Maybe I should also mention that the total countertop for the island will be about 26"-26.5" x 85".

It sounds like this will be a do-it-yourself project. If you are using a skilled granite installer I would expect them to know how to support the countertop. We acually have new laminate (180 FX). I had the carpenter enlarge the surface near our glass stove top so grandkids couldn't reach it and burn themselves. He knew exactly what support was needed and how to do it. I believe that he put an extra piece of plywood or particle board under the normal one and added a bracing strip against the back of the cabinet under it. My stepson had a granite island counter put in and the installer used elaborate masonry scroll supports at intervals to support that.

Thanks nerdyshopper. I am planning to have someone else install the granite or marble, I guess it's more of a design question - can the island be designed this way and still have proper support for marble? Also, I was wondering generally if most people have plywood installed under the counter and how is the edge hidden?

bump

Kitchenkrazed09, plywood sub-tops are often used to support 2cm (3/4âÂÂ) stone countertops. If you are planning to cantilever the island for a sitting area, then it would be best to purchase the thicker 3cm (1 üâÂÂ) countertop. I agree with the other post in that the person who comes out to template your countertop should indicate where and how the countertop should be supported however, donâÂÂt be surprised if he requests 3-4 legs for support. The reality is some granite guys are more concerned about functionality than aesthetics, so if you have anything in particular in mind, itâÂÂs best to either have them on site or at least provide the specifications. As rule of thumb, anything over 10â for 3cm stone (granite or marble) should be supported with; L-brackets, legs, corbels or angle iron extensions. If you opt for the thinner 2cm, supports are recommended after 6â of unsupported overhang. There are more in depth criteriaâÂÂs to follow such as not having more than 1/3 of the overall top unsupported. These guidelines are more for window sills, pass-through and knee wall caps where the tops are not likely to exceed the 10â max but will be unstable due to the ratio of unsupported stone.
Hiding the sub top; most 2cm stone tops are laminated at the finished edges. This build up accomplishes two things; it makes the tops look thicker and hides the sub-top. If you laminate the edge and donâÂÂt raise the top the laminate could obstruct the draws and doors from working properly.
Hope this helps.
Caayu

Have you read the "Stone Advice" thread? It has some discussion about overhangs and supports. Basically, you need to follow the "6 & 10 Rule"...

• If your granite is 2 cm thick, then there can be no more then 6" of of unsupported span with a 5/8" subtop

If your granite is 3 cm thick, then there can be no more then 10" of unsupported span - no subtop required
If you need support, then to determine your corbel dimensions:

• Thickness of Stone - Dimension of Unsupported Span = Corbel Dimension

For example: an 18" total overhang in 2 cm would require a 12" corbel; the same overhang in 3 cm would require an 8" corbel

For 3cm stone: 18" - 10" = 8" corbel
For 2cm stone: 18" - 6" = 12" corbel

Additionally, you need to support every 30" unsupported overhang (linear/horizontal).

So, a 6' (72") span of unsupported overhang will require at least two corbels, depending on location of the corbels. For example:

24" + corbel + 24" + corbel + 24"

Here is a link that might be useful: Thread: Stone Information and Advice (& Checklists)

There are other ways of supporting the stone without using corbels.

Pieces of suitable thick enough steel can be used.

Thank you for all the responses. Lots of great information. I think I will make sure I get the thicker stone. I had read the thread on stone information, but wasn't sure how to incorporate the recommended support under the "table" section of the island. Caayu and Brickeyee, I like the idea of supporting the marble or granite with steel or an angle iron. How could this be attached to the apron that will be underneath? Thanks!

The link below lists some countertop support options. Unfortunately they do not sell to the public so once you find the right part for your application you will need to âÂÂgoogleâ it and find a distributer. I have used them several times with good results.
Caayu

http://www.chemical-concepts.com/counterbalance.php