When it comes to supporting a granite countertop with plywood, the thickness of the plywood will depend on the specific requirements of your installation. In most cases, 3/4-inch (19mm) plywood is commonly used for this purpose.
The plywood acts as a substrate or base for the granite slab, providing a stable and flat surface to support the weight of the stone. The granite countertop itself can vary in thickness, typically ranging from 3/4 inch to 1 1/4 inches (19mm to 32mm).
For smaller countertops or lighter granite slabs, you might be able to get away with using 5/8-inch (16mm) plywood, but it's essential to consult with a professional or the granite supplier to ensure it will be sufficient for your specific situation.
Additionally, to ensure a safe and stable installation, it's crucial to use the proper construction adhesives and fasteners when securing the granite to the plywood base. Always follow the manufacturer's recommendations and local building codes when undertaking such a project. If you're unsure about the best approach, it's advisable to seek advice from a professional contractor or installer who has experience working with granite countertops.