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How to Build a Concrete Countertop for Exterior Structures: Part 1 - Building Forms

Estimated Reading Time: 5 Minutes

Concrete counters are easy to make, durable, and weather resistant.

Here's what you'll need:

Materials
  • ¾” Melamine Particleboard
  • #8 x 2” Construction Screws
  • Silicone Sealant
Tools
  • Circular Saw
  • Portable Table Saw (optional)
  • Rafter square
  • Cordless Drill
  • Drill Driver and Drill Bits
  • Caulking Gun
  • Tape Measure
  • Clamps
  • Bolt Cutters or Grinder with a Cutting Wheel
For Your Safety
  • Safety Goggles
  • Gloves

In this three-part series you will learn how to build a form, mix, and pour the concrete, and then stain and finish the tops to protect them.

Concrete Countertops

Concrete countertops are not a new idea, but they remain a great option for DIYers since they are relatively easy to make and can be made to fit custom shapes and applications. On top of that, you can stain the tops in a variety of colors by using dye to tint the concrete. They are also very durable and weather resistant which makes them perfect for exterior projects. Most home centers have special concrete countertop mix in bags which makes it easy to mix your own concrete.

To build a concrete countertop, a mold is built that matches the shape of the top.  This is made either in place or can be inverted.  We chose to build ours with the inverted method. The advantage of the inverted method is that the top of the counter is formed by the mold, so no troweling is required to get it smooth. Another plus is that the tops can be built and finished inside a garage or a controlled environment, and then moved to their final location once they are complete.

Once the mold is built then the concrete can be poured into the molds. Rebar or wire is inserted to strengthen the concrete. This prevents it from cracking. Once the concrete cures, the top can be removed from the mold. Then it needs to be finished, which usually consists of cleaning, staining, and applying a finish that adds protection from spills, dirt, and weather.

This outdoor countertop was divided into four L-shaped sections that surround the fireplace. The advantage of the four sections is that they weigh less and are easier to handle. The joints provide relief between each section which will prevent cracking. Plus, the four sections provide a stylish design element.

PART 1: Build a Custom Form

The forms are built from 3/4” Melamine particleboard. The Melamine provides a surface that does not stick to the concrete.

Step 1: Cut the bases to size. We used a circular saw with a straightedge clamped in place.Step 1

Pro Tip

If you are using a set of sawhorses, add a couple of wooden strips to the tops before you cut with the circular saw. This will allow the saw blade to protrude through the cut into the wooden strip, making it easier to cut the tops. It also protects the sawhorses and saw blade from damage.

Tip 1

Step 2:  Rip the sides to width. It’s easiest to rip the sides with a table saw since they all need to be a consistent width. The width is determined by the thickness of the top. Most tops are around 1-1/2”-thick. Add ¾” to the finished width to allow for the thickness of the base. Make sure to use a push block while ripping since the strips are narrow.2

Step 3: Measure and mark the length of each side piece.

3A 

Step 4: Crosscut the sides to length. A circular saw with a rafter square will keep the cut square. You can also use a miter saw to make the cuts.

3B

Step 5: Pre-drill countersunk shank holes for the screws in the sides before you install them. This will make the sides screw up tight against the base. 

4

Step 6: Position the first side next to the base, keeping the ends flush with the base. Pre-drill pilot holes through the countersunk holes and into the base. This will keep the base from splitting when you drive the screws in. After you drill the pilot hole, drive #8 x 2” screws to secure the sides to the base.

5A       5B       5C

Pro Tip

If you need a second hand to hold a part in place, just clamp a scrap piece of Melamine under the base, with the ends sticking out in both directions. This provides support to position the sides while securing them in place.

5Tip

Step 7: Drill pilot holes and screw the ends together.

6

Step 8: Cut the remaining sides to length and secure them in place as you work your way around the form.

7

Step 9: Apply silicone sealant to all the inside corners. Then run your finger across the caulk to make a smooth radiused corner. This will give the top a smooth rounded edge when it is finished. Let the silicone sealant dry overnight before pouring the concrete.

          9A          9C9B

Now that your forms are built it’s time to add the reinforcing, mix the concrete, and pour it into the forms!

Make sure to check out Part 2: Mixing and Pouring the Concrete and Part 3: Staining and Sealing!

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