4-ply Vs. 5-ply plywood 7

[KMDPE]

Hi,
We had a city inspector stop the contractor from sheathing a roof because the plywood was 4 plies instead of 5. He says that APA requires 19/32" plywood to be a minimum of 5 plies . I have never heard of this requirement before, and everyone I talked to who works in the state of FL has never heard of this requirement. Is his point ligitmate and if so, how is 5-ply better than 4-ply if the overall thickness is the same? Wouldn't more joints (between the plies) mean weaker plywood? Maybe I'm missing something, so I appreciate any input on this.

 

[RWF7437]

http://www.encyclo.co.uk/define/plywood

 

[Ron]

Sheathing is not required to be 4 or 5 ply....it is required to be span rated. As long as it is span rated under the APA guidelines for the spacing of the supports that you have, it's fine.

 

[RHTPE]

KMDPE: What is the specific grade of the plywood? Is it domestic or an import? Does it carry an APA stamp?

Table 4 in APA PS1-95 requires 5 plies for an exterior, Structural I plywood over 1/2" to 7/8". For an Exterior C-C plugged, it would be 4 plies.

My experience with plywood & plyform has demonstrated the exact opposite of your theory that fewer plies would be stronger. The use of exterior grade heat-setting glues produces a glue line much stronger that the substrate. In fact, many of the strongest plyform panels have 7 to 11 plies.

The 2003 IBC (CT 2005), 2303.1.4 requires each panel to be identified for grade and glue type. Also, Table 2304.7(5), under 19/32" thick 4-ply panels with 24" span requires an allowable load reduction of 15 psf.

The typical plywood span ratings are empirically established by testing.

Don't know if this will help you.

Ralph

 

[KMDPE]

Ralph,
The plywood is domestic, Structural I, exposure "1" and carries an APA stamp.

Your explanation of regarding the strength of the plywood makes perfect sense, it's the same idea that some epoxy adhesives are stronger than the concrete they're bonded to.

Thanks for the info, very helpful.

 

[BAretired]

I don't claim to be an authority on the subject of plywood, but I would think an odd number of plies would mean that the grain in the top and bottom layers would be parallel. With an even number of plies, they would be at right angles.

Wouldn't plywood with top and bottom grains parallel to the span be stronger?




BA

 

[kslee1000]

BA:

Same here no clue about plywood, but assume your stated layering method is correct, don't you agree the stituations should be reversed - even layers, grins in parallel; odd layers, grins in right angle. So the end results alway turn out to be the top & bottom most grins in same direction.

Both of us could be right or wrong. Just guess for fun.

 

[BAretired]

kslee,

Numbering the plies from the top down:

1 N-S
2 E-W
3 N-S
4 E-W

The fourth ply is perpendicular to the first. If a fifth ply is added, the orientation will be N-S, same as the top.


BA

 

[kslee1000]

BA:

I see what you meant now. I don't think they lay the layers this way though. Look on the plywood, both top & bottom grins are always (?) in same direcn. So top can be the bottom, and the reverse is the same.

 

[RWF7437]

http://www.encyclo.co.uk/define/plywood

 

[BAretired]

RWF7437's reference confirms that the number of plies is usually odd:

4. Plywood
A piece of wood made of three or more layers of veneer joined with glue, and usually laid with the grain of adjoining plies at right angles. Almost always an odd number of plies are used to provide balanced construction.

7. Plywood
`Plywood` is a type of engineered wood made from thin sheets of wood veneer, called plies or veneers. The layers are glued together, each with its grain at right angles to adjacent layers for greater strength. There are usually an odd number of plies, so that the grain on the outside plies runs in the same direction.

However, it appears that both 4 ply and 5 ply are in common usage according to this site:

http://www.abag.ca.gov/bayarea/eqmaps/fixit/ch3/tsld051.htm

BA

 

[Ron]

Plywood can be a little confusing, because of the way the manufacturers define the layers/plies. A layer is not the same as a ply. Plywood will always have an odd number of layers, but one layer might consist of two plies running in the same direction. So what we see as a "4-ply" plywood, is actually 3 layers, with the middle layer consisting of two plies running in the same direction. The top and bottom layers will also run in the same direction (parallel to the length of the sheet), but since they are separated from each other, they are 1 ply layers, each.

So using the convention above, the 4-ply plywood would be:

NS
EW
EW
NS

The 5-ply sheet would be:

NS
EW
NS
EW
NS

5 plies, and 5 layers

 

[kslee1000]

"Question: Plans call for five- ply plywood roof sheathing but the lumber yard sent out four-ply. They say they won’t have five-ply in stock until next month. What is the difference between the panels?



Answer: In general, building code provisions and APA’s use and application recommendations for floor, wall and roof sheathing do not make a distinction for panel construction. With the exception of roof sheathing applied parallel to framing building code and APA load span tables, treat 3-, 4-, and 5-ply plywood panels equally.

For example, consider a 15/32 or 1/2-inch C-D Exposure 1 (CDX) plywood meeting the requirements of U.S. Product Standard PS 1-07: Structural Plywood. For 15/32 and 1/2-inch the Standard provides for 3-, 4-, and 5-ply panel construction.

A 4-ply panel is actually a special case of 3-ply construction. The 4-ply panel is composed of two face veneers or layers, and a cross band of two parallel-laminated veneers, which create their own layer. Therefore, 3-ply and 4-ply panels are both 3-layer constructions."

 

[kslee1000]

The above excerpt was copy from APA help desk posting. Link is attached below.

http://www.apawood.org/pablog/index...difference-between-fourply-and-fiveply-panels

 

[kslee1000]

Ron:

Indeed, you are ahead of us.

 

[BAretired]

Indeed.

BA

 

[kslee1000]

KMDPE my want to pay specific attention to the first two paragraphes of the "Answer" on the excerpt above, as they are directly related to his openning question.