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JESTERJD 07-21-2008 12:26 AM

Underlayment for Granite Countertop
I have finished installing my pre-made cabinets for my big Wife-inspired Kitchen remodel. and it is looking great.

I have already bought my Granite slabs and am planning on having a countertop guy come out and install the Granite countertops. The countertop guy said he wants 5/8 inch plywood down on the cabinets. I have bought the 5/8 inch plywood and cut it into acceptable pieces.

My question is the pre made cabinets walls are only 1/2" thick and are a particle board composite. I am worried about screwing the plywood down into these cabinets sides and am really not sure how I am supposed to attach the plywood to these cabinets. AM I suposed to screw up into the plywood threw the corner braces on the cabinets?

Any help on doing this would be much appreciated.


Ron6519 07-21-2008 07:01 PM

The cabinets should have bracing across the top or corner blocks. Screw up from the bottom with the correct length screw into the plywood.

Termite 07-21-2008 10:18 PM

Screws through the corner supports would be best, but if that isn't possible, some liquid nails would do an adequate job. Remember, uplift isn't an issue, and that super-heavy granite slab will not wiggle much!

JESTERJD 07-22-2008 10:33 AM

Underlayment for Granite countertop.
Thanks for the replies guys, There are corner braces at the corners of each cabinet, these are also made of particle board thou, but I guess that is what they are meant to do, I just worry about them breaking. Also thanks for the liquid nails suggestion.

One more question. How big should the plywood be, should it extend over the cabinets at all or just be flush with the top, since the granite will extend out on its own an inch or so?


47_47 07-22-2008 02:21 PM

If the braces are low, install scrap wood as spacers until the braces are just below the sides of the cabinnet. Predrill the particle board corner braces from the top for the screw shank. Screw from underneath. The screws just have to be tight enough to act as clamps, until the adhesive cures.

With a granite top, I believe the plywood top should be flush with the cabinets. Ask the installer what he wants.

buletbob 07-25-2008 06:19 AM

Plywood under a granite counter top ???? Is it necessary ? how thick is your top a 1/4". All my tops are sold as 1-1/4" And in the twenty years of installing the stone we never had to install plywood over the base cabinets. We installed some extra supports over the dish washer and behind the stove, in the sink base cabinet. but never over the cabinets. you will be raising the height of the top for one thing. and another you will be seeing this plywood from the front and sides sitting down at the table looking at the cabinets.
I understand practices are different from state to state, that's why I'm questioning the plywood BOB.

Termite 07-25-2008 07:42 AM

Like Bob, I've never heard of doing this. I have a hunch the granite guy can lessen the slim possibility of a "structural" problem in the slab by installing a continuous substrate. It certainly isn't necessary on most cabinets with most tops.

dhag 07-28-2008 09:37 AM

I agree. I would never nor have I ever seen a substrate go down for a granite slab. Mainly because I don't know how you finish the edges of the plywood as it will be visible under the slabe. Some sort of moulding I suppose. If that's the case, don't overhang the plywood. Another idea would be to "inlay" the plywood by screwing blocks to the inside walls of the cabinets and installing the plywood inside the cabinets so it's flush at the top.

loggerville 08-23-2009 10:16 AM

Granite countertop
Often you need clearance for cabinet doors and drawers for granite with bullnose edging. The the bullnose will hang down the thickness of your plywood therefore you will not see the plywood at all.

You did not mention what edging you have therfore be careful that you have all of your clearances for appliances,cutting boards etc.....

Mop in Hand 08-23-2009 01:02 PM

Depending on how thick the granite is. A plywwod substrate may be the way to go. Some granites sold are only 3/4" thick, therefore a plywood base is advisable. 5/8" works, however in the remodels I've done, I use 3/4". The bullnose on 3/4" granite usually extends down 3/4" giving it the look of
1-1/2". Another reason to use plywood as a base: If the cabinets are not installed completely level, the granite installer will have the opportunity to shim anywhere to level the granite.

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