Ceramic Tile Advice Forums - John Bridge Ceramic Tile

Welcome to John Bridge / Tile Your World, the friendliest DIY Forum on the Internet


Advertiser Directory
JohnBridge.com Home
Buy John Bridge's Books

Go Back   Ceramic Tile Advice Forums - John Bridge Ceramic Tile > Tile & Stone Forums > Tile Forum/Advice Board

Sponsors


Reply
Thread Tools Search this Thread
Unread 02-09-2010, 05:47 PM   #1
Peppuz
Registered User
 
Join Date: Feb 2006
Posts: 15
Kitchen Island Makeover - Overhang Questions

I'm planning a kitchen island and counter makeover, removing particle board and formica, and replacing with plywood (3/4" + 1/2"), ditra, and 3/8" grantite tile. The island has overhangs on 2 sides: 14" x 40" and 14" x 48". I've read that for 3mm granite, rebar is glued into dado chambers in the plywood. Is this also advisable for 3/8" granite tile, or would corbels suffice? Is there a rule of thumb on the size and spacing of the rebar. (Also, is rebar advised over the dishwasher, which is a 24" span). Thanks in advance everyone!
Puz

Name:  newisland.JPG
Views: 1756
Size:  2.7 KB

Name:  newisland2.JPG
Views: 2922
Size:  22.0 KB
Peppuz is offline   Reply With Quote
Sponsored Links
Unread 02-09-2010, 06:10 PM   #2
bbcamp
Registered User
 
Join Date: Jul 2002
Posts: 30,274
Corbels will be enough. Place them about 24 inches apart. The should support the overhang to within about 2 inches from the edge.

You can brace the area over the dishwasher with 1x lumber placed across the opening. One at the front and one in the middle. A ledger screwed to the wall supports along the back.
bbcamp is offline   Reply With Quote
Unread 02-10-2010, 06:34 PM   #3
Peppuz
Registered User
 
Join Date: Feb 2006
Posts: 15
Thanks for the reply. The links to the similar threads are also excellent with good discussion. Very helpful!!
Peppuz is offline   Reply With Quote
Unread 02-10-2010, 06:54 PM   #4
Muddman
Potter, Contractor, Philosopher
 
Muddman's Avatar
 
Join Date: Apr 2009
Location: St. Louis
Posts: 918
If you didn't want corbels, I would use a piece of sheet steel, like 1/4", easier than trying to embed rebar.

For the dishwasher, I wouldn't worry about it. I have never done anything extra in that area in my installs, and have never had a problem. It is only a 24" space, the same amount of space you have unsupported front to back in the open spots of the cabinet bases. A wood piece could get in the way of a dishwasher install
__________________
Gregg

www.muddmandesign.com
Muddman is offline   Reply With Quote
Unread 02-16-2010, 06:58 AM   #5
Peppuz
Registered User
 
Join Date: Feb 2006
Posts: 15
Thanks Muddman. I've checked out 1/4" steel at the salvage yard. Pretty thick stuff; what's the best way to cut it? Also, in what layer is the steel placed? Between the ply? Then, how is it attached? Or do you think the rebar might be better for a weekend DIY'er?
Thanks for your suggestions.
Puz
Peppuz is offline   Reply With Quote
Unread 02-16-2010, 09:15 AM   #6
Muddman
Potter, Contractor, Philosopher
 
Muddman's Avatar
 
Join Date: Apr 2009
Location: St. Louis
Posts: 918
I don't think rebar will be easier. Because you have to cut it, and then physically bind it to the top, like embedding it in a layer of mortar. If you don't do this, it will ad negligible strength. Just laying it in there won't do much.

If you did sheet steel, make a template to the dimensions you need, then bring it to were ever you get the steel from, and have them cut it for you. you should have the steel go onto the supported area enough so that the weight balances well.

What I would do is lay down your 3/4" ply first, screwing it into the island. Then lay down you steel, taking some plywood of equal thickness and cutting it out to fill in whatever area there is that doesn't have the steel, so it is level.

Then I would get some epoxy adhesive that works for both steel and wood, and glue your steel to the 3/4" ply. Then do you 1/2 ply layer, screwing it to the areas with wood, and gluing it over the steel.

You may want to have them drill a few holes in the steel for you. Then you could run screws threw them attaching the two layers of ply together, which would help pull the top together

Then you can attach the ditra over that.
__________________
Gregg

www.muddmandesign.com
Muddman is offline   Reply With Quote
Unread 02-16-2010, 12:58 PM   #7
astrojeff
Registered User
 
Join Date: Feb 2007
Posts: 131
Some numbers for perspective

Let me add some engineering numbers to the corbels versus steel reinforcement debate here. These are more or less theoretical numbers from a mechanical engineering text, and I have only done tile overhangs with corbels, so even if I'm not fully correct here, I think some numbers would put the comparison in perspective. In case you don't feel like reading all this junk below, my point will be that not only would corbels suffice, they can be way more effective than adding beams or rebar. You should only add steel beams if you really can't fit in corbels, and if you really know what you're doing. Read on if you want to know why I think this to be the case:

Your deflection on an overhang is dependent on a few factors, the most important being the length of the cantilevered surface (14" in your case), the thickness (5/4" total plywood) and material (wood) of your overhang. Your tile thickness does not go into the equation since it has no tensile strength, and the top of your overhang will be loaded in tension.

Assuming you have a weight on the end of your overhang (a person leaning or hanging on the outer edge), the downward deflection of the outer edge is is proportional to the CUBE of the overhang length. That means that twice the overhang has 8 times the vertical deflection! This is a strong argument to anyone to minimize that overhang as much as possible.

The deflection is inversely proportional to the stiffness of the overhang material (engineering modulus of elasticity). Steel is approximately 20 times stiffer than wood, so the same thickness of steel would bend 1/20th as much as wood under the same loading (argument for using steel?).

BUT your deflection is also inversely proportional to the CUBE of the thickness of your solid overhang (area moment of inertia). So that means that a 1.25" wood surface deflects 125 times less than a 1/4" piece of wood (5 times as thick = 5^3). So comparing wood versus steel, a 1.25" thick overhang of wood is six times stiffer than a 1/4" thick piece of steel (125x stiffer due to thickness, but 1/20th the stiffness due to a softer material, 125/20=6.25). And that's assuming you have a whole countertop of 1/4" steel, not just a few thin beams of metal. Ok, now it gets a little more complicated when you laminate materials together--they have a somewhat multiplicative effect rather than an additive effect, so laminating two equally stiff materials together makes a surface 4x as stiff--assuming they are fully laminated--as compared to twice as stiff if they are not fully laminated. This is why Greg, I believe, has emphasized laminating or epoxying the metal beams to your wood, not just laying them together. But just changing your ply thickness to two 3/4" ply sheets (fully laminated together), will almost double your stiffness compared to 3/4" + 1/2" ply (1.5^3/1.25^3 = 1.72x stiffer).

All that considered, I would argue that embedding steel beams into your wood surface at worst doesn't really do much, and at best is not very cost effective. You might as well just make your plywood surface thicker and save a lot of time and money. To get any stiffness out of your steel, you need to use a specially shaped beam, such as an I beam or a hollow square of steel, which puts all the bending resistance on the outside edge, but it's going to be thick and hollow rather than thin and solid. For that reason I would also argue that your most important line of defense against deflection are 1) minimizing your overhang as much as possible, and 2) using corbels when possible. If you use securely attached corbels, they have almost an infinitely small deflection due to their large effective thickness! If you can't use corbels to stiffen your overhang for whatever reason (knee room, etc), then pay close attention to the design and thickness of those steel beams you are laminating to your plywood and ask yourself if they really have less deflection than plywood, or if you're just putting metal in there because metal "sounds" strong. Seriously, take a 1/4" by 8' metal beam or standard 8' piece of rebar and try to bend it versus bending an 8' sheet of standard plywood--I think you'll agree that the metal is relatively floppy compared to plywood and isn't going to add much unless you are very strategic (from an engineering perspective) about how you do it.

Since I'm speaking from theory here and not practice, I'd love to hear thoughts from someone who's compared corbels versus embedded beams versus nothing (or just thicker plywood). Have you actually taken deflection measurements with and without embedded metal, and what type of metal beam did you use? Can any other engineer here support or refute what Iv'e said?
__________________
-Jeff
astrojeff is offline   Reply With Quote
Unread 02-16-2010, 01:04 PM   #8
bbcamp
Registered User
 
Join Date: Jul 2002
Posts: 30,274
Well written, Jeff. You want my job?
bbcamp is offline   Reply With Quote
Unread 02-16-2010, 01:30 PM   #9
astrojeff
Registered User
 
Join Date: Feb 2007
Posts: 131
I don't need any new jobs, but thanks, Bob!

Here's a picture of my own kitchen counter with the bar overhang. It's a 9" overhang with corbels every 24" and two laminated sheets of 3/4" ply underneath. It was going to be a 12" overhang, but I built a small wall behind the cabinets to shorten the cantilever and to give a stronger nailer for the corbel, since the cabinets are pretty weak in back. This is a picture of how happy my wife was when I later installed the peninsula vent hood with lights. My point? I stood on my tile overhang, lifting the 70 pound vent to do the vent installation. My countertop was rock solid, and perhaps overbuilt, but that's peace of mind for you. In my opinion, always assume somebody is going to stand on the edge of your countertop and jump a few times. I certainly do with mine!
Attached Images
 
__________________
-Jeff
astrojeff is offline   Reply With Quote
Unread 02-16-2010, 01:34 PM   #10
Muddman
Potter, Contractor, Philosopher
 
Muddman's Avatar
 
Join Date: Apr 2009
Location: St. Louis
Posts: 918
I don't disagree that corbels would be stronger, and easier.

I think it is an aesthetics choice. Some people, me included, don't like the look of corbels. When I do concrete countertops I will engineer them to support overhangs, but that is a different ballgame.

I saw it done this way (with steel plate) once for a similar overhang and it seemed to work. But you may be right that simply laminating together more plywood, instead of steel, would do the job too.
__________________
Gregg

www.muddmandesign.com
Muddman is offline   Reply With Quote
Unread 02-16-2010, 03:08 PM   #11
astrojeff
Registered User
 
Join Date: Feb 2007
Posts: 131
And I completely understand that there are situations or personal preferences for which corbels are not appropriate.

I just want to make sure that folks understand that there is a lot more care that needs to be taken when designing a cantilevered overhang versus an overhand that is braced with corbels.
__________________
-Jeff
astrojeff is offline   Reply With Quote
Unread 02-16-2010, 03:26 PM   #12
Muddman
Potter, Contractor, Philosopher
 
Muddman's Avatar
 
Join Date: Apr 2009
Location: St. Louis
Posts: 918
__________________
Gregg

www.muddmandesign.com
Muddman is offline   Reply With Quote
Unread 02-17-2010, 07:28 AM   #13
Peppuz
Registered User
 
Join Date: Feb 2006
Posts: 15
This is a great discussion; wish I had taken a few more physics classes! From what I've read on this board, it does makes sense to shorten the overhang, and using 2 pieces of 3/4 ply can be easily done with corbels for support.
I am curious about the corner where the the 2 outer edges of overhang meet, because it would no longer be a 12" overhang, but an ~18" overhang at the point of the corner. I could chamfer it, but it seems that an angled corbel toward the corner and a 3x3 counter leg or post at the outside corner would be effective. Any thoughts on this setup?
Thanks again.
Peppuz is offline   Reply With Quote
Unread 02-17-2010, 08:46 AM   #14
astrojeff
Registered User
 
Join Date: Feb 2007
Posts: 131
I tend to chamfer my outside corners (I think you can see that in my picture)--they seem to be less bump-into-able that way (and the outside corner seems less useful to me anyway), but it does make the tile layout more complicated. I could see that an angled corbel or a support leg would work just fine--not sure you'd need both at the same time, though--I think I'd try to stick to one style or the other.

A single leg might look a little out of place, but there are ways to make legs look more stylish, for example if they match the material and style of your cabinets, and you can put a small arch or beam connecting two legs that stiffens the area between them, requiring fewer supports, and it can also be part of the style.
__________________
-Jeff
astrojeff is offline   Reply With Quote
Reply

Stonetooling.com   Tile-Assn.com   National Gypsum Permabase


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Kitchen Makeover Muddman Professionals' Hangout 17 10-13-2009 12:23 AM
Island overhang info request kmarsh Tile Forum/Advice Board 18 09-16-2007 12:47 AM
Kitchen Counter Overhang seanile Tile Forum/Advice Board 16 02-11-2006 09:58 PM
No overhang on kitchen countertops???? bjw1 Tile Forum/Advice Board 5 02-08-2005 09:13 AM
Overhang on granite tiled kitchen counter top DCCarp Tile Forum/Advice Board 3 11-04-2003 08:58 PM


All times are GMT -6. The time now is 02:43 AM.


Sponsors

Powered by vBulletin® Version 3.7.4
Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.
Copyright 2018 John Bridge & Associates, LLC