Box Frame Construction for Granite Counter Top

      How to provide structural support for a granite peninsula countertop extending away from a cabinet. February 12, 2009

I'm hoping someone can help me out with an issue I am having. I have a 2'x2' base cabinet installed and I am looking to have a 7' granite peninsula leading off of it into the kitchen (7' total length, 5' from the cabinet edge). The counter will be supported by two legs at 6' from the wall (4' from cabinet with a 1' rounded overhang).

How would you build the support box between the cabinet and the support legs? The customer is aware that the support box will raise the height of the counter top about 3". After consulting the granite installers they say it needs to support about 300 pounds. 2x4 box? Any suggestions would be greatly appreciated.

Forum Responses
(Cabinet and Millwork Installation Forum)
From contributor F:
300 lbs isn't really that much weight. I think if you built something like a torsion box you could easily support that. It's really going to come down to how substantial the legs are. You’re going to want them pretty strong and made in a way they can't be knocked out accidently.

I would think about dropping the box as opposed to raising the granite. Basically attach the box to the side of the cabinet with the top of the box flush with the top of the cabinet. I'm thinking this would give you a nicer look.

From contributor M:
First I'd have to agree with contributor F and drop the box flush, and the legs do need to be substantial at least like a heavy table type leg. I've done a few of these setups and set them up like a table, with a skirt 3'' or 4'', running from the cabinet to the legs and from leg to leg. You can hide a sub structure behind the finihed skirt blocking against the cabinet etc. I've even run corner or angled braces behind the legs just like a table set up. I’ve had great results as this is very strong.

From contributor J:
The Tile Council of North America recommends that floor joists to support natural stone tile be engineered to deflect no more than 1/720th of their length. Since stone counters are thicker, I imagine the TCA standard would be more than sufficient. If the stretchers supporting the top were 5' long, then the allowable deflection under the expected load would be 60" / 720 or 0.083".

My calculations indicate that two northern red oak stretchers, 5 feet long and 3/4" thick should be no less than 3.15" tall to support a midpoint load of 300#. Bump the stretchers up to 4" high and they should support a midpoint load of over 400#.

From the original questioner:
Thanks so much for the input. After reading that you have had some success with your suggestions, I am going to look into lowering the box. That would certainly be the ideal solution. I'll let you know how it goes.

Contributor J - thanks for your suggestions, but some of that structural engineering is a bit over my head. The legs I'm using are 3 1/2" solid turned pedestals that should be more than sufficient for the weight I'm looking at.

From contributor J:
That's okay. The engineering is pretty close to being over my head, too - that's what makes it fun. Good luck with your project.

From contributor F:
One other suggestion I'll throw out there. You may want to come up with a plan for securing the pedestals to the floor to keep them from being accidentally kicked out. Better safe than sorry, especially with 300 lbs of stone.

From contributor M:
I agree with the others. I just did a similar but less overhang granite top recently. I basically built an apron off the side of the cabinet. For the legs, I located them, then drilled the hardwood floor on center, used 1/4" dowel screws, and screwed the legs down.

From the original questioner:
I definitely did secure the pedestals with double sided fasteners. I had installed hardwood, so they had a good surface to bite on. The suggestions here worked perfectly and the structure is very sound and sturdy and looks great. It is much better than if it had been raised.

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