In the United States, the Steiner Tunnel (UL 723 [2008, “Test for Surface Burning Characteristics of Building Materials,” Ninth Edition, Underwriters Laboratories Inc., Northbrook, IL], ASTM E84-10 [2010, “Standard Test Method for Surface Burning Characteristics of Building Materials,” Annual Book of ASTM Standards, Vol. 4.07, ASTM International, West Conshohocken, PA.]) is an important fire test apparatus used by the building codes to assess the flammability and smoke generation characteristics of building products (e.g., insulation, sheathing materials, foamed plastics, wood-based products). This paper examines how various tools and methodologies can be used to quantify, improve, and control measurement uncertainty in fire tests, such as the ASTM E84 tunnel test. This paper also assesses uncertainty in the Steiner Tunnel using both a Gage Repeatability and Reproducibility (Gage r&R) study and historical results for reference materials. The improvements discussed were achieved by using a widely known Lean/Sigma principle—the define, measure, analyze, improve, control (DMAIC) method. Benefits derived from the DMAIC method include: (i) an improved understanding and control of the sources of variation; (ii) the development of process control charts to monitor Steiner Tunnel performance; and (iii) a reduced need for calibration and re-verification of Steiner Tunnel performance. The Gage r&R results show that the major contributor to uncertainty in flame spread and smoke developed results is primarily due to the performance difference in the test samples, and not due to the measurement device or the person conducting the test. The uncertainty, in the form of the standard deviation, is also shown in the results of each of the materials tested in the study. Monthly data for the historical reference material, red oak, and a proposed reference material, heptane, are shown to demonstrate stability of the measurement system over time. The heptane test is presented to help measure and reduce uncertainty of the smoke measurement in the Steiner Tunnel.