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five-ply vs four-ply (or odd vs even plys)

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  • chodges31711
    This from apawood.org 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
    Message 1 of 1 , Apr 24, 2008
      This from apawood.org

      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.

      In conventional applications where face grain is across supports,
      all three constructions of equal Span Rating are intended to do the
      same job. This is reflected in APA recommendations and in building
      code provisions, where no distinction in ply-layer construction is
      made for a given Span Rating. It is recognized, however, that there
      may be certain very specialized applications in which a property,
      not usually important, assumes particular importance. In these
      cases, it is sometimes necessary to specify a particular ply-layer
      construction in addition to the thickness and grade.

      A typical application of 15/32 and 1/2-inch CDX is roof sheathing.
      For example, consider a conventional application where the panel
      face grain is across supports. In this case let's say that the
      minimum stiffness of 5-ply panel is 100 percent. If so, the minimum
      stiffness for 4-ply would be 108 percent and 3-ply would be 121
      percent. All building code provisions and APA application
      recommendations for roof sheathing are equal for the three
      constructions and are based on the minimum, which is 5-ply.

      On the other hand, there are some roof sheathing applications in
      which the face grain is placed parallel to supports. In this
      situation, if we again say that the minimum stiffness of the 5-ply
      15/32 and 1/2-inch is 100 percent, the minimum stiffness for the 4-
      ply is 65 percent and 3-ply is 28 percent. Unlike when the face
      grain is across supports, this difference is significant enough that
      ply-layer construction needs to be specified when face grain is
      parallel to the supports.

      There are several properties of plywood panels that are of interest
      to design professionals, depending upon the application. The
      relationship between the ply-layer construction of panels varies
      depending upon the property of interest. In addition, published
      mechanical properties are calculated based on several allowable
      panel constructions. Refer to APA Panel Design Specification, Form
      D510, for mechanical property design data.
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