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Unread 02-25-2010, 04:00 PM   #1
Zep2525
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Granite Tile Overhang Support?

Great forum, lots of good advice. If anyone could help, I'd appreciate it.
I'm currently building a countertop/bar, rectagle 104"x36", on three standard 26" deep cabinets creating a 10" overhang. Im using 18"X18"X1/2" tiles with bullnose finishes. This is my plan for the base - 2 sheets 3/4" ocd ply with 1/4" densshield (water barrier).
Question - Will this base support a 10" overhang, or do I need additional corbels, steel supports etc.?
Also, should I use regular cement board or hardibacker instead of the densshield? Will the cement board give me a more rigid base? I've also read that thinset doesn't set to the densshield very well.
Any input is appreciated.
Thanks.
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Unread 02-25-2010, 06:04 PM   #2
Brian in San Diego
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Welcome, Kevin! I don't like your plywood choice even though I've never heard of ocd plywood. (Is it anything like obsessive compulsive disorder in humans? ) Any plywood in a tile installation must have no face grade less then "C" and must have exterior glue so it will either be designated as "exterior" or "exposure 1". Both are O.K.

I don't think I would trust a 10" overhang without additional support most likely in the form of corbels. Any force applied out at the end of that counter could result in problems. I could be wrong and if I am CX will be along shortly to correct me and that's O.K. too. Happens alla the time.

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Unread 02-25-2010, 07:37 PM   #3
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Welcome, Kevin.

What Brian said about your plywood. Don't know what that is. I'd want some AC or BC exterior glue plywood for that top.

Ten inches is pushing it a bit, I think, but if a fella were careful to have his face grain of both layers oriented perpendicular to the cabinet face at the overhang, and glued his layers together real well, and kept his larger friends and relatives from sitting on the edge, he might could get by with it.

That leaves the problem of rigidity in the direction of the outside edge of the top, though. I'm prone to using wood edging for such tops and would wanna glue a 2 1/2" or 3" edge across there to contain my tile top and provide vertical strength to the edge.

Still won't support the attempted sit of a full sized MIL, but might last a long time otherwise.

Safer to add supports as BSD suggests, but for the right customer I'd likely give it a try.

My opinion; worth price charged.
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Unread 02-25-2010, 09:26 PM   #4
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Thanks for the quick reply. The 3/4" ply was marked ""outdoor", just assumed that's what the "O" in OCD stood for. I'll keep looking around.
As for the overhang, I'm not going to risk it. I think corbels might be the easiest choice. But, if I were to install steel suppports, what would you suggest? I've read some forum members talking about using 1/4" flat steel pieces under the ply, or even sandwiched in between. What would you suggest?
Thanks again.
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Unread 02-25-2010, 09:33 PM   #5
Brian in San Diego
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Kevin,

You were probably looking at sheathing which is fine for roofs and outside walls but no bueno in a tile installation. Steer clear of the word "sheathing" on any plywood you might consider using. Like CX said BC sanded is a good choice as long as it's exterior or exposure 1.

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Unread 02-26-2010, 12:00 PM   #6
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Thanks. Any thoughts on the metal supports? I'd rather not do corbels (aka. knee knockers) with only a 10" overhang. I've read quite a few posts with different layouts...Matel laying flat (size?), standing on edge, 90 degree bend, angle iron around edging. Any advice on what's best for my plan?
Kevin
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Unread 02-26-2010, 03:19 PM   #7
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We just had a very similar question recently about using metal inserts versus corbels. I won't go into the horrible details again, but you can read my opinion in this thread: http://www.johnbridge.com/vbulletin/...ad.php?t=81633

In general, corbels are much safer. There may be ways of inserting a metal beam into your countertop, but I'm not so sure it's actually going to help unless it's relatively thick and designed as a beam to reduce deflection. In other words, I don't think that 1/4" steel strips will be any stronger than just using thicker plywood. If there's some way of rabbeting small (+/-1") diameter steel I beams or square beams into your cabinet tops so that the dual plywood layers sit on top, then I think you may be in business. Or perhaps you can find some metal braces that function as corbels but that are much lower profile--like some industrial-grade shelf brackets.

Another thought is that you could build a framed wall at cabinet height on the overhang side of your cabinets. First of all, that will shorten your overhang a little, but still leave some legroom for bar stools or whatever. That may be enough to make your overhang not be as prone to problems. But for security, you could fashion some very low profile custom wood corbels, for example by using a wedge of a 2x4 tapering to the outside end with an embedded and screwed metal strip across the top bridging the top of the stud with the wood wedge. In that case, the framed wall together with the wedge function as a corbel--the thicker wood wedge produces your resistance to bending and your metal just allows the top to withstand the tension. I'm just trying to find a way of making a less-visible corbel-like support and still give you lots of support. You could build a custom low-profile corbel right into the cabinet, but typically your cabinets won't be as strong as a stud wall, so it requires a much wider support for the same resistance to bending.
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Unread 02-26-2010, 03:45 PM   #8
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Sorry if I'm just going on about something you're not interested in, but it's much more interesting than my work at the moment...

I attached a picture of what I meant in the last post. You can embed the beam in your plywood as well, but make sure that it is actually stronger than the plywood--1/4" steel is not stronger than 2 sheets of 3/4" plywood--it really should be a thicker beam, which makes it less elegant to embed in your plywood without creating problems. I'm sure you could figure out some sort of other similar ways of adding support, I'm just kind of brainstorming here because it's interesting. One more thought--a metal t-beam might be easy to attach to the bottom of your plywood like picture #1 and cut into an unobtrusive top part of your cabinet--can you get those at the big box hardware store?

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Unread 03-01-2010, 10:42 AM   #9
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Jeff, Thanks for the input. Good stuff. I think I like option 1. I was planning on bolstering my cabinets a bit anyway, so this might be my best bet. I'll look around at the DIY store and see if I can find some square steel rods.
Also, After reading your other post, it seems that adding an additional layer of ply might give you more rigidity anyway, right? If you would sandwich another 1/2" ply in between your existing two sheets of 3/4" (if an additional 1/2" depth is not an issue), would that give you the same strength as the steel beam extensions? Or, at least enough to feel safe with my 10" overhang spanning 8' ?
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Unread 03-02-2010, 09:31 AM   #10
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Kevin: It appears to me that adding an additional 1/2" could potentially double your countertop stiffness, if it is very well laminated into the other two plywood sheets. I have no threshold criterion for what the minimum stiffness should be, so I'm just going on intuition that it seems like that would be stiff enough. But as you mention, it's going to add to your countertop thickness. My countertops are all made of two 3/4" sheets and they look thick but not too thick in my opinion--some folks prefer the thinner look, though.

You can get the same increase in stiffness with some type of steel rod or beam, but it depends on the thickness of the material, so I can't really make a comparison between that option versus a thicker coutnertop. The point in a beam is to get the largest amount of mass as far from the center as possible in the direction of bending. That's the idea of an I beam--most of the mass is on the outer edges of the I, with a "web" connecting the top and bottom to hold them apart but laminated together. A hollow square channel is the same idea, but harder to manufacture. Solid is fine too, but the metal on the inside isn't doing much except for making it weigh more. The worst configuration is something thin and wide, like 1/4"x1" steel laid flat. If you can lay it on it's edge and support it very well, it will also markedly increase its strength. Think of a strip of paper on its side versus laying flat--one way bends really easily, the other way is relatively strong. And some sort of lamination (or lots of screws) within the beam and between the beam and the countertop is important too. Think of the difference beween a paperback book and a paperback book with the pages glued together, which is basically a block of wood. That lamination has a multiplicative effect on strength, whereas just laying things on top of each other has an additive effect. A hundred sheets of loose papers are just a hundred times stiffer than a single piece of paper. But a hundred pages glued together is thousands to millions of times stiffer.


Also remember what CX said: if you decide to go the thick plywood-only route, orient your plywood so that the outer grain is going perpendicular to the cabinet face, most importantly the top piece since that one is under tension when someone sits on the edge. If you do sandwich a 1/2" layer in between, that one doesn't matter as much since it experiences little bending forces--make that the layer that runs with your cabinet to keep the edge from warping. I'm not sure about the bottom layer--it'll still be stronger with perpendicular grain like the top, but it may not be quite as critical since it's not under tension, which is where cross grain is at its weakest. I know the temptation will be to orient them all parallel to the countertop since it's an 8' top.
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Unread 03-02-2010, 08:04 PM   #11
Zep2525
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I did find some 1"x1" sqaure steel hollow beams at the local DIY store. I may use two or three of those instead of the extra layer of ply. But, I'll have to drill some holes to attach them with . Still havn't decided.
Anyway, what's the best way to laminate the ply sheets without warping/bending? Should I use regular wood glue or maybe thinset? Seems like thinset may hold better, but may be more difficult to keep the sheets perfectly flat.
Thanks agian.
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Unread 03-02-2010, 08:26 PM   #12
Brian in San Diego
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Kevin, use a fully spreadable glue like Titebond II. Doesn't hurt to drive some screws into the wood to draw it together.
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Unread 03-03-2010, 11:47 AM   #13
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I just buy one of the gallon containers of Tighbond II, pour it out over the base layer and trowel it with an adhesive trowel for vinyl floors (or a credit card with small 1/8" v notches). If you only need a smaller bottle, I still advise taking the lid off and troweling it versus squeezing it out--it'll really make your hand tired and you'll get better coverage with a trowel. I have the top layer predrilled every 6-8" or so so I just lay the screws in and start driving them and it pulls the two sheets together very easily. My sink cutouts always confirm that I have full lamination all the way through. Glue does squeeze out and make a mess, though. It seems to always end up being sufficiently flat when I'm done--the two sheets of ply tend to cancel out each others' warping. But do it on a flat surface. If you do three layers, You may either have to do two at a time or predrill the top and second layer, somehow keeping them lined up together--screws would really tend to tent up between layers adding voids if you're not careful.

I think those 1" square steel square beams should be plenty. I'd put them every 2' or so. Shouldn't be too hard to drill a few holes through with a metal bit and a little machine oil.
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