You can add OSB or plywood to create storage on some garages with built roofs with rafters, joists and stringers. You should not place loads on engineered trusses. The load carrying capacity is limited by the size and span of the joists that you will attach the OSB onto. You can increase the load capacity by constructing additional joists to transfer the loads to the load-supporting walls. If you are uncertain how the load from OSB and dead storage loads will be carried, consult with a structural engineer or contractor.
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