228) Bathroom Concrete Countertop #1

by Gail on April 15, 2017

You know how you just get used to your home and you stop noticing its deficiencies?

Well, that is what happened with the concrete countertop saga. Friends who drop by say they’ve never noticed we had temporary plywood counters, when I tell them that the countertops are the last big thing to do.

Some family members, however, have been slightly more observant/persistent. No names mentioned, but it must have bugged her enough that she invited all her friends (and one of her brothers) to join in two weekends of concrete countertop production. This spring! Yoiks!

That means I’d better get onto Prototype #2. Read about the mess I made on Prototype #1 here. That was more than two years ago. Clearly, I’m getting lazy.


As before, I was able to make the mold out of the plywood template (temporary countertop) with very little adjustment (I only needed to trim the width with the chop/jig saw because the wainscotting was added to the wall after the cabinet was installed.)

1. Filled and sanded the screw holes – the key to a smooth countertop, I’ve been told, is a perfectly smooth mold. In theory, there would be no grinding or sanding required, if I packed the mold firmly enough.

2. The local building supply store did not have the melamine 4×8 sheets to make the sides of the mold, so I bought a 12″ shelf, which was just enough to make four strips, 2 1/4″ wide. I was able to make use of a damaged one, so it was 1/2 price.

3. Drilled screw holes along the bottom and into the plywood and up the sides to prevent the mold from splaying from the weight and packing of the concrete into the mold.

4. Screwed it all together.

5. Taped side joints, about 1/4″ apart.

6. Applied black silicone to the side joints, smoothed it with gloved finger, removed the tape, re-finger-smoothed. (Lesson learned from Prototype #1 – the tape left a ridge that I didn’t like.)

7. Taped both sides of bottom joints, 1/4″ away from joint.

8. Applied black silicone to the bottom joints (more generously on the front edge that we will see), smoothed it with gloved finger, removed the tape, re-finger-smoothed.

9. Marked and bandsaw-cut 2″ styrofoam sink knockout. Was able to use a damaged piece of styrofoam at a reduced price.

10. Because I was short 1/4″ of depth, I placed 1/4″ spacers under the mold, inserted the 2″ styrofoam, checked it was level, then siliconed it in place.

11. Let the silicone dry overnight. Added tape to the styrofoam to ease unmolding.

At this point, Ben, a young man who wanted to learn about concrete countertops happened to drop by. How very convenient for me! He was a great help for concrete mixing, placing, and photographing (not-to-mention vacuuming!)

12. Spread a thin layer of olive oil on every mold surface. This was a learning from Prototype #1 – the countertop was sticky to unmold that time.

13. I placed some special pieces of beach glass in white, pale lavender, and the pale turquoise of the bathroom’s accent colour. Our daughter and her friends collected beach glass from all over British Columbia for me, and I added pieces collected with our dear friends in Pugwash, Nova Scotia, and from right here on the Sunshine Coast.

No idea what I’m doing here. No idea at all. Did not use silicone to stick the glass down because the olive oil would interfere with adhesion. This concrete countertop business is full of mystery. By the time I find out all the questions to which I have learned answers, we will be finished making countertops.

14. Cut a mesh of 2″x2″ steel grid to fit into each end of the countertop for reinforcement. The local building supply did not carry 1/4″ rebar, so I bought a threaded steel bar to reinforce the too thin sections center front and center back. Bound the front one with wire to the grid. This is probably an overabundance of caution, because the fibre in the concrete mix should give enough strength/stability and the cabinet itself is well reinforced with 3/4″ plywood. I did find, however, that Prototype #1 got a fine crack in it near the front edge.

15. Mixed 1 50-lb bag of white countertop mix with about 5 liters of water (more than the bag instructions suggested). The countertop mix, from StarPatch in Burnaby, British Columbia, is made of concrete plus polyester fibre. Ben and I mixed it until it appeared dry, like oatmeal, but when bumped and vibrated in the wheelbarrow, looks like liquid as the water rises to the surface. I used a wheelbarrow, because the bucket and drill-mixer I used in Prototype #1 was much harder to mix in the dry powder.

16. Started packing into the corners with rubber-gloved hands, patting and packing for about 1/2 hour before mixing the second 1/2 bag.

17. We packed 2/3rds of the depth before adding the reinforcement, laying it on top of the firm concrete. Did not suspend the reinforcement like I did in Prototype #1. I think it may have impeded the packing, causing more depressions than I want in a countertop.

18. Packed the top 1/3 in the same way, patting and pounding and pressing with our gloved hands.

19. Using a board, checked to see if the mold was full but not overfull. Had to mix a tiny bit more in a bowl to fill in the depressions.

20. Ben “floated” the surface with a metal trowel. I tried to assist with a plastic trowel, and it only picked up and dragged the coarse sand across the surface. So, I picked up the edger, and softened the edges with it – this is the bottom of the slab.

21. Ben left. I left. When I came back two hours later, the concrete was leather hard. I re-edged and re-floated the top (i.e. bottom). Then I left it to cure for the three days suggested by Bendixon at StarPatch. I’ll do another post for the unmolding, finishing, and installation.

So far, time required for me to prepare the mold was 4 hours, and for Ben and me to pour and finish was 3.5 hours. Professionals could do it in half the time, I’m sure.

Cost so far: Concrete mix: $40.00, styrofoam: $7.00, melamine shelf: $8.00  TOTAL: $55

Tune in next week for episode #2.





{ 1 comment… read it below or add one }

Joy April 15, 2017 at 5:26 pm

A time to work without rain? Rare day. Very clear tutorial Gail. Looking forward to your next post. Nice that Ben arrived to learn/help out.


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