Ground roughness effects were considered in Monte Carlo calculations of the protection factor of a cylindrical concrete structure exposed to an infinite plane 60Co source (∼1.25 MeV gamma) assumed to be representative of fallout. The “buried-source” model with source depths of 0, 0.5, and 1.0 in. was used to simulate smooth, rough, and very rough ground, respectively. The simple structure had a 10-ft radius and wall thicknesses of 20, 40, and 80 lb/ft2. The quantities calculated included the distributions of energy and of angle of the photons incident upon the structure and at detector heights of 3,13, and 23 ft inside. Ground roughness was found to enhance the protection factor of the structure 1) by reducing the incident radiation intensity and 2) by altering the energy and angle distribution of the incident radiation. The first effect was generally about twice as strong as the latter. Both effects are largest for the 3-ft detector height where, for all wall thicknesses, the protection factors were increased by a factor of ∼4.5 by very rough ground. Calculations performed with the engineering method including ground roughness effects accounted for only ∼75% of the protection factor enhancement, but a simple modification produced results that generally agreed within 10% with the Monte Carlo results. The modification consists of adding a fictitious height to the detector height before evaluating the directional response function Gd(ω,H).