40" Granite overhang.

cjsdad

I'm building a kitchen island, 66" x 114", 3 cm granite and have never done this kind of work before.
I need help on how to support the end, which will be used as a table.
Approx 40" x 66" will overhang. There are 72" of cabinets up to the overhang. On the far end are a sink and dishwasher.
My wife would like to not use legs for support and I'd like to not use corbels either.
So, I'm thinking of cantilevering some square tubing over the length of the cabinets and almost to the edge of the overhang, spaced at about 18". Then using some metal plate, 1/4" or 3/8", (48" x 90") under the entire slab, attaching it to the tubing (either weld or screws) and filling in around it with plywood. Then covering with some trim under the granite edges.
Anyone think this will work? have a better way?
Thanks

Scuba_Dave

I put a 17x73" piece in as a hearth that weighed over 200 lbs
Your piece will be at least 50% bigger/heavier

That's a lot of weight
Then you have to consider several people leaning on it or the person that will decide to sit on it

You will need a massive amount of support without legs

RegeSullivan

Without calculating it I would guess 1/8" 1X2 square tube would probably do it but I would move up to 1X3 just to be sure there is no sag. I don't see a need for the plate. It would only add weight and you could accomplish the same effect by running square tubing down the middle of the structure. My bigger concern would be the entire thing tipping. You would need to provide extra support for the cabinets (at the end where the overhang starts) and attach it in such a way that a couple of people leaning or sitting on the over hang does not rip it off the cabinets or the cabinets out of the floor.

A small metal fab shop in you area should be able to build this easily. I would figure 750# plus the weight of the slab and a 100% error factor if the shop asks how much weight will be on the over hang. So if the slab is 250#, build it for a 2000# static load and that should handle just about anything that would go on in the kitchen. Be sure to mention it needs to be stiff, the less flex the better.

This may sound like overkill but you do not want cracked granite in your future.

Rege

cjsdad

Thank you Rege
I had thought about the tipping. They tell me the slab wieghs ~1000#. There will be about 35% overhanging, 350#. That leaves approx. 650# holding it down, plus the cabinets. If anyone sits on it I'll have to kill them.
I will take your advice.
Sounds like I may have to support it farther out than just the start of the overhang? What do you think?

RegeSullivan

If you can do it with out changing the look you are after, run a center support maybe 16" inches out from the center of the cabinets. If you made it (maybe 1" wide) and attached it to the center support it would make this much more stable and I don't think you would notice it from a standing position. Plan on people sitting or standing on this at some point, you will regret it if you don't.

Ron6519

Sorry, but I do not think your scenario will work. Supporting that much granite will require legs. There isn't a granite company that will install/guarantee an installation like this. The liability is too much.
I think you might have under calculated the weight
Ron

Wildie

How about angled support, rather than legs?

cjsdad

I appreciate all your replies.
Rege and Wildie, I think we will go with angled support legs, about 16" out from the cabinets.
Just so you all know, I am the installer/owner.builder. No, I've never done this work before and I'm building the whole house. Always wanted to.
My background is machinist/millwright, and I have access to a full blown fab shop, so I'll be building the support too.
And Rege, I've always liked overkill.
Thank you for all your help
I'll post a picture when it's made, then you can tell me what you think.
Should be ready to install in about a month.
Thanks again.

Wildie

Nothin' wrong with being a newbie!

I just finished building a roof over a concrete porch, that was my first time.
The inspector congratulated me and said that he wished that all his inspections were like this!

I'm an electrician by trade, so carpentry is'nt my forte!

RegeSullivan

Glad to hear you are going ahead with the project and I look forward to seeing your pictures.

Rege

cjsdad

Well, it's done!
Thanks again Rege and Wildie.
I went with 1x2 tubing. The installers loved it.
The plate at the bottom of the legs will be under the floor tile, just the legs coming through.
Here are some pix, hope they come thru
Thanks again.

RegeSullivan

Wow - That looks great!!! Job well done.

Rege

Wildie

Nice work! congrats!

Michael Thomas

Per the standard reference, the Marble Institute of America "Dimension Stone Design Manual":

3/4" material must be on underlayment, has a max span of 24" and a max overhang of 6"

1-1/4" material has a max spam of 24”, 10" max overhang,

more needs brackets, legs or other rigid support.

At an inspection I've seen 1-1/4" stone at a 8" overhang sitting on the floor, broken off because someone stood on it to change a light bulb.

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Scuba_Dave

Very nice, Thanks for the pics & update

Now keep everyone OFF that granite !!