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Discussion Forum

Plywood strength

| Posted in General Discussion on November 13, 2004 10:14am

The finish carpenter and myself are in conflict about the strength of plywood.  We need to cantilever 5/8 inch CDX plywood over the end of a pony wall (34″ tall) for our counter top.  My understanding is that the grain of the plywood should run perpendicular to the wall direction for maximum strength.  The finish carpenter says it doesn’t matter as there are 4 plies to the plywood and therefore there is no strength bias in either direction.  I think the inner plies are of a “higher grade” ie.  less knots and therefore have more strength than the outer plies.    What do you all think?

Foxtrotkilo

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  1. User avater
    hammer1 | Nov 13, 2004 10:23pm | #1

    5/8" CDX for a countertop? The C stands for the grade of the face veneer, the D is for the backside veneer and X is exterior glue. The inner plys are likely less than D with voids. Use a different material for this job.

    Beat it to fit / Paint it to match

    1. foxtrotkilo | Nov 13, 2004 10:29pm | #4

      Sorry, my mistake.  The plywood is for subtop to be under granite.     Foxtrotkilo

  2. User avater
    IMERC | Nov 13, 2004 10:25pm | #2

    Perpendicular is strongest in this application...

    How much cantilever...

    Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming

    WOW!!!   What a Ride!

    1. foxtrotkilo | Nov 13, 2004 10:30pm | #5

      8 inches.  The plywood is a subtop for granite.  FK

      1. User avater
        IMERC | Nov 14, 2004 10:13pm | #10

        FK...

        Why worry about the strenght... 8" needs some sortta support...

        Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming

        WOW!!!   What a Ride!

        1. Snort | Nov 14, 2004 11:50pm | #11

          We've been using 3/8" x 5" steel plates, spaced every couple of feet. Screwed w/ 3" #12s, & some PL Premium. Very lo profile, way strong. Don't worry, we can fix that later!

          1. Snort | Nov 15, 2004 12:01am | #12

            Right<G>, like this: Don't worry, we can fix that later!

          2. JohnT8 | Nov 15, 2004 07:40am | #13

            I can't see the steel plates in that picture ;)

            jt8

          3. Snort | Nov 17, 2004 02:03am | #14

            Ha, I told you they're low profile<G>...here's the view from Australia :-) Don't worry, we can fix that later!

  3. User avater
    IMERC | Nov 13, 2004 10:27pm | #3

    But you should have used 3/4 AC Fir in a 7 or 9 ply...

    now rip it out and do it right..

    Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming

    WOW!!!   What a Ride!

  4. WayneL5 | Nov 14, 2004 12:58am | #6

    If it's regular plywood with all plies about equal in thickness then it's a good bit stronger with the grain running perpendicular to the supporting edge, that is, the grain is sticking out in the direction of the cantilever.

    If it's veneer plywood with only very thin plies on the surface, then it's stronger with the next underlying plies running perpendicular to the supporting edge.

  5. blue_eyed_devil | Nov 14, 2004 01:51am | #7

    Fox, it's significantly stronger. Don't ever take structural advice from a trimmer...most don't even own a framing square!

    blue

    PS No attacks please...I'm only jiving....I'll accept good natured jabs at my trimming ability however.

    Warning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!

  6. 4Lorn1 | Nov 14, 2004 03:23am | #8

    It has less to do with the number of plies in either direction and a lot to do with where those plies are within the plywood. Think of it like an I-beam.

    Plies on the interior of the I-beam have less mechanical advantage. Mostly they only have to handle shear forces associated with keeping the top and bottom surfaces connected so they act as a unit.

    Those closest to the surface, top and bottom, carry the weight. Grain running the direction of the span handles this stress much more effectively.

    1. foxtrotkilo | Nov 14, 2004 05:23am | #9

      Thanks for the excellent explanation.  I knew the outer plies determined the strength but now I can print your explanation out and mysteriously leave it where the finish carpenter can read it.  

      Thanks again,  Foxtrotkilo-  this forum is great!!

    2. dIrishInMe | Nov 17, 2004 02:35am | #15

      I have an opinion on the original question, but no facts to back them up so I'll stay quiet on that however,

      You said:

      >> Think of it like an I-beam.

      Plies on the interior of the I-beam have less mechanical advantage. Mostly they only have to handle shear forces associated with keeping the top and bottom surfaces connected so they act as a unit.  <<

      Not sure if I can go along with that analogy.  I thought an I beam got it's rigidity from the web, and the top and bottom flange were just there to stabilize the web.  Here is an idea: think of a flitch beam...  where does it get it's strength?

      And just to clarify - IMO, this stuff has nothing to do with plywood... Matt

      1. JohnSprung | Nov 17, 2004 02:59am | #16

        >  I thought an I beam got it's rigidity from the web, and the top and bottom flange were just there to stabilize the web. 

        No, the web is just there to hold the top and bottom flanges in place.  The top is in compression and the bottom is in tension.  It's the extreme top and bottom that contribute the most to section modulus.  Note that the flanges are generally a bit heftier than the web.

         

        -- J.S.

         

      2. WayneL5 | Nov 17, 2004 05:07am | #17

        When a beam is loaded in bending, the forces near the middle of the span are tension in the bottom surface and compression in the upper surface.  The web carries essentially no load except to hold the upper and lower flanges in position.  That's why webs in steel beams can have lots of holes in them near the middle of the span.  Usually holes are not made because they cost more in labor to cut than they save in material, but if weight were a factor, such as in an airplane, holes are made in the web.

        Near the ends of the beam, or in a really short beam, there is nearly no bending force, but rather vertical shear.  Sheer forces are carried by the web and the flanges do almost nothing.  In fact, in commercial steel construction you'll notice that the flanges of steel beams are not even attached to columns.  Only the webs are.

        A flitch beam is made from a flat plate only for convenience.  It could be done with less steel if it were made in the shape of an I-beam, but it would be an awkward, impractical shape.

        1. dIrishInMe | Nov 17, 2004 07:00am | #18

          OK - most of what you and JS said makes sense. 

          Riddle me this though... How come with I joists, dimentional lumber, and floor trusses, if you want a stiffer floor, you go to a taller member ie: a taller web? BTW - can you tell me the weight of a piece of steel that 1/2" x 9" x 14'?  This is not just a quiz - it's for a real application.  I had a little steel book around here somewhere, but havn't seen it in several years. Matt

          1. blue_eyed_devil | Nov 17, 2004 02:35pm | #19

            Thats heavy stuff! I worked a steel supply shop for three years right out of high school. Most of our "fish plates"  (flitch plates?) were 1/4 x 9's x 17' for garage headers. Occasionally we'd have one of those 1/2 special order deals....not fun!

            I don't know the weigths per foot any more...try US steel or weights and measures .com.

            Aww..just wait..someone in here will know it, they always do!

            blueWarning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!

          2. dIrishInMe | Nov 17, 2004 04:03pm | #20

            I found a steel weight calculator:

            http://www.mesteel.com/cgi-bin/w3-msql/goto.htm?url=/info/carbon/convert/metalcalculator/metalcalculator1.htm

            Matt

            Edited 11/17/2004 8:06 am ET by DIRISHINME

          3. BenM | Nov 17, 2004 04:28pm | #21

            214 pounds.  A taller member is stiffer.  You could increase the stiffness with bigger flanges but it's more efficient to make it taller.  But you do lose headroom so sometimes it's a tradeoff.

          4. WayneL5 | Nov 18, 2004 01:52am | #22

            The further apart the flanges are from one another, the stiffer the beam.

            Steel has a density of 490 pounds per cubic foot.  Ben did the math for you.

          5. JohnSprung | Nov 18, 2004 03:09am | #23

            > Riddle me this though... How come with I joists, dimentional lumber, and floor trusses, if you want a stiffer floor, you go to a taller member ie: a taller web?

            Because section modulus is proportional to the square of the height. 

             

            -- J.S.

             

  7. Hooker | Nov 18, 2004 03:43am | #24

    My response to the original Q.  This might have been mentioned.  8" isn't all that much for the granite to stick out.  Assuming a 2x4 wall, 1 1/2 overhang on the other side, 8 inches is very little.  What does the granite guy say?  I'd be incined to use other supports.  How is the ply planned to be finished?  A job with granite tops certainly needs a nice finished product under the tops. 

    My advice is to skip the ply and use a decorative angled support.

    ADH Carpentry & Woodwork

    Quality, Craftsmanship, Detail

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