A fire protection sprinkler system includes a horizontal barrier that covers two adjacent racks, and the vertical flue space between the racks. The horizontal barrier is provided at a predetermined height, and has a width that is at least equal to a width of the rack, and a depth that is at least equal to a sum of a depth of the rack, a depth of the other rack, and a depth of the vertical flue space. In addition, the horizontal barrier can be a sheet having one or more apertures. The system also includes a rack level fire protection sprinkler connected to a fluid supply conduit, and being disposed in the vertical flue space below the horizontal barrier. The rack level sprinkler has a K-factor of 11.2 gpm/(psi)1/2 or greater, and is vertically spaced from commodities stored on the rack and the other rack covered by the horizontal barrier.