Concrete House Numbers

• Make a new 8.5″ x 11″ file and then create two 4″ x 6″ rectangles. Choose a font and center it in the 4″ x 6″ rectangle.
• Print out the numbers on regular paper at full scale.
• Cut out the paper numbers with scissors or a razor blade. Be as accurate as possible. You will skip this step if using a laser cutter to do the cutting.
• Center the paper numbers on the gasket rubber and trace around them with a permanent marker. Cut out the number with a razor blade or utility knife.
• Make the cuts as straight up and down as possible.
• If you have access to a laser cutter, use it to cut the rubber. It has a tendency to burn the edges but this can be cleaned up with denatured alcohol and won’t show up in the finished concrete.

Step 2 – Making the Form

• Cut the baseboard: In this example, we’ll use a 15″ x 11″ baseboard to lay out 4 numbers. The baseboard has to be waterproof (melamine-coated particle board works well). It could be a piece of plastic, glass, or anything similar.
• Cut the foam walls: Insulating foam (pink or blue) works well for the form walls because it has a tight surface, unlike the styrofoam used for packaging – but that will also work. A layer of clear packing tape will help cover up that rough surface.
  •    The strips are 3/4″ tall, 1″ wide, and vary in length. The total thickness of the concrete numbers should be at least 3/4″; any thinner and they might crack during demolding.
• Attach the foam walls to the baseboard: To lay out the grid, it might help to make a 4″x6″ template to get the right spacing. Use spray adhesive to attach the foam walls to the baseboard.
  •    Lightly mist one side of the foam, but if you spray too close the aerosol will melt the foam. Don’t spray adhesive on the baseboard – it will leave a residue that you’ll have to clean up. The smoother the casting surface, the smoother the concrete. Any texture, even a misting of spray adhesive, will show up in the finished concrete.

Step 3 – Glue the Numbers in Place

• When the grid is laid out, affix the numbers to the baseboard. THEY SHOULD BE BACKWARDS. This is very important, and you will only make that mistake once.

Step 4 – Mix and Cast Concrete

• Check out the Mixing Concrete by Hand Guide for tips on making a small batch of concrete.
• Pour the concrete into the form and press it in by hand.
• When the forms are full, smooth the top with a trowel.
• Insert any cables or hardware that you want to cast into the pieces for hanging at this point.

Step 5 – Demold

• Remove the foam form walls.
• Gently lift up on the concrete, under the rubber, with a plastic putty knife.
• Remove any remaining pieces of rubber with an awl.