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Not long ago I read in an DIY outdoor kitchen book that many builders are choosing to place granite directly on top of their counters without support because the granite is so heavy it doesn't need it.
Your granite supplier should be able to advise you based on the stone chosen, the thickness of stone chosen and the grade as to how much support the slab will need.
Hardiebacker and plywood will give little to no support on a horizontal surface, just fyi.
Hardiebacker is just a bonding material for mortars and thin sets that can be easily attached to other structural supports.
Plywood, unless it is sub flooring sheets, provides very little structural support between the bracing holding it up. And in an outdoor kitchen situation it will get wet and rot away in a few years.
All the kitchens we do, we place piers of cmu block, or angle iron supports as to our fabricators recommendations. Or the fabricator will install steel dowels into the back side of the stone that will help give it the necessary strength.
So any extra stretchers you'd add to support ply or hardi etc would support the granite, anyway. I can't figure any granite, of any quality could not span 28" safely. [Of course I didn't mention my span].
Ah - PT ply will rot when horizontal because it's ply, unlike PT deck boards.
A: ..."piers of cmu block, or angle iron supports..."
B:... "install steel dowels into the back side of the stone.."
Can't visualize A or B.... can you re-word it, please?
if I was using solid granite slabs of thickness greater than 1 1/4 inch - I wouldn't worry about it spanning 28 inches. If you are using a thin veneer, you'll need a substrate or base material to support the thin and vunerable stone.
Trick is - along long streches - you need to provide support along the length. - if you you don't the weight of the slab may cause the slab to break.
I made my base out of fairly thick metal studs and then laminated hardibacker to about an inch thick (with thinset between the layers - probably overkill) and applied granite tile to that. I'm very happy and it's super sturdy.
good luck - If I had the money, I would have gone the solid granite route. Send us pics!
To be safe 3cm granite needs a well supported perimeter with almost no deflection in it. Meaning the support will not flex or bend. Hardi backer or durock is not needed. Simply a few spots silicon to keep it from shifting. If location dictates only three being supported then the maximum span on the unsupported should be no more than 30"
Granite is a beautiful and elegant stone,It is available in many stylish colors and finishes, which makes it easy to coordinate with your kitchen design or layout. The natural character of granite creates exquisite aesthetics for your kitchen.