Why do you stagger plywood on roof?

This will help keep everything square and aligned while the sheathing panels are laid down and fastened. It is common for framing supports, such as roof trusses, to be spaced 24 inches on center. Blocking can help correct areas with twisted framing, as well as provide surface for the panels.

nachi.org - Inspecting Roof Panel Sheathing - InterNACHI®

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by Nick Gromicko, CMI® and Ben Gromicko

This article covers some tips and details for home inspectors to keep in mind when inspecting the lumber roof sheathing and the general structure of the roof. Be sure to check with your local building code requirements and manufacturer recommendations related to roof sheathing. Lumber roof sheathing and lumber roof panels are terms used in the International Residential Code that refer to panels manufactured with fully waterproof adhesive. They include plywood, oriented stand board (OSB), and composite panels made up of a combination of wood veneers and reconstructed wood layers. Plywood panels are manufactured by gluing together three or more cross-laminated wood layers. OSB panels are made from multiple layers of wood flakes oriented 90 degrees to each other. Sheathing provides the base for attaching the roof covering and adds strength to the framed structure. Thickness The minimum thickness of lumber roof sheathing should conform to Table 1 below. Table 1. Minimum Lumber Roof Sheathing Thickness Minimum Net Thickness

Rafter or Beam Spacing

5/8-inch 24 inches 1-1/2 inch tongue-and-groove 48 inches 60 inches 72 inches You may be able to measure the thickness of the roof decking material with a simple ruler, as seen in the photo below taken during an actual inspection. Allowable Spans for Wood Structural Panels for Roof Sheathing The maximum span for roof sheathing is limited by the stresses and deflections caused by the design loads. The inspection agencies include a span rating label on the panels, which indicates the maximum spans for that panel. The labels above are common APA rating labels for roof sheathing. The rating appears as two numbers. The first number is the maximum span for roof sheathing, assuming that the panels are installed with the long dimension across three or more supports, with the edges supported. The second number is the maximum span when the panel is used for floor sheathing. The label on the far left is a common rating label for sheathing installed on roof trusses spaced 24 inches on center, which is widely recognized as an economical construction method for residential roofs. Rafters Rafters should be straight and in alignment during the installation of sheathing. This will help keep everything square and aligned while the sheathing panels are laid down and fastened. It is common for framing supports, such as roof trusses, to be spaced 24 inches on center.