4. Scoop grout onto the edge of a grout float and drag the float at a slight angle across the gaps between the stones to fill them evenly with grout. Allow the grout to dry for 10 to 15 minutes.

5. Wash off the flagstones with a damp sponge. Rinse the sponge in a bucket of water and continue to wash the surface of the flagstones until the sponge wrings out clear water.

6. Measure the trim boards and cut four pieces to length on a miter saw. Cut the ends at 45-degree angles so that they fit around the hearth assembly like a picture frame. The back side of the trim boards should measure 36 inches. The front side of the trim boards will be slightly longer.

7. Fit the trim boards around the hearth assembly. Hammer finish nails through the trim boards into the plywood base every 6 to 8 inches along the length of each board.

8. Center a nail set over each finish nail head and tap it with a hammer to set the nails 1/16-inch below the wood's surface. Fill the nail holes with wood putty using a putty knife. Allow the putty to dry 24 hours and sand it flush with the surface of the trim boards using a fine-grit sanding block.

9. Apply a wood finish to the trim boards with a paintbrush. Brush on a second coat after 4 hours if another coat is desired.

Tips & Warnings

• Select flagstone that is varied in color to create an natural looking hearth.

• Wear eye, hand and face protection when working with thinset cement and grout. Wear eye protection when operating a screw gun.

References

• "Mother Earth News"; Super-simple Woodstove Hearths; Mike Skelly; October/November 2009
• Hearth.com: Installing a Woodstove - The Basics on How to Install a Wood Burning Stove