Support for Granite Island Overhang

Old 09-21-17, 05:22 PM
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Support for Granite Island Overhang

We are in the process of installing an island in our kitchen as shown in the image. This is a single slab counter top, made from white ice granite, 3 cm thick.

The rectangular portion is 42 x 64 and sits on the base with about 1 and 1/2 inches overhang on each of 3 sides. The circular part is 52 inches in diameter and sits atop a 6 inch post.

The question is: What is the best way to support the circular portion of the counter top. It's not like it needs any sort of support from the base cabinets like a bar overhang might need (since the granite is essentially in balance on top of the post). But, there is essentially 26 inches of granite from the post outwards that needs some support.

I have contemplated using 1/2 x 3 inch steel support brackets in the shape of an 'X'. Horizontally in the diagram, right to left, the bracket would run from near the outside edge of the circular part to the base in the 3 inch area... 2 x 4's in this area to fasten to. Vertically, top to bottom in the diagram, the second bracket would run from outside edge to outside edge. There will also be a circle of 3/4 inch plywood under the granite.

Will this be sufficient support for the granite? Any other thoughts of how support might be accomplished without having to resort to more than one leg in the center? Thank you in advance for your assistance.
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Old 09-22-17, 02:19 AM
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That is a big piece of granite,!

Not being a granite expert what does the supplier recommend?

I would think they would have best recommendation and if they say it needs support they we can come up with some ideas.
Old 09-22-17, 05:34 AM
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Supporting the weight is easy. Preventing the slab from twisting if someone leans on an edge will be the tough part. I would make an "H" or box/square shaped frame of steel. Attaching one end of the H or box to the cabinet will add a lot of torsional stability to prevent it from twisting. I would use square tubing as it's much more resistant to twisting than an open profile like angle or bar stock. I'd start thinking about 1 1/2" square 14 ga as a minimum and probably go even thicker wall.
Old 09-22-17, 07:17 AM
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That post will need some serious support from underneath the floor.
Old 09-22-17, 10:13 AM
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I agree with PD on how the support should look, there will be some welding involved.. 3cm granite can have a max overhang of 10" and usually there is no plywood under the granite. 2cm requires underlayment.
The installers I work with use 3/4" x 1-1/2" rectangular steel tubing and the tubing is recessed/notched into the sides of the cabinets.
You will need to add some support/wood rails inside the exposed cabinet side panel so the notches are hidden.
Here's a crude drawing:
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