Support for Granite Island with 20" Overhang

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Old 11-17-14, 01:26 PM
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Support for Granite Island with 20" Overhang

I am building a new home with an 11' granite island that has a 20" overhang. The island has two 5" legs and a 1x3 rail that connects the two legs. There is a seam in the middle of the island. I am concerned as to whether this is sufficient support for the granite. It seems that the lack of additional supports where the seam is would be an issue? There are no additional supports underneath the granite. Any feedback would be greatly appreciated as we are being told by the builder that it is sufficient.
 
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Old 11-17-14, 02:44 PM
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Your granite supplier should be able to tell you better but some stones are stronger than others so your choice of stone may affect it somewhat. Also, the seam is a weak point. Even though it's usually epoxied I would not count on it for much structural support and am nervous having it in the center where you'll have the most flexing.

Personally I think a 11' long for a 1" x 3" to provide much more the cosmetics. Unfortunately, a child may end up sitting on the counter. A big guy over for a party or dinner may lean down hard on it or heavy boxes may be plopped down on it... If you don't want a leg or brackets in the center I would consider using steel. A flat bar of cold rolled steel vertically behind the wood 1x3 would not be visible but would add a lot of strength and stiffness. If the steel or welding shops in your area don't have cold roll bar stock hot roll will do or square or rectangular steel tubing.

Have the steel shop get a piece out for you and support it on blocks 11' apart. Step on the middle to get a feel for how much flex even steel has. When installing it I generally shim the center slightly so the steel is pre-loaded to help offset sag under it's own weight and to provide support for the stone without it having to flex before receiving support from the steel.
 
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Old 11-18-14, 06:48 AM
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Thank you Pilot Dane! Please let me know how to determine the required thickness for the rolled steel/plate for adequate support? At this point, I believe they will look for something cheap and not necessarily strong enough unless I am very specific. Also, is it possible to use steel L brackets? Thanks again!
 
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Old 11-18-14, 12:26 PM
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Cheap L brackets from a home center are really only intended for shelving. The trick with granite or any other stone is that it can not tolerate any flexing. I would go for cold rolled bar stock 1/2" x 2 1/2" the cold working hardens, strengthens and stiffens the metal but it's a less common metal for many welding shops to keep in stock. In Sarasota I'm sure you have good access to about anything and a stick may have to be ordered, but most steel and welding shops have a steel truck making a delivery pretty regularly. I just checked an online retailer and it was $137 for a 12' long piece. You can also just stop into a welding or steel fab shop and tell them what you're trying to do and what I suggested. They may say we don't have that but this may work.
 
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Old 11-18-14, 05:48 PM
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It's important to know if the granite is 3cm or 2 cm.
3cm is ~ 1-3/16" thick, it sits directly on top of cabinets and has no underlayment. 3cm can't be unsupported more than about 10"

2cm is ~ 5/8" thick. 5/8" plywood is laid down on top of cabinets and counter sits on that. 2cm has built up edges that make it appear thicker and hides the edges of the plywood under it.
If you have 2cm, you will probably see that the plywood substrate also extends 19", helping to support the 20" overhang.
Even this is not ideal, but if the builder trusts it, you should be OK.
 
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Old 11-18-14, 07:00 PM
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This a new install, correct? No need to cheap out now. There are several makers of heavy duty brackets that can easily be installed. Your countertop fabricator/installer should be well aware of them.
 
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Old 11-19-14, 05:47 AM
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Many people recommend the flat steel bars. I've never seen them used.
Here's what we use, we call them bars. They are either recessed into the plywood or the cabinet is notched a little to accept them:
They are almost always hidden after installation:

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Old 11-19-14, 06:25 AM
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It's rectangular tubing and it will work but because it made from hot rolled material it is less stiff pound for pound. Rectangular tubing might be difficult to find in 2 1/2" height to hide behind your 1 x 3 wood unless it's truly 3" then you can find 3" high tubing. Either way I'd go with 1/4" wall for that distance. Even though there is a lot more metal there it would be slightly less expensive than cold rolled bar.
 
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Old 11-19-14, 07:10 AM
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Thank you everyone for your replies. It was very helpful in discussion with the builder. We appreciated going in armed with information
 
 

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