A consortium study, Energy Resistance of Commercial Roofs, was developed by the National Research Council Canada (NRC) with support from roofing contractor associations, roofing and accessories manufacturers, insulation manufacturers, and roofing consultants to develop scientifically supported performance data on energy resistance of roofing systems that are constructed according to field practices. This study has two major tasks. This paper presents and discusses the results from Task 1, in which 36 roofing assemblies representing seven climatic zones in North America were designed and tested to evaluate their effective thermal resistance and to validate their performance relative to the code requirements. The test matrix comprised mechanically attached roofing system, partially adhered roofing system, and adhesive applied roofing system designed with three different insulation types—polyisocyanurate, expanded polystyrene, and stone wool. The test protocol involved measurement of the thermal resistance of the insulation component and effective thermal resistance of the roofing assemblies at four different mean temperatures representing a very cold climate to solar-heated climatic conditions. All the testing was conducted at NRC's Dynamic Roofing Facility–Energy. The experimental research highlighted two influencing parameters that are currently not considered in the energy design of the roof. One is the temperature dependency performance of insulation and its impact on the effective R-value, and the second is the thermal bridging of fasteners. The measured data showed a decreasing trend of the effective R-value with increasing fastener density. The combined effect of insulation performance and thermal bridging lowered the effective R-value of some of the tested assemblies by 2% to 20% below the target design values. The current design practice needs to acknowledge the thermal bridging effects of fasteners and plates and accommodate them appropriately at the design stage of the roofs. Excluding this could lead to repercussions on the overall energy performance of the roof.