DIY Network

How to Precast a Stone Fireplace

These step-by-step instructions show how to use precast stone to convert a dull-looking fireplace into a stone masterpiece.

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precast a stone fireplace

Step 1: Prepare the Materials

Determine how much stone will be needed. Take careful measurements of the surface to be covered, and add the square footage.

Tip: Most manufacturers will have the popular models in stock, such as ledge stone or river rock, but if you have to order some more exotic styles such as castle stone, add a couple of weeks to the order for some lead time.

Purchase all of the supplies: stone (figure time for ordering), moisture-resistant barrier such as roofing felt or Tyvek to cover the wallboard, expanded metal lath and mortar. (Mortar type N pre-mixed is easiest to work with and is pre-tinted.)

Tip: Always order extra material.

Make sure you have all the tools needed. (See the materials list, above.)

Step 2: Prepare the Fireplace Area

Take off any existing trim around the fireplace, and the fireplace front glass.

Put on the weatherproof barrier (Image 1) to keep moisture from the mortar from seeping into the wall, which could cause the wallboard to rot if left unattended. Use a hammer and nails or a stapler.

Attach the expanded metal lath to studs in the wall. (You may need to use a stud finder.)

Tip: make sure to wrap the metal lath all away around the structure in one continuous run (Image 2). If two ends are butted together over time a crack can form going all the way down the structure.

Tape edges and cover fireplace opening and surround floor with plastic.

Step 3: Apply the Mortar

Mix mortar until it's firm and moist (just add water to type N pre-mixed mortar).

Tip: Mix mortar to the exact right consistency — not too wet or too dry.

Cover the entire surface with a thin scratch coat of mortar, and be sure to apply enough to cover the mesh.

Let the scratch coat of mortar set up overnight.

apply enough mortar to cover mesh

Step 4: Lay Out the Stones

Set a few bricks on top of the mantel to act as a spacer for the mantel stones to be installed later (Image 1).

Lay out all the pieces of stone on the floor like a puzzle and then you'll see what you have to work with (Image 2).

Tip: The stones need to be put up in a good variety (Image 3) -- thick next to thin, light next to dark, etc.

Step 5: Set the Stones

Pick a corner stone and butter it with mortar over the entire surface of the stone and along the edges (Image 1). Use a special metal spacer on top of a brick (Image 2) to create a gap and place the stone. Put the stone on, then apply it with moderate pressure and hold it for a few seconds, give it a wiggle to set in then take hands away slowly.

Work in sections, putting up the stones and mortar the joints in stages. Do it in about 5 to 10 square feet at a time so the mortar doesn't dry out between joints.

Tip: If the mortar on the wall gets too dry, spray water on it with a spray bottle.

Measure the space between the corner pieces and look for a stone that will fit snugly in the gap. Check to make sure it will fit.

Step 6: Cut the Stones to Fit

If a stone is preferred over another for a specific spot, cut the stone to fit. Hold the saw at an angle and rock back and forth.

Nick the edges up with a hammer to create a more natural stone look.

nick edges with hammer for natural stone look

Step 7: Fill the Joints With Mortar

After the stones are on, squeeze mortar into the joints by using a grout bag (Image 1).

After the joints have been filled with mortar, wait an hour or two to let the mortar set up just a bit then use the 1/2" tuck pointer to both pack in and scrape away the excess mortar. The mortar should be dry enough so it just crumbles away. Keep going until the edges of the stones are exposed.

Tip: To protect the outer walls, be sure to apply some masking tape (Image 2). Once the mortar is dry, remove the tape.

Use a whisk broom to brush away any loose mortar.

Step 8: Apply Stones to the Mantel

Cut all mantel stones to appropriate size.

Lay a bed joint down on the mantel with strips or mortar about 5/8" thick along the outer edge, and then along the inside as well (Image 1).

When the area is well covered, lay the stone in place, wiggle it gently to set it, and then level it. Leave a 1/2" gap to later fill with mortar when you lay the next stone (Image 2).

Tip: When applying hearth/mantel stones, appearance is extremely important. Use the cut edges toward the wall or in the mortar joint, leaving the natural edges exposed (Image 3).

Step 9: Apply Stones to the Hearth

Follow the same process for the hearth.

As the last hearthstone (Image 1) goes into place, tap the stone with your fist to set it in the mortar.

Go back and fill in the gaps in the wall and the joints in the hearthstones. Then, just squeeze out a thin bead of mortar into the gaps between the stones.

Remember: If the hearth platform has an overhang, make sure it's less than 1/2" (Image 2). If not, the stone could break if someone stands on it or if a heavy object is placed on it.

To finish the job, use a tuck pointer handle to clean and smooth the joints between the stones and give it a good wipe with the whisk broom.