This article is an overview of the National Fire Protection Association (NFPA) 654 layer thickness criteria, including the 2011 Tentative Interim Amendment, and potential misinterpretations that could result in failure to recognize the hazard. Potential misinterpretations include allowing excessive floor area coverage, failure to account for layers thicker than the threshold, and failure to account for layers on equipment. The thickness criteria are applied to real-life example situations, illustrating application of the equations. Practical guidance is provided for establishing an initial housekeeping plan, monitoring accumulation, and documenting compliance for both existing and new installations. (C) 2011 American Institute of Chemical Engineers Process Saf Prog, 2012