How to Build a Concrete Fireplace Hearth
Materials and Tools:
two 3/4" pieces of plywood at 24-1/4" x 6"
two 3/4" pieces of plywood at 88" x 6"
one 3/4" medium-density overlay (MDO) at 86-1/2" x 22-3/4" (Medium-density overlay is an engineered plywood that has been coated with a resin to give it an extremely smooth finish.)
two 3/4" MDO at 88" x 4"
two 3/4" MDO at 21-1/4" x 4"
1" brad nails
90# cement mix
2"x2" board for smoothing cement
1. The first step to building your new concrete hearth is to begin building your frame. Start by attaching your MDO sides to the plywood sides. Your MDF will sit on the inside and will actually be touching the concrete. Do this for all four sides of your frame.
2. Once the sides are made, attach them to the MDO bottom so your MDO sides sit on top of the bottom piece, and the plywood sides cap it at the ends. Attach the pieces by nailing through the plywood and into the edges of the bottom MDO piece.
3. Since your concrete hearth will be extremely heavy, add supports to the bottom side of your frame. Turn your frame over so the bottom faces upward and add 2x4 supports by attaching with countersink screws.
4. Turn your frame over again and place caulking along the seams of your MDO and smooth it out with your finger. This will create smooth and clean edges for your hearth.
5. Once the frame is ready, begin cutting down your rebar pieces. Rebar will serve as an inner support for the concrete, making it much stronger. Attach your rebar with wire ties.
6. Drill 1" screws half way into the outside of the plywood. Use your wire ties and attach one end to the rebar and one end around the screw. This will hold your rebar frame in place, suspended inside the frame.
7. Mix your 90# concrete with a concrete mixer. Once it is ready, fill the MDO frame until it is completely full.
8. Use a concrete vibrator to remove any air bubbles that may be trapped inside the concrete mix.
9. Once the concrete has started to set, take a spare 2x2 board and smooth out the concrete, making it nice and flat.
10. Let stand for three days, or until completely dry.
11. Once dry, remove the wire that was holding the rebar in place, then carefully begin removing the screws, plywood and MDO frame.
12. Grab some friends to help you install your new concrete hearth.
13. Stain and seal your new hearth to make it looking great for years to come.