Granite countertops are made of natural stone slabs that have been regulated to specific sizes. They come in three standard thicknesses -- 3/4 inch, 1 1/4 inch and 2 inches -- and can come in a wide range of durability. Whether installing the granite directly on cabinets, or installing it so it overhangs the cabinets at a breakfast bar, the stone requires correct support.
3/4-inch Thick Granite
Granite that is 2cm, or 3/4 inches, thick is a less expensive option that gives homeowners the ability to have a less expensive stone and the potential to build a thick, decorative edge. This thinner granite must be supported even directly on top the cabinets to prevent cracking or breaking of the stone. Medium-density fiberboard (MDF) or plywood can be installed in the cabinet frames. The granite rests on top of the support, extending past the cabinet by approximately 1 inch.
The most frequently installed thickness of granite is 1 1/4 inch, or 3cm, thick. This thickness, as well as 2-inch-thick granite, will not crack when being installed directly on top of cabinets unless the granite is prone to fissures.
If the granite is a Class A, B or C stone, it will not require supports. Class D granites, which can be extremely prone to fissures and weak spots, may require plywood or MDF support in addition to the cabinets. Speak to the fabricator about the strength of the stone in question, its rating and whether it has had resin injected into it for additional strength. Stones with a fiberglass mesh on the back are typically weaker and should be supported.
Most 3cm granites are strong enough to extend approximately 10 inches past the cabinets without cracking. For granites extending past 10 inches, corbels need to be installed no more than 30 inches apart across the entirety of the counter, beginning at each end and moving inwards.
If corbels are not desired, or a significant overhang is desired, a 1/8-inch steel plate can be bolted onto the cabinets before the granite is installed. The plate should extend to within 4 inches of where the granite will stop. The granite is then epoxied onto the plate, which holds it securely in place without corbels or fear of cracks.
3/4-inch and Class D Granite Overhangs
Granites that are only 3/4 inch thick can overhang the cabinets by a maximum of 6 inches without cracking. After 6 inches, they must be supported either by corbels spaced every 36 inches apart or by a steel plate. The granite will be supported by plywood and the plate will be bolted to this.
Class D granites, which are prone to fissures and may be reinforced with fiberglass mesh on the back, should also not extend more than 6 inches unsupported. Corbels or a steel plate are adequate to support the granite.
In the case of either weaker stones, 1/4-inch flat bar can be embedded in the plywood used to support the stones to help support an overhang as well. The bars run from the front edge of the granite to the back of the cabinets and are spaced 12 to 18 inches apart.
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