Ah, so many various answers. IMHO, plywood is just too thick of a product. 3/4" probably won't bend, however, it's too bulky.

Knowing what I now know about granite, the solution is to do a metal plate to support the granite. The fabricator will glue it to the backside of your granite. This is what you want for your overhang. I have an 8' x 4' counter with 18" overhang on each end. The metal, which is probably close to 1/8" thick is UNBELIEVABLY STRONG. I took a photo of the underside a few years ago before our island paint job was redone, so excuse the 'greenish' cast on the corbels.

PS This is a divisive topic. You'll get a lot of different responses. But I have to tell you, I have two stories about overhangs on islands. Both stories came from clients I had over the years. (I was a hair dresser.) One client's kiddos were 'diving' and flipping off her counter when she was not at home. They were the gymnistic types. Apparently, too much pressure caused this granite to break on the end. This is how I learned about the metal plate being needed. The second client had a corian islan overhang. Her husband popped hisself on the end of the island to sit and talk. As he swung his feet back and forth, he felt the crack give way. He just got off the counter in time before it, too, crashed to the floor.

So, when I had an overhang of 18", my fabricator, in a new state and town, DEMANDED that he do the plate, or he would not do the island over a 10" overhang.

PSS Those corbels are not to be trusted with granite. They are only held on with a few long screws. Consider corbels as decorative in purpose only.

Sherilynn- You are right - Overhang issues and the use of
steel, corbels and or plywood IS a divisive topic for many...

Here's how I answer the question:

The Marble Institute of America has inter-nationally recognized
guidelines for design and implementation of assemblies that
are used by people all the time. Theses guidlines are used
as a benchmark for good construstion practices.

Here's what the MIA says:

Plywood subtops should be used as a substrate for stone
less than 1" in thickness (2CM = 3/4"), and are NOT required
for stone greater than 1" in thickness (3CM = 1 1/4")

Corbels should be used if an overhang is greater than 6" in 2CM,
and greater than 10" in 3CM stone.

Metal reinforcement can and should be used to increase the
rigidity of a stone assembly.

Im my opinion, IF you are doing a 3CM stone, plywood will
not be required as a substrate/subtop. You can adhere your stone
directly to the cabinets.

IF you are wanting an overhang of 20" - you should use
corbels that are 10" from the face of the cabinet. I know that
you probably don't want to use corbels for aesthetic reasons,
BUT - IF you want an assembly that is within MIA guidelines -
THAT is how it SHOULD be done -

Now... as far as the steel goes, you CAN use steel in the
underside of the stone - this will help to strengthen the overhang
and increase the strenght of the top. However, you should
never sit on or do gymnastic type vaulting manuevers off
of the edge of the countertop.. this is a no no...

IF you are going to use steel, I would suggest using
an "L" channel or a "box" channel of steel - NOT a flat
bar - flat bars - like plywood - WILL bend, but if you
turn the steel 90 degrees (and make it like an "I" beam,
you'll have much greater strength... make sense???

What many folks do, is they lamminate a strip of stone on the
perimeter of the top, and use 1" x 1" "L" channel - the
lammination hides the steel from view, and does a good job of
supporting the overhang - I'll try and post a pic later today...

Remember though - overhangs should NOT be sat on, as the
added weight can break the stone.

my .02 cents worth...............

hope that helps ya

kevin

Kevin M. Padden MIA SFA
Fabricator, Trainer & Consultant to the Natiral Stone Industry
www.azschoolofrock.com

thankyou sherilynn, kevin ,
I will see any of the fabricators are willing to do either. Most of them around here say its the customer's responsibility to provide adequate support.

SheriLynn, I have to say your island looks very nice, even from the underside. I tend to like contemporary, but I guess if its done well any style can look beautiful.
I understand the steel plate, it seems to me that instead of making grooves for steel bars they made a large groove for granite ? Or did they just glue the plate to the granite with no modification to it ?

Kevin,
I tried to look up the box I beam thing, not much luck in understanding how it works. Is this something that fabricators do routinely ? Is there some specific thing that I ask for so they get the idea ? I am looking forward to the pics hopefully I will be able to show it to them and see who will do it.
The cabinet already has a veneer backing - does that matter ?

The metal plate on my island was glued to the entire full length of the granite.

Please excuse my typos in the above post. I could not sleep this morning and had been up for a few hours and did not proof what I published.

We have an 18" overhang at one end of our island, 3 cm granite, and our fabricator did not put anything under it.

Consider the stiffness of a dinner knife blade held with the knife horizontal. It is much stiffer to a vertical force when oriented with the sharp edge down than when the flat of the blade is horizontal. The problem with a thin vertical support is twisting, which is avoided with the box shape, and to a lesser degree with the L shape.

Unstated above is what spacing the corbels should have, and what stiffness they should have to keep the stone out of the fracture limit (1%?) of its flexural strength. Presumably the MIA groundrules address these questions.

kas

I'll tell you what, if the granite installers also installed the corbels, then there might be some kind of 'warrantee', as they say.
However, no one is going to guarantee forever your granite. From my perspective, having personally talked to two people whose counters did break and crash, we believe that the metal glued to the bottom of our island granite as a type of assurance and protection in the event that the overhang broke. Instead of crashing to the floor, causing harm and damage, then hopefully, should some misbehaving go on and IÂm not home that if the granite did give way then hopefully, someone would have time to remove themselves, or an animal underneath the counter, or a small child in time before a tragedy occurred from falling granite.
I failed to say that there was a 'spotter' for the gymnast above that slipped and some of the stone crushed his foot when he tried to away from the islandÂs falling stone. I also did not mention that tile was ruined, too.

IMHO, If you know you have rambunctious or mischievous kiddos, or adults that like to sit or stand on counters, then anything that can be installed under an overhang to delay the falling of granite, would be better than nothing installed at all.

Our 8 foot wide island with 15" overhand has 5 metal bars (about 2" wide by 3/8" thick) embedded in the plywood under it. Ours is 2 cm granite. That works out to 1 about every 2 feet. A box beam gives greater resistance to flexing per weight of beam, but those bars are pretty strong - they aren't going to flex to any degree with a person's weight.

I like that there is nothing in the way of people's legs when they sit at the bar and that the support goes to within a couple of inches of the edge - corbels generally stop a lot earlier. Also, we have cabinet doors on the middle 4 feet under the overhang which leaves no place for corbels.

Thanks Kevin and cloud_swift, I will need to make use of your information in the future.

cloud_swift: how "perfectly" planar did you try to get the bars, or did you just depend on the granite weight to deform the high ones or their underlying plywood until the load on each was somewhat equalized.

kas

Kas,
Your last post seems to imply that a plywood subtop is also needed. why would that be ? as long as i use the box channel beam that kevin mentioned, spaced close enough i should be ok no ? i am thinking of reducing my overhang to 18 inches... even so in the corner where two 18 inch sides meet, the overhang will be 25.5 inches... the latest fabricator is going to use 6 bars and guarentee it for 10 years. donno how he can do that.

kaseki, the cabinets were installed very level - our cabinet guy was good about that. The plywood was flat on top of them and screwed down to the cabinets. The router used to make the channels in the plywood made them all level. The bars themselves were quite flat and uniform. The hardest part was probably laying them out to cover the overhang well while avoiding the cooktop and sink cut outs.

ani_t: I intended to make no assertion about the need or not for plywood. I'm sure that I know little more than you do specific to granite support. However, I are an engineer and can see areas where answers would allow calculation of the viability of different support schemes.

In fact, I was considering using bars alone as knee-saving corbels for a counter overhang application (supporting possibly soapstone, which has a lower Young's modulus than granite). The bars would have to be mounted to the supporting counter wall in a way that allowed adjustment for leveling, while resisting forces that would tend to cause flexure of the bars or distortion of the wood framing. Use of plywood in addition to bars has merit; but I wouldn't claim that it would apply to every case.

cloud_swift: Thanks for the instructive photos. Mechanical engineers, which I'm not, would find analyzing your support structure to be a challenging homework problem.

kas

Kas, kevin
all this talk about flexural strength is going over my head but i get the gist of what you are saying...
, the only thing I am concerned about is getting the strongest support for my granite and find a fabricator who will do it. I dont know what they will do to level the top, cabinets are level, but the plywood subtop is not.

they dont want a plywood subtop (one even told me that a plywood subtop with 3cm granite was dangerous because plywood flexes! )
I think they are going to put the bars into the granite itself. do you think
that should work good ? especially if I ask them to use the box channel ?

The fabricator's plywood subtop viewpoint seems to me to be valid only if cabinet wood is significantly stiffer than the plywood. Otherwise, their argument would apply equally well to the supporting cabinets themselves. Exterior plywood has pretty stiff resin between layers, so its flexibility where weighted by the granite should be low. On the other hand, how well plywood would support an overhang is questionable.

It would seem to me that a box channel beam with section height large enough to provide good stiffness would be large enough to weaken the granite if a slot that deep were to be cut into the granite to hold it. A shallow slot for locating the beam would be ok, but all beams and slots must be precise so support is equalized.

If, as implied by cloud_swift's photos above, the plywood is mainly a means of retention of the beams, which do all the supporting, then the fabricator's plywood flexibility argument is seriously, er, weakened.

I would imagine that the following would work as a conceptual approach. Adhere beams to the granite as required for support. Adhere plywood or other wood in between the beams that is barely lower in height. Use the wood as the point of attachment for whatever covering trim is desired.

For a bar overhang without using typical corbels, I think I could use polished or brushed stainless steel beams (thickness TBD) set into the supporting half-wall framing. A scheme to keep the lifting ends tied down and of uniform height would be needed, as would a means to equalize the pivot side heights and keep the beams from compressing the wood there.

More simply constructed, if flatness is ensured, is the steel plate method used by sherilynn shown near the top of this thread. I might opt for a thicker piece, depending on some analysis. Other approaches could be equally valid.

kas

Here's a question...if you router the bars into the plywood doesn't that reduce the strength of the plywood?
The GC's I have gotten bids from say that they router the steel bars into the top of cabinet, then put the 5/8" plywood on top of that secured with some kind of construction glue and bolt it to the cabinets. Then the installers put the granite over that. Out here (Calif) the granite is 2 cm so everyone has a plywood base.

I'm bumping this up...

bumping

Thank you, thank you for posting this! OMG I have a breakfast bar in my kitchen which my two kids eat at daily. It is not supported like the ones above.

My kids will be eating at the table until I know for sure that our breakfast bar is stable. Thank you again.

How do you stabilize a granite overhang that is already installed?

I think sufficiently stiff gusseted L brackets could be installed onto an existing wall or cabinet construction if care was taken to assure their mounting strength, as well as their flexural strength. If for a bar wall, then mounting to the studs would be required with sufficiently strong screws. If mounting to a cabinet, then some interior reinforcement might be necessary.

Brackets fabricated from cold-rolled steel bars and painted, or from stainless steel bars would be adequate, given appropriate gusset design and bar section.

Uniformity of support could be an issue if the counter top and wall are not the same angle as the brackets, or the granite underside is very rough. Some shimming or epoxy fill might be needed to assure sufficient support. None of the two-corbel designs typically pictured, except for sherilynn's armor approach, meet the 16-inch support rule of Kevin's message.

Knee room can only be obtained by making the L bracket out of sufficiently stiff sectional material that a large gusset is not required. (A corbel is one big gusset.)

kas