Swingman wrote:
"bdeditch" wrote in message
My daughter wants me to build her a Kitchen Island that will be approx
11' by 8' ( L shaped). It will hold a stove top and will have a back
splash behind it. On top of the back splash she wants to have a
granite Counter top. My concern is what is going to hold up the 1 foot
over hang that she wants to have. I am sure that I can make the back
splash approx 6" wide but what should I use to hold up the over hang?

In short, you need a design that incorporates support, but not knowing your
design, it is almost impossible to advise you.

I would draw up a proposed plan and head to a local cabinet shop and see
what they advise based on your drawing. Here's why:

IME, and simply put, you need _support_ of some type for the overhang,
regardless of the material you are using for your countertop.

A 3/4" plywood substrate is quite sufficient to support a 3/4" thick granite
"countertop" overhang of 12", providing the plywood substrate is
sufficiently deep (front to back) to be properly "cantilevered" from the
supporting base/island cabinet.

In doing a granite overhang on an island, I try to use the old "1/3 to 2/3"
cantilever rule as a bare minimum, and much more if I can get away with it:

IOW, as a MINIMUM for 12" overhang, I want at least 36" of granite over +/-
36" of well fastened plywood substrate, supported by a minimum of 24" of
countertop.

Your problem is that your "countertop", in this case the top of your
backsplash, is only 6" deep and you're attempting to "cantilever" 12" of a
roughly 18" deep slab of granite.

You're concept of how a cantilever works is okay if you're talking
about materials with the same modulus of elasticity. Unfortunately
wood and granite behave far differently. Wood and plywood can take a
fair amount of flex. Granite has no such flexibility. A 3/4" plywood
sub-counter is certainly "strong" but it is not stiff in the way
granite is stiff, and it is brittle. In other words the plywood will
simply flex and the granite will be forced to take all of the load and
it will crack. It doesn't matter how far back that plywood sub-
counter extends, it will not change the amount of flex as it can not
change the modulus of elasticity of either material.

If you want to omit knee braces, corbels or any other type of visible
support, you need a stiffer material with a higher modulus - steel.
The steel should be thick enough that you can stand on the end of its
full cantilevered length. In other words the steel supports should be
designed to take _all_ of the load so that the granite can do what it
is there for - look pretty - without being required to be structural
as well.

Robatoy's suggestion of using a thicker, engineered (man-made) stone
with let-in steel is certainly a good way to go, but it is also
frequently a more expensive way to go as letting into the stone
requires more labor and entails more risk. The thicker stone adds a
lot more weight to the countertop, which may or may not be a problem
structurally, and you're paying for that extra stone that you'll never
see. The engineered stone also limits the choices you'll have in
stone. The idea is to add strength and stiffness. Cutting into a
thin slab creates stress concentration points, and let-in braces are
weakening the stone in order to strengthen it - that doesn't make
sense to me.

R's comment about idiots doing stupid things on stone cantilevered
countertops should not be discounted. I once swung a hammer at an
idjit painter's ankles that was _standing_ on the cantilever! He
jumped off, and started yelling at me, and I told him that the only
reason I didn't swing at his head was because I couldn't reach it.

Cantilever's are one of the few things where I tell people to
purposefully over-build things, as deflection and stiffness are of
paramount importance. Doubly so with a stone countertop.

R