Are Sub-Bases Needed with Granite Tops and Undermount Sinks?

You can clamp the sink directly to the granite. But some installers do use a plywood sub-top. February 2, 2011

Does a 3/4 MDF base need to be installed prior to granite tops being set onto base cabinets? If so is it necessary to go back and trim this 3/4 exposed MDF edge?

Forum Responses
(Cabinetmaking Forum)
From contributor L:

From contributor K:
I always use a sub-base for granite counter tops. How else can an under mount sink be installed? Use 5/8" cheap plywood not anything 3/4".

From contributor F:
Undermount sinks are not fastened to plywood or anything else wood. They're fastened to the granite, and yes they're fastened by the granite installers. Granite sits right on top of base cabs.

From contributor K:
The way I have seen it is the sink is held up against the bottom of the granite, till the glue dries, by the plywood. The sink is not fastened to the plywood, no, only to the granite. So how do you think a sink is installed? Also, if you are building frameless and maybe using The Blum method, you have to space the granite off the cabinets or you can't get the drawers open. I'm sure many other 32mm systems require a spacer under the granite. This is all assuming 2mm granite with doubled edge.

From contributor B:
I found back in my custom cabinet days the easiest way to mount under-counter sinks was to place a scrap piece of wood/stick across the sink cut-out once the tops were mounted. Then place a bar clamp up thru the drain hole and use it on the scrap to hold the sink up against the bottom of the counter until the epoxy dried.

From contributor H:
You do not need a plywood base for the sink. The granite is drilled and inserts are put in around the sink. Then they screw in undermount sink clams and hold the sink.

From contributor J:
My experience is that a granite top with a laminated edge requires a 5/8" ply base. The lamination hides the ply. The newer, thicker granites (about 1 1/4", I forget the metric number) are glued directly to the cabinets, no sub base required.

From contributor F:
Contributor H has it right. Iím not sure why they're using thinner granite in other places, but the "newer" 1-1/4" thick granite has been the standard here since long before I started my business a decade ago. I haven't ever seen a plywood top done.