R602.8.1 Materials. ¹  Except as provided in Section R602.8, Item 4, fireblocking shall consist of 2-inch nominal lumber, or two thicknesses of 1-inch nominal lumber with broken lap joints, or one thickness of 23/32-inch wood structural panels with joints backed by 23/32-inch wood structural panels or one thickness of ¾-inch particleboard with joints backed by ¾-inch particleboard, or ½-inch gypsum board or ¼-inch cement-based millboard. Minimum 16-inch vertical batts or blankets of mineral wool installed in such a manner as to be securely retained in place shall also be permitted as an acceptable fire block. Batts or blankets of mineral wool shall be permitted for compliance with the 10-foot horizontal fireblocking in walls constructed using parallel rows of studs or staggered studs.