How to Install a Flagstone Fireplace

Flagstone is a traditional material that has been used for outdoor fireplaces for hundreds of years. Add an outdoor fireplace to your yard for ambiance and warmth on cool evenings. Installing an outdoor fireplace is much easier than an interior one and is generally subject to fewer building codes and regulations. Be sure to check your local building codes and regulations before beginning construction on your flagstone fireplace.

Things You'll Need

• Shovel
• Aggregate gravel
• Concrete, either premixed or powdered
• Wheelbarrow
• Concrete trowel
• 2x4-inch lumber
• Plywood
• Fireproof bricks
• Concrete flue insert
• Fireproof mortar
• Metal lathe
• Hammer-type staple gun
• Stone mortar
• Flagstone

Instructions

1. Build the base for the concrete. Dig out the top 8 inches of soil in the site where the fireplace will be built. Level the soil and cover with 4 inches of aggregate gravel.

2. Mix the concrete following the manufacturer's instructions or purchase premixed concrete that will be delivered to your site.

3. Install concrete over the aggregate rock base. Dump by the wheelbarrow load or have the concrete company dump it onto the rock base. Smooth the top using a concrete trowel.

4. Build the frame for the firebox and flue. Use 2x4-inch lumber to construct a frame the size you want. The flue is the interior lining of the chimney. Box in the firebox/flue framing with plywood.

5. Line the inside of the plywood firebox with fireproof brick and mortar. Install the brick using traditional brick laying techniques. Butter the bottom of a brick with mortar and set in place. When installing the second and subsequent courses of brick, begin with a brick cut in half so they are offset in the traditional bricklaying pattern.

6. Install the formed concrete flue inside the plywood frame. Set it on top of the top layer of fireproof brick, securing it with mortar at the bottom of the flue and along the sides.

7. Staple metal lathe to the outside of the plywood fireplace form. Use a hammer-type stapler and affix staples 4 to 6 inches apart in all directions.

8. Apply a scratch coat of mortar to the metal lathe, using a concrete trowel. Let the mortar set up for the time recommended by the manufacturer.

9. Apply flagstone to the outside of the fireplace, over the metal lathe and scratch coat of mortar. Butter the flat backside of a piece of flagstone with mortar and set it into position, beginning at the bottom of the outside of the firebox.

10. Apply flagstone to the remaining exterior of the fireplace, placing the stones as close together as their shape allows. Use globs of mortar, if necessary, to support stones above gaps formed by their irregularity. Some gaps will remain between the stones; this is normal.

11. Apply mortar to large openings between the flagstone pieces when the fireplace form is completely covered. Use your hand to glob mortar into the larger gaps. For smaller gaps, put mortar into a pastry bag and "pipe" it into the cracks. Hold the bag with one hand near the pointed end to guide the flow of mortar and squeeze the top part of the bag with your other hand.

12. Let the mortar set up for at least 48 hours before using the fireplace.

References

• How to build a fireplace on an outdoor patio