If you look carefully at your picture, you will see that there is a support rail just under the stone edge running between the two legs. It is wood, but may have a steel beam behind it. You would have to provide skirt support such as this, between your legs, and between the legs and the base cabinets.

For such long lengths, add in some angle iron support to the table apron between the posts, and add cross bracing as well.

launche:

In order to answer your question, we need to know the depth of your cantilever. Most 3cm granites can be cantilevered 9" or so and 3cm engineered stone 14" without additional support.

My reading of the OP's post is that there will be two legs holding up one end of the 8 x 4 island, then nothing between that and the 2 x 4 cabinet at the other end, so basically 6 ft x 4 ft of unsupported counter.

Yes that is correct there will be a 6 ft x 4 ft seating section of unsupported counter.

Thanks live_wire_oak your diagram makes sense to me.

The project is a new construction home and the builder hasn't done an install like this. His sub contractor may have a proper plan but I don't want to chance that. The GC has suggested maybe we'll just install a another set of legs in the middle and that's want we don't want. We want to keep the seating options flexible. I don't want to take the chance of them just doing whatever they want on the spot especially just for the sake of ease and making their deadline.

They have been receptive to our input on things and will try to do it how we want if it is a sound approach.

The picture threw me off, sorry.

launche, I'd recommend a 2" 18 gauge steel square tube frame with the welds ground flat, painted flat black and suspended between the legs and the cabinet.

A 6' unsupported span in 3cm granite would be criminal. Seriously, someone could be killed.

Is that 3/4" thickness solid wood for the stretchers and crosspieces? Is every thing dead flat level and plumb? If so, that should be OK for a lighter less dense stone, Some of the darker and denser stones weigh more though, so I hate to say it, but it kinda depends, I'm a belt and suspenders gal, so I always want some steel in for a really long span like that. Wood can flex. Granite can't. When those two things meet, you can sometimes have cracking occur.

I am not an expert by any means, but it strikes me that your GC's plan is insufficient. I would absolutely be better safe than sorry, and Trebuchet's suggestion is a good one that still allows you the seating flexibility while addressing safety issues.

If your countertop material comes with any kind of warranty it will likely be invalidated if you install it without the recommended supports, FYI.

Sorry, I need a better picture.

That looks somewhat flimsy to me. The suggestion made above for welded metal supports should really be considered. As Trebuchet pointed out there is a real safety concern here. Improper support could result in major injuries.

I spoke to the builder and there may not be time to fully explore welded metal supports but they will beef up the wood sub framing and add a center post. Still a work in progress.

Sadly there's not much out there to show them as examples that I've seen. I've sent them these pictures as far as wood bracing is concerned. I know not as good as metal bracing but hopefully sufficient.

This post was edited by launche on Thu, May 22, 14 at 14:36

Given the area, you'll have 432 lbs of granite between the cabinets and the legs. You need to add to that a minimum of the weight of about 2 people - about 300-350 lbs. Adding just a bit to round it off for the child who decides to tap dance on the counter, the load your builder should be anticipating is about 800 pounds on those supports.

The worst parts would be the active weight of the tiny dancer in the middle of the slab or people getting up from a meal and pressing down on the edges at the same time.

If both people were on the same side and rising at the same time, 1/2 of all of their combined body weight would push down within a foot.

So I guess I'd ask the contractor if two of their guys could tap dance on top of what they are building. It would be about the same thing (sharp load changes in different places).

Here is a link that might be useful: Counter Balance

It is between the builder and the stone supplier. Talk to the stone supplier if you are concerned about a specific application.

I came to gardenweb some time ago for a similar issue. I got all kinds of expensive "NEEDED" fixes that were hundreds or more dollars to do.

I talked to the stone supplier, and the installer. They said my design was adequate and no additional support was needed. Three years later still standing.

So I would say this is not one to crowd source.

Different layout but maybe helpful to you? That is a 2x6 false wall we built behind the cabs. They are the counter balance supports and we have a 15" overhang:

"It is between the builder and the stone supplier."

With the exceptions of preschoolers, I can't think of many people more unqualified to design supports for stone. A stone supplier knows all about importing, transportation, and sales but is unlikely to have much framing experience. A builder has a huge conflict of interest which is getting things finished as quickly and cheaply as possible.

A perfect example of builder conflict of interest is their lack of installing an uncoupling membrane between the concrete slabs and the tile on houses built here in Florida. I get calls monthly on "tented" tiles, those which have lost the fight with the concrete and have been shoved into a pyramid. Why didn't the builders install the uncoupling membrane? Because it can't be seen, they can't charge for it, and because by the time the tiles tent, their checks will have cashed. They even do the cost/benefit analysis on these types of items. Often it's less expensive to just repair the damage than to do it right the first time.

Might my recommendations be a little overkill? Perhaps. But I'll gauran-dang-tee it will never fail. They're your kids; make your call.

This post was edited by Trebruchet on Fri, May 23, 14 at 9:52

Thank you bmorepanic for the thought out response, very helpful.
I've seen the counter balance and the like products but most seem to address overhangs. I wonder why they don't have a product that covers a larger expanse, that's what I need.

The stone supplier in this case is not a full service vendor and shifts the responsibility to the builder. Obviously, we are not pleased either party can't easily find a solution as a matter of common day to day business practice. Of course things revolve around time and expense for a given solution. The builder is agreeable in working with us within those constraints.

I agree largely with Trebruchet, our issue as far as getting the best solution revolves around time as we are fighting scheduling as we come to the end of completion. We all know builders cut all kinds of corners. My own best advice that I did not take was to hire a independent GC to work on my behalf and monitor the build. It's the best thing and I regret daily not having done that, the stress relief alone is worth the cost let alone the potential for a better built house.

In the end I'm sure the island will have good support but maybe not the optimal support method. I've spoken to the GC, he understands my concerns and I've given him my thoughts of what could be done given what we are working with.