I started to put a barn door 34x84 between a bathroom and an adjacent bedroom. The bed room is 10ft tall x 11ft wide. I cut the sheet rock in the bath room, however when I began to cut the bedroom side I ran into 5/8 plywood behind the sheet rock. The entire wall has the plywood. Im not sure why. It is an internal wall.
The BC building code requires some 2 storey buildings to have "braced wall banding" and its intent is to provide stiffining within the structure against seismic sway and wind loads, it is often needed when outside walls exceed opening allowances for windows or doors. for example a second floor bedroom that has large widows and patio doors in one wall would loose its rigidity and to compensate an interior wall could be sheathed to provide the needed strength. It could be structural, city drawings submitted with the homes building permit would tell you. City or municipality will charge 30 bucks or so for a copy.
best of luck
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