Pour a Concrete Countertop for an Outdoor Bench

In this episode of the garden potting bench build I’ll make the form and cast the concrete countertop, then add the sink and upper and lower shelves. In the previous episode I built the frame, poured footings to secure it to the ground, and added a corrugated metal roof. This is Part 2 of the series.

See the Youtube Video and download the Plans.


Background for Pouring the Concrete Countertop

YouTube player

(transcript of the Build Video Part 1)

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof

This video will focus on pouring the concrete countertop,  setting it on the top rails, installing the sink and drain plumbing, and adding the top shelves, then finally adding the bottom shelves with the corrugated panel back wall.

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
3D model of potting bench

Concrete Countertop Form

The cast concrete countertop will be poured in a shallow form made with a melamine coated board and one by walls.

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Concrete countertop form

The back of the countertop will be notched around the center back 4×4 support. I’ll be casting two holes during the pour: one for the sink, and the other for a waste shoot.

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Formed concrete countertop 3D model

Make the Countertop Sink Knockout

I need to create a hole in the countertop for my DIY sink I’m making from a stainless steel mixing bowl. I didn’t have any pipe or anything else the right size for this so, I’ll have to make a disc from scratch. 

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Cutting a disc with jigsaw

I glued two squares of 3/4″ plywood together to give me a 1 – 1/2″ tall disc. Which is also the thickness of the counter I’m making. When the glue dried I scribed a circle with a compass. Then rough cut this disc with a jigsaw.

Sand the Blank Smooth

I slightly tilted the table of my disc sander to create a draft angle on the disc. Then I rotated it to sand it round and smooth.

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Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Sanding the knockout on the disc sander

I gave it two quick coats of spray polyurethane to seal it from the concrete. To keep it from absorbing any water and hopefully make it easier to knock out after the concrete hardens. 

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Coating wood disc with spray urethane

Build the Concrete Countertop Mold

The bottom of the countertop form is made from 5/8″ thick Melanie coated board. I wanted to buy the 3/4″ thick board but in my haste I pulled the 5/8″ from the stack by mistake. It will do but the 3/4″ is stiffer so its preferred. 

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Cutting the base of the form with a jigsaw

Anyways, I cut this to the final width and length of my countertop. I’ll mark the cut-out for the rear middle leg of the bench and cut this with the jigsaw.

Concrete Form Walls

The walls of the form can be made from strips of melamine but I had some one by spruce around so I ripped them to width, cut them to length, then gave them a quick two coats of spray polyurethane. 

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Coating wood form walls with spray urethane

The walls are screwed to the base with self drilling cabinet screws.

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Attaching walls to the form with screws

I marked a line on the walls so the screws would end up in the middle of the melamine.

Round Waste Hole Formed from Pipe

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Tapered pipe cut from larger section

The waste hole will be formed from a section of plastic drain pipe I found. I cut a section near the end of this pipe with a handsaw. This sectional had a nice taper to it as it reduced in diameter. I thought that would be just perfect for the waste shoot.

Attach Blanks to Base of Concrete Form

The plywood disc that will form the sink hole is screwed to the base of the form from underneath.

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Attach sink hole disc to bottom of form

The plastic pipe will he held in place with a few blocks of wood and a metal scrap.

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Hold downs for pipe

Seal all inside edges of Concrete Form

A bead of latex based caulk is added to all corners of the form and around the pipe and plywood disc.

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Caulking the concrete form with latex

I smooth this bead and remove the excess with my finger. I’ve been using latex caulk for most of my form work as I find it easy to remove later and it’s not so overly strong. And it’s usually a bit cheaper than pure silicone.

Cast your own Concrete Countertop for a Potting Bench with a Galvanized Corrugated Metal Roof
Caulking the concrete form with latex

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