How to Build a Concrete Countertop
Learn how to construct and install a concrete slab countertop for a kitchen island.
Making your own concrete countertop is one of the more challenging — and rewarding — DIY projects. If you’re a rookie, keep your goals simple. Plan for basic color treatments and plain edges. The key to success is building a sturdy, leak-proof mold in which to pour the concrete. Plan to spend a couple of weekends on this project. It takes about a week for the concrete to cure, so work in a space where the countertop will not be disturbed.
- table saw
- concrete stamp
- bolt cutters
- chemical resistant protective gloves
- orbital sander
- concrete mixer
- liquid release agent
- galvanized structural stucco wire
- eye protection
- muriatic acid
- 1" melamine-coated MDF boards
- 2" screws
- ready-mix concrete
- release agent
- masonry sealer
- silicone caulk
Step 1: Build the Mold
To determine the size of your countertop, measure the base cabinets that the countertop will be sitting upon (Image 1). Add an extra 3/4" for each overhang. Our island countertop will be 3’ x 4’.
The key to a great countertop is a well-built mold to pour the concrete into. Start with a 4' x 6' piece of 1" thick melamine particleboard. Set it firmly on a pair of sawhorses. Measure and mark the exact dimensions on the mold base (Image 2), then cut to your dimensions using a circular saw.
Cut the sides of the mold. Measure and mark four 2-3/4" wide strips. Use a table saw to make the cuts. We cut our four strips to 4' long even though we will be cutting the shorter sides down a bit.
Attach the longer sides first. Drill 2" pilot holes every 6 inches along the bottom of the side piece and into the edge of the large horizontal piece of melamine. Then secure the two pieces with 2" wood screws. Repeat for the other long side. For the shorter sides, trim the strips to size and attach them in the same way you did for the longer sides. Use a carpenter's square to check corners, and adjust as necessary.
To determine the size of your countertop, measure the base cabinets or your existing countertop. If measuring the base cabinets, add an extra 3/4" all the way around for an overhang.
Measure and mark the exact dimensions on the mold base, then cut using a circular saw.
Step 2: Make Cutouts
If you need to have a cutout in your countertop for a cooktop or a sink, measure and mark it on the mold's base. When determining the size of the cutout, take into account the thickness of the side pieces of the melamine mold. Do a dry fit of the cooktop or sink to make sure the size is correct.
To start the cut, drill pilot holes on the inside corners of the portion that will be removed. Then insert a jigsaw into the holes and cut from hole to hole along the edge marks. Make sure your cut lines are straight so you won't have large gaps between where the horizontal and vertical pieces of the mold will meet.
Then measure and cut the side pieces for the cutout. Attach them the same way you attached the outer side pieces by butting them against the edges of the base inside the cutout (Image 1).
Carefully clean the mold of sawdust and other materials. The bottom of the mold will be the top of the countertop, so it’s important the concrete sets on a debris-free surface (Image 2).
Run a small uniform bead of 100-percent silicone caulk in all the inside corners and seams of the mold. Smooth the bead with a caulk tool and let it dry thoroughly for 24 hours (Image 3). You can also use the tip of your finger to smooth out the caulk. The silicone will seal the joints of the mold and prevent the wet concrete from leaking out of the mold.
After cutting out the necessary sections, attach the sides to the inner part of the cutout.
The bottom of the mold will be the top of the countertop, so it’s important to clean the melamine so the concrete sets on a debris-free surface.
Run a small uniform bead of 100-percent silicone caulk in all the inside corners and seams of the mold. Smooth the bead with a caulk tool or your finger and let dry thoroughly for 24 hours.
Step 3: Build Support Frame and Prep Wire
When the outer mold is done, you’ll need to build a support frame to surround it. The concrete you’ll be pouring is heavy — about 10 to 15 pounds per square foot so the extra support is important to prevent the edges of the mold from bending due to the weight of the concrete.
Cut several 2x4s (we used three, if your countertop is bigger, use more) 3-1/2" longer than the length of the mold. Lay the boards underneath the mold with 1-3/4" overhanging each end.
Cut two 2x4s 3-1/2" longer than the shorter sides of your countertop. Attach these two pieces to the overhanging sections of the 2x4s you placed under the mold — center them so you'll have a 1-3/4" overhang on each side. You’ll want these side pieces of the support to be snug against the mold. Attach them from underneath using 2" screws. Cut two more 2x4s to fit against the long sides of the mold to complete the support frame. DO NOT attach the support frame 2x4s to the mold itself. The 2x4 support pieces should be attached to each other and the mold should lie within the frame. And again, the frame should be tight against the mold to prevent the heavy concrete from pushing the mold out of shape (Image 1).
The last step in preparing the mold for the concrete is to cut a section of galvanized structural stucco wire. This will be added to the concrete during the pour to add strength and prevent cracking. Use metal snips to cut the wire to the shape of the mold so that it comes about 1 inch off the edges all around (Image 2). Cut the wire before you mix the concrete so it is ready when you pour the concrete. You can find galvanized structural stucco wire at most concrete supply stores.
A sturdy frame made of 2x4s all the way around the mold will keep the heavy concrete from bending it out of shape.
Use clippers to cut the stucco wire to size. Do this before mixing the concrete.
Step 4: Mix and Pour Concrete
For our 12-square-foot countertop, we needed three 60-pound bags of ready-mix concrete. Add water to the concrete and mix with a shovel per the manufacturer's instructions.
If you want to add color to the countertop, now’s the time to add pigment to the mix (Image 1). Pigment additives come in powder or liquid. Liquid pigments are easy to measure and mix, especially with small concrete batches like this one. But don’t forget to account for the amount of water in the pigment when measuring the water for the concrete. Controlling the amount of water added to the concrete mix is critical to producing consistent color. Refer to the manufacturer's guidelines.
Mixing the concrete correctly is critical to its strength and durability. When it achieves the texture of peanut butter it’s time to add it to the mold. Remember that the concrete at the bottom of the mold will become the top of the concrete slab.
Using a small spade or bucket, pour the concrete into the mold, pressing and compacting it as you fill the mold to a depth of about 1" or half full (Image 2). Set the galvanized wire into the concrete, be sure that it does not touch the edges of the mold (Image 3). The wire will keep the concrete from cracking as it dries and it will also add strength to the slab.
Continue to fill the mold on top of the wire, tamping the concrete with a trowel, as you go along to ensure it is well packed. Your objective is to slightly overfill the mold. The level of concrete will drop slightly in the mold as it settles. Smooth the concrete surface with a hand trowel. This will draw the aggregates to the top.
To settle the concrete, use an orbital sander without sandpaper against the sides of the mold. The vibration from the sander will help bring air bubbles in the concrete up to the surface (Image 4).
When finished, gently cover the countertop with a sheet of plastic or damp burlap to protect it from dust and dirt (Image 5).
Let the concrete cure at least one week — the more it cures, the stronger it gets.
Add water to the concrete and mix with a shovel per the manufacturer's instructions. If you want to add color to the countertop, now’s the time to add pigment to the mix.
When the concrete is ready, pour it into the mold about halfway full and then smooth it with a trowel.
Lay the stucco wire onto the concrete. This will add strength and prevent the concrete from cracking.
Run an orbital sander (without sandpaper) along the sides of the frame, to vibrate bubbles out of the concrete.
When finished, gently cover the countertop with a sheet of plastic or damp burlap to protect it from dust and dirt.
Step 5: Remove the Mold
Remove the 2x4 support frame from the sides and ends of the mold. To do this, carefully drill two 2" screws equal distance apart, halfway into each side of the mold. Be careful not to drill all the way through — you don’t want to disturb the mold edge. Remove the screws holding the mold sides to the base. Then use the claw end of a hammer and the new screws to pry each side away from the concrete slab (Images 1 and 2). Take your time with this step. You don’t want a misstep that will cause a chip or any breakage.
Get help from another person or two to flip the slab over. Remove the cutout sides using the same technique as you did for the outer pieces. The weight of the slab usually makes removing the melamine base an easier task (Image 3).
Carefully use the claw-end of a hammer and the new screws to pry each side away from the concrete slab.
Remove the screws holding the mold sides to the base, then use the claw-end of a hammer and the new screws to pry each side away from the concrete slab.
Get help from another person or two to flip the slab over. Once the sides of the form are removed the base piece of the melamine should easily be pulled off the concrete.
Step 6: Finish the Slab
When the slab comes out of the mold, it’s going to have imperfections. Use an orbital sander to smooth out the surface and edges. Wear a respirator; this is a dusty process. Start with 100-grit sandpaper. Keep one hand on the top of the sander as you polish the edges. Be prepared to use lots of sandpaper. Work to progressively finer grits, finishing with 220-grit. Sand evenly, checking the smoothness as you go. Keep sanding and testing until each edge and surface feels smooth to the touch (Images 1 and 2). When done, wipe the slab with a damp rag to remove any loose grit and concrete dust.
Prepare the surface for finishing by etching it with a solution made from 1 ounce of muriatic acid mixed in 1 gallon of water. Wipe the surface thoroughly with a sponge dipped in the acid solution. Wear eye protection, a respirator and acid-proof gloves. If you’re working inside, open doors and windows for better ventilation. Rinse the slab with fresh water to remove the acid mixture and then let it dry completely.
Apply a concrete sealer using a sponge or brush. Work in long strokes from one edge to the other. Let the sealer dry then apply a second coat, working at right angles to the first. Keep applying coats until the concrete won’t absorb any more liquid. Let it dry thoroughly, about 30 minutes.
Wearing a respirator, use a sander to smooth the concrete countertop. You'll need to work progressively from 100-grit sandpaper, finishing with 220-grit.
Sand the concrete evenly, checking the smoothness with your hand. When done, wipe the slab with a damp rag to remove any loose grit and concrete dust.
Step 7: Install the Countertop
Prepare the installation by running a thick bead of silicone caulk around the upper edge of the cabinet. Bring the countertop in, set it in place and press down gently to seal the caulk.