Flat Plate Concealed from Viking Corp. Created: December 2, 2010 Viking Corp. introduces a new 4.0 (58) K-factor residential, flat plate concealed, horizontal sidewall sprinkler. It installs into a standard 2 in. by 4 in. stud wall. Want to know more about this product? Click here to request more information about Flat Plate Concealed from Viking Corp. .