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Discussion Starter #1
The last time I did a roof, 1/2" CDX had 5 plys. Now, I can only find 3 ply or 4 ply. For the 4 ply, what direction do the inside layers run? The two outside layers go the traditional 8' lengthwise. Are the two inside layers perpendicular to the outside (and therefore parallel to each other)? Seems like this is no better than 3 ply with an extra glue layer, but it costs more.
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Mother, please! I'd rather do it myself!
 

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I can't answer your question in regards to direction. I suppose I could if I went and looked at a ply on my truck, but you can do the same in the store.

I can tell you I notice a definite difference in rigidity between a 3 ply and a 4 ply. If we are replacing plywood on a roof we almost always use the 4 ply CDX, except if we are boarding over something like spaced decking, in which case we don't need that rigidity, so we opt for the 3 ply.
 

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Thank you to GBR for the massive info resource! I had come across these pages when I was initially looking for the answer. TMI; it is easier to ask somebody than to do the research.

Thank you to Grumpy. You are right about the stiffness. I went into the Depot today to buy the decking, and tried bending the sheets. The 4P is definitely stiffer than the 3P. I asked the guy who was working the lumber aisle (a furloughed builder trying to make a few buck until the economy picks up again). He had a very credible explanation: "You see the row of knots on the outside layers? Well, the inside layers also have similar rows of knots (often fallen out and not plugged). This is a very weak point when bent in the short direction. The knots in the 3 ply inner layer go all the way through the layer. The 4 ply offsets the knots so you have at least one layer of solid grain running across the 4' dimension."
 
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