For the last two years, research at the US Environmental Protection Agency’s National Homeland Security Research Center has included laboratory scale experiments to determine washoff coefficients from urban surfaces (e.g., concrete) for spores. This work is being conducted as preparation for any emergency response to a large-scale release of a biological agent that results in the contamination of a wide area. Understanding the spread of contamination as a result of precipitation is important in order to efficiently and effectively sample and decontaminate structures. The spores used during the studies are simulants for the causative agent of anthrax. This poster will detail some of the lessons learned from designing, constructing, and testing the rainfall simulator being used for this ongoing research. The rainfall simulator is a 7.62 m (25 ft.) tall structure made of pvc pipe, plastic sheeting, a nozzle manifold, a water reservoir/pump, and a containment berm. Many different nozzles types and nozzle configurations have been tested for determining the range of droplet sizes and their spatial distribution. The poster will provide data and insights into configurations better suited for indoor washoff research. The poster will also explain the value/challenges that different measurement tools including plastic bins, heat maps, and a laser disdrometer have provided during characterization of rainfall intensity, droplet size, rain velocity, and kinetic energy. The poster will compare coefficients calculated from the washoff experiments to coefficients used in the literature when using the stormwater management model (SWMM) to model water quality in urban areas.