How to Build a Rock Fireplace Hearth

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Fireplaces actually have two hearths: the inner hearth and the outer hearth. The inner hearth---also called the fireplace floor---is an integral part of the structure, while the outer one (the one we usually mean when we refer to the hearth) is often built or put into place after the rest of the fireplace is finished. This outer hearth functions as the all-important interface between the fire and the open floor in front of the fireplace.

Things You'll Need

  • Tape measure
  • Pencil
  • 2-x-4s or 2-x-6s
  • Wood saw
  • Drill
  • Drill bit
  • 3-inch deck screws
  • Screw driver bit
  • Framing square
  • Scrap wood for temporary cleats
  • Cinder blocks if needed
  • Stones
  • Stone hammer
  • Stone saw (if necessary)
  • Mortar mix
  • Mixing container
  • Brick trowel
  • Mason's jointer
  • Straight edge
  • Vinegar
  • Wire brush
  • Decide on the hearth's length from side to side and its vertical depth. It should extend well beyond the fireplace opening on either side. Depth will be determined by the thickness of the stones you are using. Width from front to back---in other words from the floor to where the hearth meets the fireplace's inner hearth---is commonly required by code to be at least 16 inches.

  • Make a simple three-sided wooden form out of the 2-x-4s (or 2-x-6s if your stones are too thick for the 2-x-4s). Do this by measuring, cutting and attaching with butt joints where the ends of the boards meet; basically you're overlapping the boards' edges rather than cutting angled miters. Measure so the inside dimensions of the form will be the outside dimensions of the finished hearth. The form is only three-sided because, when set in place, the rear edge will be formed by the front of the fireplace's inner hearth. Use 3-inch deck screws to attach the form's joints and pre-drill the holes with a bit somewhat smaller than the screw diameter.

  • Set the form in place. If the substrate under the hearth is wood, screw temporary wooden cleats down at the form's corners to hold the form firmly against the inner hearth. If the substrate is masonry, place cinder blocks at the form's corners and anchor them with additional weight to secure the form firmly against the inner hearth. Make certain the form is square.

  • Dry lay the stones in the hearth form to get a sense of placement and fit. Reshape any stones for a better fit with either a stone or brick hammer or a stone saw with a masonry blade. Make any cut with the stone saw from the underside (the side that will not show) at a slight angle pitched closer to the center of the stone on the underside and away from the center on the top side. Use the stone or brick hammer to chip the newly cut edge slightly. This will camouflage any unnatural edge.

  • Once you're satisfied with stone placement and fit, make note of which stones go where (take a photo if it helps) then dismantle and prepare for mortaring. Mix mortar according to directions and trowel mortar into the form. Level it to a depth that will receive the stones and bring their top surfaces flush with the inner hearth. Place the straight edge onto the inner hearth and extend it over the hearth you're building to arrive at the proper level. If the stones vary in thickness, make the mortar depth such that the thinnest stone will be at the correct level. Thicker stones can be pressed deeper into the mortar to compensate for their thickness.

  • Carefully trowel mortar between the stones. Avoid getting mortar on the stone surfaces as much as possible and clean off any that does get on there while it's still wet. Using the mason's jointer, smooth and contour the mortar between the stones. Once satisfied that the stones are placed where you want them and the mortaring is complete---with all the stones' surfaces flush with the inner hearth---allow the mortar to cure undisturbed according to the recommendations on the packaging.

  • After the recommended curing time has passed, remove the form and carefully clean off any thin mortar excess that might be along the edges.

Tips & Warnings

  • If you have a single stone of the right size and shape, or perhaps two that together can form the hearth, you can reduce or eliminate the shaping and fitting of separate stones. This is a matter of preference and availability of building materials. After the mortar is cured, if you do have some mortar stains on the stones, first try wire-brushing vinegar onto the stain. You might find this to be effective and not have to resort to harsher chemicals. There are other acids you could use, but they are hazardous. See the warning below. Whichever cleaning agent you use, avoid getting it in the joints since it will degrade the mortar.
  • Professional masons use muriatic acid for cleaning brick and stone. This is a very strong and very dangerous acid. It's probably worth it to just live with a few mortar stains rather than risking serious injury with this acid. Phosphoric acid, though still requiring caution and careful handling, is somewhat kinder and gentler. If the vinegar doesn't work to remove stains from the rocks, you may want to try phosphoric acid with all due safety precautions.

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