Granite top counter advice needed

[Rick Potter]

I am building a 32" high peninsula cabinet on one side of the kitchen, which will have a granite top. The top will hang out on one side of the cabinet so the wife can slide a chair under it occasionally. This part of the top hangs out 18" total, and will have two 4X4 turned legs holding the end up. These legs are 38" apart C/L.

The plan is to use 3/4" plywood as the top/underlayment, which would cover the cabinet, and extend the 18" on one side. There will be a skirt between the legs, but it will be arched, and only 1 1/2" high in the center, as the designer/wife wants it that way.

My question is: Will 3/4" plywood with that 1 1/2" skirt be enough to support the granite top for the 38" span?

Thanks,

Rick Potter

[Mark Bolton]

Your best bet will be to ask your granite fabricator but my answer is without a doubt yes. In our area they will commonly overhang 12" unsupported. They would not even blink at doing the 18" with only a leg and apron at the perimeter (no 3/4" ply). We dont run a full top on any of our cabinets for granite so If you wanted to you could omit the 3/4" top all together in my opinion.

I would just put a call in to your granite fabricator and they will give you a go/no go but I wouldnt even blink an eye at it.

[guy young]

If you bond the plywood to the granite the granite is actually supporting the plywood. The granite stops the plywood from bending, not the other way around. So the plywood is really only there to make you feel better.

If you are using 3cm granite, I would not do the job. The plywood will probably not save the sheet from cracking if your legs and front support (which I assume to be the cabinet) are not all on the EXACT same plane. And make sure the floor is not flexible. At which point, why use the plywood.

If you are using 5cm granite I would ONLY do the job if the granite is not VEINED and I would try to move the supports to 36"

Rick is 100% correct about 12" overhang being the limit for unsupported. 36" spans are the limit for supported. I personally have never installed a top with more than either one of these limits, so I cannot tell you it will definitely crack. I have made and installed some crazy granite counters when I was doing it for a living.

[Mark Bolton]

If you are using 3cm granite, I would not do the job.

Guy,
Not questioning your installation background. I have asked installers many times about support and tops/alignment/and so on, and with 1 1/4" tops (3cm) we have been repeatedly told that the silicone bed makes up for small imperfections. Of course they have said they have walked into jobs that were so out of line that they simply wouldnt do the install but we are talking a flat floor, cabs, and the like. Of course no one is going to guarantee floor movement but Im not quite sure I follow the issue with the cabs being "perfectly flat".

3cm tops are the standard in any commodity market and they are installing them every day on top of big box/ikea boxes?

Interesting.

[Rick Potter]

Now I am starting to worry. I have looked at numerous homes with granite countertops and never seen one over 3/4" thick, not counting the edge treatment. Many were 5/8". I have gone to several granite places, and all their pre-made slabs are 5/8 to 3/4 thick. If I need it to be 1 1/4" thick, that really changes things.

If I doubled the 3/4" ply, would that help?

[Mark Bolton]

Rick,
Again, it will all depend on your fabricator. They will be the one to give you the answer. There are a few shops around here that are doing laminated work similar to what you are talking about. Thin sheets, thickened only at the edges. If this is your only options I'd say you'll be doing some re-engineering.

Our top suppliers are running 1 1/4 as the default. You have other things at play as well. You call out an 18" overhang which I am assuming is the top itself. The leg and apron support will shorten that actual span by perhaps 3-4" at a minimum as you will have an overhang beyond the legs (perhaps an inch and a half or more) then the distance back to the apron. You could easily shrink your apron/leg back a bit OR add some rails back to the peninsula cabinet to provide a bit more support.

But again, your top shop is going to be the best source for information on this.

[Sandra Mart]

If you bond the plywood to the granite the granite is actually supporting the plywood. The granite stops the plywood from bending, not the other way around. So the plywood is really only there to make you feel better.

If you are using 3cm granite, I would not do the job. The plywood will probably not save the sheet from cracking if your legs and front support (which I assume to be the cabinet) are not all on the EXACT same plane. And make sure the floor is not flexible. At which point, why use the plywood.

If you are using 5cm granite I would ONLY do the job if the granite is not VEINED and I would try to move the supports to 36"

Rick is 100% correct about 12" overhang being the limit for unsupported. 36" spans are the limit for supported. I personally have never installed a top with more than either one of these limits, so I cannot tell you it will definitely crack. I have made and installed some crazy granite counters when I was doing it for a living.

right...

[Jim Foster]

As an FYI, I have a granite counter-top that one area is about 30" wide and overhangs the end by 12 inches. All the granite in our kitchen is fastened down to the cabinets with standard silicone sealer, along the 3/4 inch plywood outline along the tops of the cabinets. Our installer said the bond would be so strong, the cabinets probably would not survive if we tried to remove the granite at some point. The granite is over 1" thick, maybe 1 1/8. We were originally going to have this 12" overhang deeper at around 18" and were going to get some large angle brackets to fasten to the cabinet as extra support. After seeing the kids lean on this for over 10 years, I think 18" unsupported might have been fine, but your installer should be able to verify what they have been successful with.

Note: Our granite is not veined in any visable way as some are, so it may be less prone to cracking. Also, I would question the legs. If the floor moves, they may not be supporting the slab the way you expect it to. I like the idea of the angle brackets because then they can be continuous with the cabinet surface regardless of floor movement.

[Rich Riddle]

Will your granite be the 2 cm or 3 cm variety?

[Rick Potter]

I don't know, Rich. I have a call in to the guy who will be doing it. I also called two other granite places. The first said 5/8 to 34 will be fine. The second said their standard is 2 or 3 cm.

Hopefully, I will have a definitive answer in a couple days from the recommended firm I called. We have a good tile man who recommended him.

I am glad I asked the panel. I had no thought of putting thicker granite on, but if that's what it takes, so be it.

Rick Potter

[Rich Riddle]

Rick,

When I installed granite, 3 cm was used. We purchased a house with the same granite (Uba Tuba) with 2 cm granite atop 3/4 plywood. The thinner granite developed all kinds of little chips in it. Would be glad to post pictures if you would like to see the lesser grant and the edge problems.

[keith micinski]

I don't see how the three centimeter granite spanning 18" with an apron and a leg support would be an issue at all. Also bonding the plywood with silicone to the underside of the granite absolutely adds strength to the entire sandwich. Think of a glue lam beam and then think of one that's not glued together but just stacked there. I'm not saying the 3/4 plywood is as strong as the granite but it isn't just "being supported" by the granite and a waste of time. Also over 18" you are going to have an apron that's 1 1/2 thick at its thinnest and then 3/4 plywood over that. 2 1/4" of support for 18". This would hold hundreds of pounds before it deflected enough to matter and I would be willing to bet you could do this and not even need the legs, although I get they are there for aesthetics.

Use silicone sparingly to adhere everything and you might be able to remove this stuff at a later date. Whatever you do don't use liquid nails, which I run into way more then I like, You cannot remove 2 or 3 cm granite with out destroying it at least I have never been able to?

[Rick Potter]

Thanks Folks,

I talked to the granite guy today, and he is going to look at the job tomorrow afternoon. The tile guy came yesterday, and he wants me to do some cabinet underlayment a bit differently, so once the granite installer tells me what he wants we can go for it.

Yes, tile and granite. Guess that makes this a hybrid kitchen. Just have to be different, I guess.

I will let you know what he says.

Rick Potter

[Rich Riddle]

I have seen some very good looking kitchens with granite countertops along side of tile backslashes. They can look great.

[Jeff Duncan]

Of course the granite guy is the one to get the best advice from and so take this as secondary recommendations.

First off 18" is pretty deep and over what my granite guys recommend for un-supported, however you have legs so that should make up for a lot of it. It's then just a question of the span between the legs. Plywood as an underlayment without additional support is not going to do much. Granite is far stiffer than ply and would break before the ply would offer much resistance. So I wouldn't bother using it at all. One thing that is used more and more frequently is steel. You can get steel plates and recess then into the top of the cabinets extending out 6" - 8" or even more if necessary. You would still have to be wary of granites with veining or similar as they can be more prone to cracking in un-supported areas.

As far as thickness goes the thinner stuff is only used on vanities and similar small applications around here. The thicker 1-1/4" or so granite is the standard for kitchens. Sounds like things may be different on your coast though and you may not have much choice?

good luck,
JeffD