Concrete Kitchen Countertops
Making Concrete Counter Tops in Molds
Concrete kitchen countertops can be made either in place on top of the existing cabinetry or can be poured into a mold and finished off-site before installing on top of the cabinets. This article will describe the basic process for making concrete countertops in molds.
Building the Concrete Counter Top Mold
Making a concrete counter top mold means building a mold that is upside down and backwards from what the finished countertop will look like. This is because the concrete will be poured into the mold and then left to sit for a couple of days before being turned over and out of the mold. The concrete resting on the bottom of the mold will become the finished surface of the installed countertop.
Make a pattern for the concrete mold by laying down strips of cardboard or thin strips of wood onto the perimeter of existing cabinetry. Allow for an overhang over the front of the cabinets. Use a hot glue gun to attach the strips together.
Allow 1/8” gap between the mold pattern and any walls to allow a bit of wiggle room when installing. Also lay down strips to mark where any openings for sinks will be. Carefully label pattern, indicating top, left, right, front and back of countertop. Indicate location of any faucets or soap dispensers as well as the centerlines of any sink knockouts.
Each length of counter will require its own mold. Ideally, no molds should be longer than ten feet long. Even at this length a few people will be required to flip the concrete-filled mold.
The easiest material to use for mold making is white 3/4” melamine-covered particle board. The final counter top surface will be very smooth without any troweling required. Flip the pattern upside down onto an appropriate size piece of melamine and trace the outline. Saw along the outline with a saw blade appropriate for melamine.
Use only eraseable pencil to write on the melamine. Some felt pen markings will transfer to the finished counter. On the edges of the mold, transfer the pattern's markings of left, right, front and back, and label inside with an arrow. Be careful not to scratch or nick the melamine since all imperfections will transfer to the countertop surface.
Every five feet or so there should be a break in the concrete. Pieces longer than this will be more difficult to install and will be prone to stress cracks. To create a break in the concrete, vertically insert a piece of painted aluminum flashing into the mold. To do this, route out a channel into which the flashing can be inserted. Cut the flashing so that its top is flush with the mold's side walls. As long as the flashing is painted, the concrete will not stick to it.
Creating Concrete Countertop Knockouts and Openings
For sink openings, trace the pattern piece that came with the sink onto a piece of dense styrofoam such as that used in insulating walls. The styrofoam will need to be the exact thickness of the finished countertop. With a band saw, carefully saw along the line drawn on the styrofoam.
If an undermount sink will be installed, try to keep the sawn edge as smooth as possible. Wrap the sawn edge of the styrofoam with packing tape, keeping the tape completely smooth. Any wrinkles in the tape will create small dented lines in the finished exposed sink edge. Have the tape start and finish on the edge of the sink knockout that will be closest to the front of the counter. Imperfections in the final counter will rarely be seen here.
Make all faucet soap dispenser mold knockouts out of PVC pipe or wood of the diameter required for the fixtures. Keep in mind that the faucet knockouts will need to be big enough on the underside of the counter to accommodate wrenches for installing sink fixtures.
Oftentimes, it will be necessary to use two different diameter pipes stacked one on the other – the first, sitting on the bottom of the mold, is a narrower size to fit the actual faucet hardware and the latter sitting on top of the narrow one, is the widest to accommodate installation tools.
Attaching the Concrete Counter Top Mold Sides and Sealing
Once all voids have been created and attached to the mold surface, it is time to attach the side walls. Cut melamine sidewall pieces to fit around the perimeter of the counter mold. The sidewall height should be such that the inside mold height is the same as the desired counter top thickness. Pre-drill holes and turn in drywall screws every 6” along the outer edge of the mold and along the corner butt joints.
Now the mold must be silicone sealed so that water does not leak out the edges of the mold after the concrete is poured. Using painter's masking tape, mask all around the inside edges of both the mold surface, the sidewalls and any dividing channels. Leave about 1/16” gap between the tape and the edges all the way around.
Using a caulk gun, carefully squeeze silicone into the inside edges of the mold. Use just as much as is needed to fill the space between the two adjacent pieces of tape. Use a moistened finger to smooth the silicone to a gentle convex shape. Any bumpy spots in the silicone will result in a bumpy edged counter. Work quickly since once the silicone starts to skin it can not be worked any longer. Remove tape once the silicone has been smoothed.
Pouring the Concrete Kitchen Countertop
The time spent making a mold accurate and well sealed will reflect directly on the quality of the finished concrete counters. Once the mold is complete, it is time to move on to pouring the concrete counter tops. Continue reading the next article on pouring concrete counters in this series.
Related Articles:
- Making Concrete Counter Tops: Required Concrete Countertop Supplies and Sources for a DIY Project
- Finishing Concrete Counters: How to Cure, Grind, and Polish a Concrete Counter Top
References:
- Cheng, Fu-Tung. Concrete Counterops. Taunton Press. 2002.
Concrete Counter Tops
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