Opinions vary on this topic, but since the weight of granite can cause it to crack, making sure you have the proper support is essential. In most cases, support is placed underneath unless the overhang is 6 inches or less. According to the Marble Institute of America, you can safely overhang granite that is 1 ¼ inches thick up to 10 inches without support. However, the cantilevered portion cannot be more than one-third the total width of the countertop.
For 3cm stone, use an I-brace support with overhangs that are 12 inches or greater, and for 2cm stone, use an I-brace support with cantilevered brackets or corbels that match the cabinets. I-brace supports are spaced every 18 to 24 inches, and other bracket systems are spaced 3 to 4 inches apart.
Most counters are installed with a standard overhang of 1 inch. This may be modified to suit personal taste, but must be stipulated before the granite slab is cut. Granite can be cantilevered up to 12 inches if it’s a large piece with sufficient support on the fixed end. It should never be cantilevered where it might receive excessive stress, such as where someone may be tempted to sit on it or use it as a stepstool to change a light bulb. In these cases, it must always be supported. It’s best to assume the worst and add proper support on any extension over 6 inches. An unsupported span of no more than 36 inches is usually acceptable as long as the stone is supported on both sides.
For granite, an edge requires additional installation where two sections meet in a corner. Additional labor is required to match the two sections. It is recommended that a dishwasher not be attached directly to the granite countertop, but be side-mounted to adjoining cabinets. Special brackets are available from the dishwasher manufacturer for this purpose. Drilling into the bottom of the stone can cause stress cracks and discolorations in the surface of the granite.
Considering the cost of granite coupled with its heavy weight, it would be advisable to support any overhang just to ensure your granite never runs the risk of cracking, which in most cases cannot be repaired.
Contact Granite Unlimited with any questions regarding overhangs or other granite installation topics.