If you are looking to extend your concrete countertop to place seating underneath, or just have a bit of an overhang, this can be done relatively easily. You can safely extend to a 12" overhang without any additional support or reinforcement. All you will need to do is let your cement board overhang the cabinets. Remember, the standard size form will add another 1-1/2" of overhang so you should let your backer board cantilever 10-1/2" past the cabinets. You will need to temporarily support the cement board so it does not flex or crack under the weight of the wet concrete. To do this, we will usually just use some 2x4 legs. Cut each eg about 1/4" short so you can use wood shims to fine tune the exact height. This will also help with removing the eggs after the concrete has cured. They should stay in for 2-3 days while the concrete sets up before removing them. The cement board will stay in place, as the drop down in the forms will completely cover it.
If you are going beyond a 12” overhang or want to be extra cautious, additional support will be needed. We offer a full line of decorative and hidden support brackets. As always, If you have any questions about creating an overhang, please feel free to contact us.