All you wanted to know about plywood

https://www.fpl.fs.fed.us/documnts/fplrn/fplrn059a.pdf
This is pretty teckie - Is there someone here who can scan this and answer these questions...

[EDIT] - found these charts too https://www.tecotested.com/techtips/pdf/tt_plywooddesigncapacities
Looks like the plywood is 3 times as strong when laid one way vs the other.

1. Obviously 5 ply is better than 3 ply - can I get more plys in less height and have it be stronger? Looking to go less than 3/4 inch plywood for weight.

2. Laying it down, if i have a long span, do I go parrallel to the span with the outer grains or perpendicular to the span with the outer grains?

Maybe you don't even need to read this to know the answer - Unless it's numbers, I'm clueless.

Will

wasilvers said:

https://www.fpl.fs.fed.us/documnts/fplrn/fplrn059a.pdf
This is pretty teckie - Is there someone here who can scan this and answer these questions...

[EDIT] - found these charts too https://www.tecotested.com/techtips/pdf/tt_plywooddesigncapacities
Looks like the plywood is 3 times as strong when laid one way vs the other.

1. Obviously 5 ply is better than 3 ply - can I get more plys in less height and have it be stronger? Looking to go less than 3/4 inch plywood for weight.

2. Laying it down, if i have a long span, do I go parrallel to the span with the outer grains or perpendicular to the span with the outer grains?

Maybe you don't even need to read this to know the answer - Unless it's numbers, I'm clueless.

Will

Click to expand...

1. yes you can get more ply's out of 1/2", But its probably what you already know, which is marine ply and also high end mohaganey which is more expensive than the marine.

2. You can lay it any way, makes no difference considering each ply is at a different orrientation than the other.


Clear as mud huh? :lol: hope this helps, I tried to be simple.

Perchin - according to both links, plywood has a strenght axis and is much stronger along that axis than the other way. Something about the compression and elasticity of the outer layers - if I read it right. Of course, the more ply's you have, the less this matters.

"Strength axis is defined as the axis parallel to the face and back orientation of the grain (veneer), which is generally the long panel direction, unless otherwise marked." from page 2 of the second link. For a 3 ply piece, the differece in bending is 370 lbs vs 92 lbs - quite a difference. If I read it right.

I'm guessing that if I have a 24 x 12 opening, for the most strength, I would have the grain of the outer layers run in the same direction along the long span. I only ask because I have some bigger doors to span on the front deck and want to make it as rigid as possible.

Since I posted this, I went to Menards (like Home Depot) - they had some 5/8 plywood - 40/20 span rated for $22.47 a sheet. Based on the second link above, this is rated over 600 lbs a square foot AND save about 35-40 lbs on the decking. (That 35 lbs would make up for the extra bracing I put in on the front casting deck #-o

I use two different kinds of 1/2" plywood at work. One is Luan (a kind of mahogany) and the other is called Sandy (or Sandee). Both are 7-ply. They cost a little more than AC fir but a lot less than marine grade plywood. If you look for cabinet grade materials you might find plywood with more plies than typical materials used for sheeting. I can't remember if the glues are interior or exterior but you'd have to put on some kind of finish in any case to be able to use in your boat. Places like HD usually only carry sheeting materials so you may have to go to a real lumber yard. I get what I use for work at a yard that specializes in cabinet making materials.

bobberboy - Thanks! I know of the real lumberyard here, the place where everyone else gets their wood from. Kind of pricy to buy direct (I think they just don't want the hassle). The one I found is rated exterior 1. I don't know if you can get better without buying treated?


Dang it, I think I was wrong, it should be facing the shortest span for most stregth. Here's another link
https://www.gp.com/build/DocumentViewer.aspx?repository=bp&elementid=3805 (page 2)


You know what - screw it, I'm going to put some out on a long span and step on it. See how 225 treats it one way or the other.

bobberboy said:

I use two different kinds of 1/2" plywood at work. One is Luan (a kind of mahogany) and the other is called Sandy (or Sandee). Both are 7-ply. They cost a little more than AC fir but a lot less than marine grade plywood. If you look for cabinet grade materials you might find plywood with more plies than typical materials used for sheeting. I can't remember if the glues are interior or exterior but you'd have to put on some kind of finish in any case to be able to use in your boat. Places like HD usually only carry sheeting materials so you may have to go to a real lumber yard. I get what I use for work at a yard that specializes in cabinet making materials.

Click to expand...

LUAN=MAHOGANY??? Wow I must of really missed something in shop class, and my suppliers have been really pulling one over on me. :lol: The glues used in cabinet grades are interior. A sheet of real Mahogany is not cheaper than Marine grade. look below at the two links. The first being Mahogany at $127.68 a 4'x8' sheet and the next being Maringe grade ply. at $69.99 a 4'x8' sheet.

https://www.bristolvalley.com/catalog.asp?prodid=296583

https://www.overtons.com/modperl/product/details.cgi?r=view&i=76126&aID=601E4&cID=FROOGLE_76126

As for the strength by orientation......... I could be wrong for sure, just always done what was taught to me, which was cross grain is much stronger.

Perchin - same prices here, just called Alpine Plywood - $70 for 1/2 marine - or I could go with aspen, same glues, same plys, just might have more voids - for $40 a sheet and it's in stock.

But all marine grade is sold out for at least a week

wasilvers said:

Perchin - same prices here, just called Alpine Plywood - $70 for 1/2 marine - or I could go with aspen, same glues, same plys, just might have more voids - for $40 a sheet and it's in stock.

But all marine grade is sold out for at least a week

Click to expand...

10-4... I won't argue, but I'll say this... aspen is not in any test or way shape or form going to be as strong as any marine grade ply or Mahogany ply.

The luan thing was getting to me........... here look below.

https://www.insidewoodworking.com/luanplywood.html

We have used the luan a lot at work. I wonder if because it's a SE Asian wood that some consider it the same as mahogany or from the same family of trees; my dealer tells me that luan is a sub-species of mahogany. Whether it is or not doesn't really matter. It is pretty reasonably priced. At my dealer 1/2 luan is $33.52/sheet and AC fir is $34.41/sheet. More plies means more stable so it's worth the effort to get better grade materials.

for what it's worth, I bought reg. plywood at the lumber yard.....but.....painted each pc. with a high grade exterior paint, 100% , top, bottom and all edges...... still holding up fine and going on four years.

Thanks ecbob! That makes me feel ALOT better about not spending the cash on marine grade.

I made the trip to Menards and bought 3/4 and 1/2 A?X. It had the same number of plies as my marine plywood, was $35 and $25 a sheet respectively, and just looked much better (thus the A rating on one side).

And to answer the question I posed before, I cut some old 1/2 ply I had to a square and gave it the '225 pound step test'. If the grain was running parallel to the long side, it flexed ALOT. If the grain was running at a 90 degree T to the long side, it flexed a little, but noticeably not as much as the other way. It really did affect how I'm going to lay the plywood down.

Thanks all!

You can see my deck job at https://www.electric-bass.org Nav. menu then click on "fully rigged EBC boat". One note, if I fish in the rain or it gets wet....I dry it out at first chance.... I framed in for the deck with pine 2x2's but also painted them with the same high grade exterior paint. You can see that in one of the pics. The frame is placed where the vertical pcs. rest on the ribs for support and not on the hull of the boat. I did use an exterior grade glue for the carpet. Like I said, Its going on 4 years and everything is still good and sound. One little note, an easy and cheap way to open and close hatches.....get a short pc. of vinyl or marine rope. Drill a small hole in the hatch in a proper location for lifting the hatch....the hole slightly larger than the rope.......knot one end of the rope so in cannot pass though the hole.....insert the un-knotted end of the rope in the hole........tie another knot on the un-knotted end......And you have a nice and cheap pull for the hatch.....works great and the only thing on top of the deck is a small knot that you can grab to open the hatch when needed. The rope pc. ends up being only about 3 to 4 inches long.

Sometimes I over think things I guess.... I thought you were trying to figue out what was the strongest species and which way to lay it down. Anyways yes you should be fine with any plywood really, so long as its sealed up from the elements..... they use a mere 1/2" OSB on roofs :lol: