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Before building the countertops, talk to your carpenter or cabinetmaker to ensure that your cabinets can support the weight of concrete countertops, which can weigh (at 2-inch thick) between 12 and 22 pounds per square foot.
If necessary, add extra support to the cabinets. Often, 3/4” plywood sheets are attached to the top of the cabinets to give the concrete a flat, level surface to sit upon. In this case a trim piece matching the cabinets will be needed to cover the plywood edge.
It’s a good idea to build the molds and pour the countertops off-site, in a garage or workroom. It ensures a clean kitchen during this very dusty and dirty process. Avoid places that are exposed to the weather and have drastic temperature change.
The process of pouring concrete countertops begins by creating very accurate templates of the existing counter space. The molds for the countertops will be made from the templates.
There are two ways to create a template: 1). Trace the countertop base on an oversized piece of wood; or 2). Create a template by connecting strips of wood veneer. This project uses the wood veneer method (Image 1).
The template should be the exact size of the countertop base or cabinet top – not the size of the countertop that will go on it. Using wood veneer strips, secure the strips to the surface of the cabinet top with the hand stapler. On corners, use a diagonal piece for extra support secured using the hot glue gun (Image 2). If the template is particularly long or wide, add strips for cross supports. Score and snap off ends with the utility knife and square.
The template-making process is the time to decide on design features like control seams and countertop overhangs. If countertops are made in sections larger than about 8 feet, they are prone to cracking and more difficult to move. Reduce the chances for cracking by incorporating joints or seams in the countertop.
Countertop overhangs – the countertop lip that extends beyond the face of the countertop – are usually 1/2” for standard cabinets and 3/4” for flush cabinets.
Mark seam locations and overhanging edges on your template with a permanent marker (Image 3).
Measure the countertop base again and write these measurements on the template to ensure accuracy (Image 4).
Remove the template from the countertop and move it to where the countertop mold is being made.
The molds are built with 3/4”- thick melamine (Image 1), a medium-density fiberboard with a smooth laminate finish that the concrete won't stick to and which will produce a smooth, even surface for the countertop. When buying melamine, it is important to buy pieces with perfectly smooth surfaces; cracks or dents will distort the surface of the countertop.
To create the molds flip over the templates and trace them onto the surface of the melamine with a pencil (Image 2). It is important to flip the template because the concrete is poured face down – the bottom of the mold will be the top of your countertop.
Transfer your seam locations and overhangs onto the mold (Image 3).
This project has two seams requiring three molds for three sections of countertop. Trace the overhang on the edge of the mold that will have the overhang. An easy way to do this is to get a small piece of solid material the width of your overhang – such as a wooden block – and run it along the edge of the template to create a uniform overhang (Image 4).
Using a table saw, cut out the sides and bottom of the mold. The bottom of the mold is the area that was traced out; the sides should measure the length of the corresponding side of the mold plus a width of 2-3/4 inches. The width of the sides equals the thickness of the countertop – 2 inches – added to the thickness of the 3/4” melamine). Use a new, sharp blade on the table saw to make your cuts to prevent the melamine from chipping.
Construct your molds by attaching the sides to the bottom of the mold (Image 1).
The interior of the mold should be a uniform laminate surface with no exposed particle board. Attach the sides to the bottom of the mold with screws placed at each corner 6 to 8 inches apart (Image 2).
Pre-drill all holes. Drill at a downward angle to preserve the integrity of the melamine surface; any breaks in the surface of the melamine will distort your countertop when the concrete hits it.
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