Attenuation by a floor barrier of fallout gamma radiation scattered into a basement has been studied experimentally with cylindrical steel structures. These structures were 2-ft high, 2-ft in diameter, with a 4-ft-deep basement. Wall thicknesses varied between 5 and 60 psf, with floor thicknesses of 0, 10, 20, and 40 psf. Detectors in the basement were located between 0.25 and 3 ft below ground. Cobalt-60 point sources were used to simulate the fallout field. Basement reduction factors predicted by structure shielding theory were lower than the experimental results by a factor of between 1.5 and 8. This discrepancy was attributed to the theoretical floor-barrier reduction factor. A new theoretical floor-barrier reduction factor, which is a function of both the floor thickness and the solid-angle fraction subtended at the detector by the floor, was constructed within the formalism of the existing structure shielding theory. This new function agreed quite well with both experimental results and Monte Carlo calculations, over the entire range of wall and floor thicknesses used in the experiment.