Making a Bathroom Concrete Countertop

by Dave Morrison
(Lindenhurst, Illinois)

Q: -How to set up a bathroom concrete countertop mold design-what do I use to make it? Order it or off the shelf like HD or Menards?
-best mix to use?
-want top to b in flat or semi-gloss white and super smooth?
-want two rectangular bowls either top mount or under-mount or molded into form and how to put into mold if not doing other two?
-sealing necessary?
-weight? for 85 long x 22.5 deep x 1.5inches thick and do I need to reinforce base cabinets?
-making backsplash?
-do I need to add metal as a reinforcement to top
-Can it be cut with a saw or hole saw?
-is edge able to put a design into and how?
-How do all this on the cheap? Wood, rubber, etc

Thank you for your help. I plan on doing a sample first to learn and plan on doing half the top then when its done I want to do the entire top as one piece.

Thanks,
Dave

A: Dear Dave,

You're wise to do a sample first before tackling construction of the actual countertop. Concrete itself isn't expensive, so if you mess up, the materials haven't cost you too much. The reason having concrete countertops made is expensive is the skill involved in doing it well and the more artistic finishes you can get for them. Let's take your questions one by one:

How to set up mold design-what do I use? Order it or off the shelf like HD or Menards?

You can build the frame using pine or poplar 1 x 3s. Attach them with screws. For the bottom of the frame, use 3/4 inch medium density fiber board. Attach this to the 1 x 3s with deck screws. Seal all inside edges and corners neatly with caulk, and then coat the inside of the frame with paste wax for easy removal.

As for molds to create custom edge designs, well, it's up to you. If you have a piece of moldwork of the right length that you'd like to reproduce as an edge to your countertop, then you can buy a polyurethane mold kit at your hardware store and make a mold yourself, but you can also order them pre-made. You'll put these inside your frame along the edge you want to customize

The same is true if you want a custom sink basin made of concrete. You can make your own mold, but there are plenty of ready-made designs available. Personally, I think making your own molds may be biting off a little more than you want to chew since this is your first concrete countertop, but if you're up for it, then who am I to stop you?

Best mix to use?

The short answer is that you can buy premixed countertop concrete that only requires you to add water. SureCrete, Counter Weight, and Cheng Pro Formula are three of the many brands available.

If you're making it from scratch, you can use Type I or Type III Portland cement. While Type III has earlier strength gains, the 28 day strength is about the same for either one. Be aware that if you want white Type III Portland cement, sometimes you have to buy it by the pallet-load. If you mix from scratch, keep your water cement ratio low. A 0.4 water cement ratio will give you compressive strength of 5600 psi when it's cured properly. But a 0.8 water cement ratio will only give you compressive strength of 2000 psi when cured properly.

Want top to be finished in flat or semi-gloss white and super smooth?

You can get your countertop anything from sidewalk rough to super smooth, depending on how carefully and extensively you want to sand with an orbital sander to finish the job (this could take many sanding disk refills).

Want two rectangular bowls either top mount or undermount or molded into form and how to put into mold if not doing other two?

Keep in mind that integral sinks will stain more easily than an undermount or top mount sink, and they can chip. If you do use a mold, you attach it into your countertop mold, then put the mold in melamine and attach the sink mold with screws from below. When it's time to take the frame off, back the screws out before flipping the countertop over. You want the sink mold to come out with the countertop.

Is sealing necessary?

Absolutely! Concrete countertops are tough, but they do stain fairly easily. Most of them, even when well-sealed, will take on a patina over the years, but most people like this as it is a sign of its uniqueness. If the concrete isn't sealed, things like cooking oils, or even a napkin with pizza oil on it, can leave permanent stains. Acidic substances like vinegar will etch and stain too. Cheng Concrete makes a good countertop sealer. If you don't seal it, it could look like crap in no time.

Weight? for 85 long x 22.5 deep x 1.5inches thick and do I need to reinforce base cabinets?

Your countertops will weigh about 18 lbs. per square foot at 1.5 inches thick. For this thickness, you probably do not need to reinforce your cabinets underneath.

Making backsplash?

Cast and finish the backsplash separately and mount it after the countertops are put in. They should be an inch thick and should have outlet openings cast into them, not cut out. Don't try to cast the backsplash with the countertop as an integral piece. It's very hard to do well and they're harder to finish smoothly, so they'll end up looking rougher.

Do I need to add metal as a reinforcement to top?

Yes. While you can use 3/16 inch rebar made into a grid, you will probably find it easier to use galvanized wire mesh or carbon grid mesh, which is very lightweight and lies flat.

Can it be cut with a saw or hole saw?

I assume you mean for cutting expansion joints. Ideally, you should use a grooving tool to create joints in the concrete, but if you don't want to do this, you need a special "early entry" saw designed to make clean grooves.

Is the countertop edge able to have a design into it and how?

I believe that is covered in your first question, about the pre-made and DIY molds.

How do all this on the cheap? Wood, rubber, etc.?

For your "sample" concrete countertop, you may be able to make the mold from scrap lumber. If you buy premixed countertop concrete, it will cost more than if you mix it yourself from scratch. Galvanized wire mesh or rebar will cost less for reinforcement than carbon mesh. The materials for making concrete countertops aren't that expensive. It's the "sweat equity" and skill that goes into them that makes them expensive to have a professional make.

Here is more information on how to make concrete countertops correctly.

Build it well!

Alan
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