WESTERN WOODS INC. (8140-2084:0006)
This is a listed siding assembly. Listing 8140-2084:0006. This site cites OSFM BML. Match the model and orientation.
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Western Woods' board-on-board listing specifies plywood inserts and a maximum 1/8-inch gap in a vertical assembly.
In the September 2025 handbook's description of OSFM listing 8140-2084:0006, the boards are minimum nominal 8 inches wide and nominal 1 inch thick. Between them sit plywood inserts 3/4 inch thick and 3-1/2 inches wide. The maximum gap between boards and inserts is 1/8 inch. Both fasten to the wood frame with fasteners at least 3 inches long, spaced no farther apart than 24 inches on center.
The assembly needs structural plywood or OSB backing at least 7/16 inch thick, complying with PS1 or PS2. The solid-sawn siding must have no through holes or loose knots. The named wood options are Incense Cedar, Redwood, Alaska Yellow Cedar, Western Red Cedar and Port Orford Cedar. For another species, the condition is an ASTM E84 flame-spread rating of 75 or less, Class B. Nominal board sizes must satisfy DOC PS 20, and edge- or end-glued siding may replace solid-sawn members.
The handbook's recommended wall-installation practice uses two nails at each frame-support crossing for nominal 8-inch-or-wider siding, with joints nailed into studs or blocking. It calls for butt joints staggered at least 12 inches apart and flush, without gaps. That butt-joint practice differs from the limited gap beside the plywood inserts.
Use the manufacturer's product data sheets and installation instructions alongside the industrial technical guide to check this vertical insert-and-board detail. Confirm the exact listing before specifying the assembly; a board-and-batten detail does not supply the dimensions of these plywood inserts.