What's the stiffest sheet good?

[jgscott987]

I'm putting in granite countertops and need some advice on supporting an overhang. I have an island where the countertop overhangs the edge by 12 inches on three sides. The total countertop size is 66" x 36". The granite companies recommend support for anything more than a 6" overhang.

This particular application won't allow for any visible brackets, so instead I am going to lay down a sheet a plywood a little smaller than the countertop and rout-out and imbed hefty steel bars in the plywood. Specifically I'm using 3/8" x 2" x 2' steel bars every 16" or so. The bars will essentially be in a sunburst patern, radiating out from the center of the island.

This is, by the way, an accepted method of supporting granite countertops.

So to make a long question short, what do you guys think is the best sheet-good to imbed the steel bars in? I was thinking baltic birch, but I've heard that the quality has gone way down hill over the last few years (I've never used it). Does anyone know of a reference on the relative strength and stiffness of various sheet goods (MDF, particle board, OSB, plywood, etc?)

Thanks for the input.

PS- not surprisingly, the granite installer won't give me specfic advice for fear that they will be held liable if the top cracks.

[poolhound]

One of my kitchen counters has what I guess would be a breakfast bar that sticks out 12" from the cabinet. It does have 2 wooden supports and the material that is used for the substrate is OSB.

Not sure how the steel bars are inset into the support, I would hope they are screwed down and ideally also into the cabinet. I would think that with such an arrangement it should provide pleny to support. I wouldnet jump up and down on the edge though

IMHO I wouldnt even consider particle board or MDF. Neither would have the inherent strength. By elimination that leaves ply or OSB. As I mentioned mine has OSB under it. a thick multicore ply would seem like a good idea. Baltic birch is usually one of the better constructed plys. This is usually cabinet grade but seems overkill for the support.

Who do you usually buy your lumber from? You could go down to one of your local sheet goods suppliers (NOT HD) and ask them about the strength of the various options they carry.

[cabinetman]

Most veneer core 3/4" hardwood plywoods will work. Maple is a good choice and probably the least expensive.
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[jackellis]

You're only likely to have a problem if someone sits on the unsupported edge of the countertop.

One way to keep this simple is to get some 3/4" wide strips of 1/4" thick steel and screw it to the edges of plywood sections that are cut into wedges. Routing grooves is probably more trouble than it's worth, you'd need to be sure the routed grooves are exactly wide enough for the steel strips so they don't tilt, and what you really want is for the steel strips to stiffen the plywood. If you want to hide the steel, use a thin layer of plywood underneath the whole thing so it looks like it's all wood.

Of course, 2 3/4" layers of plywood laminated together might provide all the stiffness you'd need.

[jgscott987]

With two young children who are getting heavier by the minute, I'd say it is a certainty that people will occasionally sit on the overhang. I feel pretty comfortable that this will support it.

Between the relatively thick granite (1 1/4"), plywood, and the 3/8" by 2" steel bars it should be pretty stout.

Thanks everyone for your input. Will let you know how it goes.

[jgscott987]

You're only likely to have a problem if someone sits on the unsupported edge of the countertop.

One way to keep this simple is to get some 3/4" wide strips of 1/4" thick steel and screw it to the edges of plywood sections that are cut into wedges.

That would work great if plywood was actually 3/4" thick


...

[cabinetman]

That would work great if plywood was actually 3/4" thick

...

Granite is extremely strong. At 1 1/4" you may not need any support. But just to provide some peace of mind a plywood substrate without the steel would IMO be more than adequate.

Since the overhang is at its weakest at the last line of attachment, I run the plywood out to a distance short of the edge. I usually add a hardwood edge to the plywood and put on a chamfer (taper down), or a bullnose, to make the edge more decorative if seen. The plywood will underpin the entire piece of granite with adhesive. Supporting the overhang that way has never failed. Think of just a plywood overhang that distance...you could stand on just that.
.

[pierhogunn]

call the wurth group, they should have a shop in your area, and ask them, if it's stiff you want, then something like the furniture grade birch plywood would probably suite your needs

[jgscott987]

I agree that I'm probably going a little overboard on the support of this overhang. I just don't want the granite company to be able to point a finger at me if the granite cracks.

thanks again for everyone's input.

[cgallery]

So, you're going to have the plywood stop but the steel reinforcements extend to the edge of the grainite? Or will the plywood extend the full distance and cover the steel? And how far into the 12" overhang will the steel and/or plywood extend?

[cgallery]

Can you do something like this?

http://img02.picoodle.com/img/img02/...sm_81d38a3.jpg

[pelligrini]

What CG posted is similar to what was done in some condos we recently built, but we had a framed wall 43" wall behind the 43" tall cabinets. The angles helped to support a curved overhang for the 3cm granite slabs. The curve was about 12" at the ends and about 18" in the middle. The steel was epoxied to the granite under the exposed overhang. I think the shortest slab was 6'.

I wasn't too worried about anyone sitting on the overhangs at that height.

[jgscott987]

So, you're going to have the plywood stop but the steel reinforcements extend to the edge of the grainite? Or will the plywood extend the full distance and cover the steel? And how far into the 12" overhang will the steel and/or plywood extend?

The steel and the plywood will stop about 2 - 3" short of the edge of the granite.

[jgscott987]

Can you do something like this?

http://img02.picoodle.com/img/img02/...sm_81d38a3.jpg

That was my first approach, however the inside of the cabinet is pretty tight with drawers, and the walls of the cabinet are not very stout. The strength of the cabinet is in the corners. The cabinet faces would flex too much, even with plywood glued to them.

[JimD]

Baltic Birch plywood is definitely the stiffest. You don't really care about the strength, you care about the deflection. Granite is a whole lot less flexible than wood. If you get much deflection, the granite will crack where the wood would just flex.

I have a publication but it does not compare individual types of plywood. I has modulus of elasticity (goes into a deflection calculation) for MDF, particle board, OSB and plywood. I believe they are in that order but I did not go look. I am sure that OSB is closest to plywood. My personal experience is that BB is the best. I made the slats for my daughters double bed of 3/4 BB. I tested deflection versus softwood and it was a little less. I am using baltic birch for an island now. It costs a little more but it is much better plywood than what the big box stores sell in my area. All the layers start the same thickness, are nearly free of voids, and are birch - not softwood or poplar.

Jim