As energy code requirements for the thermal performance of buildings increase over time, the requirements of roofing systems are becoming more stringent. One of the requirements focuses on the minimum amount of insulation within a roofing system when it is installed in a continuous manner, entirely above the deck. However, the energy code does not clearly address the reduction of the roof system's thermal performance due to penetrations that create thermal bridges through the system. These penetrations can come in the form of fasteners used in a mechanically attached roof system or from much larger penetrations used to support rooftop equipment and accessories. It is common for roofs to contain several different types and combinations of penetrations such as roof drains, vent pipes, mechanical ducts, duct supports, mechanical equipment, roof screens, parapet bracing, ships ladders, and photovoltaic panels. Two- and three-dimensional thermal modeling will be utilized to study and quantify the impact of the thermal bridging of typical penetrations through a roof system. Results from previous studies related to the thermal impact of mechanically fastened roofs will be reviewed to gain additional insight related to this issue. A low-slope roof with a single-ply roof cover and industry standard roof details will be utilized as the basis for this evaluation. This paper will compare a variety of penetrations to a roof without any penetrations to evaluate the impact on the overall thermal performance of a roofing system. In addition, various thicknesses of insulation will be evaluated in this study.