Granite Countertop Underlayment

Depending on thickness, granite countertops may be self-supporting without underlayment. September 5, 2006

I had some 3cm granite installed in a kitchen. Is it normal not to use a plywood underlayment? They put the stone directly on the cabinets. Also, what is the best way to support the bar that has a 10 inch overhang? There is no plywood under the granite.

Forum Responses
(Cabinet and Millwork Installation Forum)
From contributor K:
You need to have a subtop to glue the granite to. This will prevent it from cracking when there are larger unsupported areas. Sounds a little sketchy to me that they didn't use subtop.

From contributor J:
With 3cm, it is standard procedure to not use plywood decks. I see it done all the time. The downside is that you have to scarf it out and fill it with Bondo where the dishwasher sets or you have to set anchors in your granite to mount the DW. Also, corbels work well for your overhang, but at 10" I don't think you need it. What you might need is some support across the front of the sink, if you build a frameless cabinet.

From contributor K:
I'm not a granite expert so I can't say you need it or not for sure. It's my experience to use it. I build and install custom cabinetry and I supply subtop, needed or not.

From contributor K:
I did a little research on 3cm granite, and you don't need it. 3cm is 1 1/4" thick, so it is sturdy enough by itself. I hope your cabinetry is built to support all that weight. I was wrong - my bad.

From contributor E:
In the UK, it's unheard of to use a sub-base with 3cm granite.

From contributor B:
3cm granite does not need underlayment. However, I highly recommend support for overhangs over 9", because while granite is hard and strong (to a point), it is brittle. I've seen several overhangs snap from not-so-hard hits. Granite companies will not stand by a top not correctly supported. Many won't stand behind them anyway, due to its nature. Ditto on dishwasher openings.

From contributor P:
Another ditto for no underlayment needed for 3cm granite. Plenty strong enough to be self-supporting. We usually provide supports for all overhangs, but our granite company doesn't require them unless it is over 10".

From contributor W:
3/4" (2cm) needs a plywood underlayment for support. This method is mostly used on the West Coast. On the East Coast, 1 1/4 (3cm) is common. 3cm is considered strong enough to not require an underlayment and is not used. As far as cantilever amount, it is generally given that 3cm can go 12" without support, but as the previous poster said, you would need to check with your granite guy as far as how far he's willing to go.

From contributor O:
The lip that hangs down in the front of the counter… Do the granite guys glue that piece on? Will it match the rest of the granite? How far does the lip hang down?

From contributor W:
3cm granite usually is not made with a lip and just sits as a slab on the cabinets. If you have 2cm, the lip is epoxied to the front of the slab to give the appearance of a thicker slab and to hide the wood subcounter. How visible the joint is depends on the type of granite, the thickness of the seam, how well the people color match the epoxy, etc. As a general rule, they are not very noticeable; however, no seam is invisible.

From contributor B:
When lesser thickness (such as 2cm) is used, they glue a lip using color matched adhesive. The glue is stronger than the granite. Usually the seam is visible, but not real noticeable.

From contributor M:
Do people still use 2cm? I know they do in Brooklyn, NY.

From contributor W:
2cm is much more common on the West Coast. A lot of companies that sell granite tops over the internet also do it in 2cm because of the weight factor.

From contributor O:
I built my cabinets to have a piece of 3/4'' plywood sit on top of the partitions and be the same height as the face frame (flush with the top of the face frame), so does this 3cm granite sit on the top of the plywood and hang over the face frame, or do I need the 2cm and a glue-on lip? I'm on the west coast. Why does the coast make a difference? I'm going to stick to furniture and not do another kitchen ever!

From contributor W:

3cm granite sits as a slab on the cabinets - there is no "lip" that hangs below the level of the cabinet top. I am guessing from your description that your ply subtop sits flush with the top of the face frame and not above the face frame (you do not see the ply when looking at the front). If so, you're all set.

If your ply sits on top (sticks out above the face frame), you're gonna be able to see it with the 3cm sitting on it - you can either put a lip to cover it (which would give the appearance of an unusually thick slab), or get 2cm with the overhanging lip.

The difference between East and West Coast is just the prevalent practice of granite guys in the given area. It does not mean that you can't find 3cm on the West and 2cm on the East. It's just that one is more common. It has nothing to do with codes either.

From contributor U:
I'm in Chicago, so I stay out of the East vs West coast. Here is a clarification of the above description. For 13/16" (2mm tops), the subtop should be a minimum of 5/8" plywood, no MDF or particleboard.

For weight calculations, 2mm weighs about 13 lbs per square foot and 19 lbs/sf for 3mm. The Marble Institute of America guidelines suggest an unsupported overhang for 2mm of 4" max, and 8" for 3mm.

For 3mm, I typically add cleats to the wall on dishwasher openings and susan corners. I have the shop pre-make 3/4" plywood cleats, two 3" rips butted and screwed at 90 degrees.

There is a good article in the November issue of the Journal of Light Construction regarding the installation of stone tops.

From contributor R:
I'm a stone product fabricator. I have used both 2 and 3 cm material. Whether or not a plywood build-up is required depends on the edge detail. Sometimes we laminate two pieces of 3 cm material together and in turn require 3 cm of plywood buildup. As for overhang depth, 4" for 2 cm and 8" for 3 cm is fair. If the 3 cm material is black, then try even 12", just don't let your kids play under it.