Building your own concrete sink and integrated countertop leads up to that moment when you knock off the form and witness your creation. It's nothing less than thrilling to work on this project upside-down and in reverse, and then see it all emerge as intended.
A perfectly molded concrete sink plus counter is simply the natural result of careful planning. If you build the forms with enough care and pour the concrete to spec, you'll have a concrete sink molded into its own concrete counter—ready to mount on a bathroom vanity or kitchen base cabinet.
Basics of Making and Pouring a Concrete Sink
You can make a concrete countertop either facing up or down. When you make it facing up, the concrete is poured into a form on top of the sink cabinet. Troweling the top to a glass-smooth finish is difficult.
That's why pouring the concrete upside-down is best for do-it-yourselfers. The smooth bottom of the form becomes the top of the countertop: no troweling necessary. The forms are built with slick, melamine-surfaced MDF. With a loosening agent added to the melamine, it's easy to release the concrete from the form.
When making a concrete sink, a sink-shaped mold is added to the countertop form. The lower part of this mold prevents concrete from entering the sink cut-out area. The upper part of the mold will create the sink basin.
A second form of melamine MDF must be built around the sink basin mold. This will allow the concrete to settle in around the basin section.
Equipment / Tools
- Circular saw or table saw
- Cordless drill
- Caulking gun
- Hacksaw or angle grinder
- Concrete mixing tub
- Garden hoe
- Orbital sander
- Water spray bottle
- Putty knife
- 3 bags countertop-grade concrete
- Concrete countertop patch slurry mix
- Faucet knockouts
- Concrete countertop rubber sink mold
- Melamine MDF, 3/4 inch by 4 feet by 4 feet
- 1 1/2-inch drywall screws
- 1 1/2-inch sink drain pipe
- Black silicone caulk
- Mold release agent (or mineral oil)
- 4 pieces of rebar
- Sheet plastic
- Concrete countertop sealer
Cut Counter Form Bottom
Cut Counter Form Side Walls
Additional strips of melamine MDF form the side walls of the countertop form. They will be attached to the edges of the counter form bottom. Because the countertop should be 2 inches thick, cut the MDF to 2 3/4 inches thick to account for the thickness of the counter form bottom. With a tape measure, verify that the MDF's nominal and true thickness are the same: 3/4 inch. Rip the strips of MDF to the lengths required to create the form walls.
Attach Counter Form Side Walls
Use the cordless drill and drywall screws to attach the side walls to the counter form bottom. Make sure that all melamine surfaces are facing inward.
Fit Sink Mold and Knockouts
Lay the sink mold upside-down on the counter form bottom. Set the rubber faucet knockout molds in front of the sink mold (in the direction of the eventual room wall). When you are satisfied with the positions, seal the sink mold in place by running a bead of silicone caulk around the lip. After the caulk has cured, add the knockouts in a similar manner. Add the 1 1/2-inch sink drain pipe to the sink basin mold.
Completing the sink mold before sealing down the faucet knockouts gives you full access to the lip of the sink mold.
With the hacksaw or angle grinder, cut the four pieces of rebar to the width of the countertop, minus 4 inches. So, if the countertop will be 24 inches wide, cut the rebar to 20 inches long.
Cut Sink Basin Mold Form
The sink basin mold form consists of four side walls (no top or bottom) that encompass the sink basin. The side walls should be spaced 2 inches or more beyond the sink basin mold. At top, the side walls should be high enough so that the concrete will cover 2 inches above the drain.
Two of the side walls should extend 2 or 3 inches beyond the depth of the counter form. The purpose is so the sink basin mold form will be able to rest on two of the counter form walls.
Add Release Agent
Spray or add by hand the release agent (such as a release wax or mineral oil) to the inside of the form and the outside of the sink basin and knockout molds.
Add the dry concrete to the concrete mixing tub. Add water as specified by the concrete mix instructions. Working quickly, mix thoroughly with the garden hoe.
Pour Concrete in Counter Form
Pour the concrete into the counter form. Have an assistant vibrate the mold from either the side or the bottom with the orbital sander as you pour. When the concrete is halfway up the counter form, add the pieces of rebar: two in front of the sink, two in rear. Continue to pour.
Pour Concrete Around Sink Basin Mold
As a continuation of the concrete pour, mount the sink basin mold form on top of the counter form. Pour the concrete into this upper form until it reaches the top. Continue vibrating until all bubbles have disappeared.
Let Cure and Remove Forms
The concrete should fully cure in three to four days. Keep the concrete misted with water during this time. Cover with plastic.
After the concrete has cured, unscrew the forms to disassemble.
Patch Surface and Sand
Mix up a concrete slurry. With the putty knife, patch holes and pits on the concrete surface. After the slurry has dried, sand the concrete surface smooth.
Add sealer to the concrete with the paintbrush or a paint roller.