A research effort is underway by the Federal Highway Administration’s – Turner-Fairbank Highway Research Center and the Forest Service’s Forest Products Laboratory in Madison, Wisc., as part of the National Historic Covered Bridge Preservation program to assess the efficacy of using ground penetrating radar to detect internal moisture and fungal decay within Douglas-fir beams. The objective of the study is to develop methodologies and tools for fast evaluation of the structural condition of timber bridges. Timber beams of the sizes typically used in timber bridge construction were cut into test specimens and assessed using a variety of physical, mechanical, and nondestructive evaluation test methods including ground penetrating radar. After initial baseline assessment, test beams were inoculated with brown rot fungus(Fomitopsis pinicola) and exposed to above-ground conditions in southern Mississippi. Test beam specimens were inspected in the field at 6, 12, and 17 months of exposure using ground penetrating radar and several nondestructive test methods. A subset of the specimens were removed from the field during each site visit and returned to the Forest Products Laboratory. Computer tomography (CT) scans were taken of the retrieved specimens. Finally, destructive Janka hardness testing provided insight into mechanical property changes at critical interior locations during exposure. The ground penetrating radar scans were compared to the nondestructive testing, CT scans, and hardness results. It was found that ground penetrating radar is sensitive to the presence of internal moisture, which is useful in locating areas of potential decay.