Cabinet top when putting granite on it

Folks:

I've made a lot of cabinets, but never one where I'm planning to put a 2' x 6' slab of 1" thick granite on top of it as a countertop.

My default plan is be to make this as a normal cabinet (I'm not worried about the strength of my cabinet) with 3/4" sides, bottom and a full plywood top under the granite. But am I making a mistake? All the cabinets that I've seen built into homes are missing the top (using the granite as the top surface). I'm thinking that's because it's cheaper - and I'd rather have a sturdier cabinet, so I'm planning on a full plywood top under the granite.

Am I missing something? Am I about to make a mistake?

Thanks for any and all advice.

I don't think you will hurt anything by using a full top but I also don't think it is necessary. I built a similar sized island for my last house and I just used a couple 4 inch wide pieces on the long side and I think I just has the 3/4 edge of the cabinet on the front and back. Granite counter tops can overhang several inches, maybe as much as a foot, unsupported. When the granite is supported around the periphery like with a cabinet, it is fine unsupported in the center. On the other hand, full support will give you margin against a vein in the granite weakening it.

My current house has 20mm granite countertops including a cantilever breakfast bar on the island I had to support with steel. I bought the cabinets but they have the same type construction I used on the island but with thinner 1/2 inch plywood. Works fine.

I built « full » cabinets (ie full width top) 3/4 inch (19mm) plywood boxes. Counter top is 30mm stone. While I have a tendency to over design, I sleep well at night. 😁

You will be wasting good plywood and making it harder to level the granite. The flat plane of plywood doesn’t add any strength in supporting the granite. Your granite installer will shim and level the top edges of the cabinets when installing it. Use some strips along the front and back and sides and you’ll be plenty strong and make it easier for the installer to locate his shims.

Dan

I agree with Dan...there's little value to putting in a full plywood top under the stone. It's the cabinet sides/front/back that do the actual support. An extra cross piece on a wider cabinet is fine, but otherwise, that would be a lot of expensive plywood that would get buried for no good reason.

I like using a full top on base cabinets. It helps in the glue-up to keep all the parts aligned and square. It also keeps the cabinet together on the trip from my shop to the installation spot.

Aside from requiring it for support of an unusual amount of weight, I have to agree with the other guys that it is a bit of a waste of material from a cabinet strength point of view. The one reason I have encountered the requirement of a full top though is for example when the main slab is 2cm thick and it has a laminated front edge that is 4cm thick. In this case, you may need to space the entire top higher so that the countertop front edge doesn't interfere with something or affect the visual appearance between the countertop and the drawers or doors top edges below. The laminated granite front edge hides the additional plywood tops edge. And now that I think about it, depending on how your granite installer installs the top you may also want to plan in advance for a future change. If the granite is bonded with adhesive directly to the cabinet sides/front/back at the top edge then removal without cabinet or top damage may not be easy or possible. If you design and build with this in mind (eg. full plywood top removable from inside cabinets, top bonded to this) then they can be managed separately during remodel without damage.

Thank you, everyone. I very much appreciate the different viewpoints. I knew I could count on this forum for some insight.

Thanks again.

When they installed the granite on our custom cabinets in our remodeled kitchen, there was no top underneath the granite on the kitchen counters or the island. The granite was set on the vertical walls, front and backs. IIRC, they used a silicon adhesive caulk to adhere them.
;

Ditto what Ken just said. I have granite throughout my house. The largest slab is 39x79 for the island with a 12 inch overhang you can slide stools under. Three cabinet boxes hold this slab up. I did specific 3/4 Hickory ply for all the cabinets and boxes throughout my house not because of the granite, but because I never wanted to ever replace these hardwood cabinets. They are STOUT! 10 years since I had the house built and that island is the same as it was brand new.