This paper describes an experimental study of the effect of atrium roof shape and atrium roof obstructions upon the daylight levels in atrium buildings. The work was carried out using physical models of atrium buildings under real sky conditions, and was divided into three stages. The first part of the study investigated the effect of the atrium roof on available daylight levels in the atrium. The second part examined how obstructions created by structural elements beneath the atrium roof influenced the roofs transmission characteristics. Finally, a comparison was made between the observed reductions in transmission and those predicted by applying the plan area obstruction created by the structural roof elements. It was concluded that for overcast skies the influence of atrium roof shape on transmission losses was small if the plan area roof obstructions were similar. Applying the plan area obstruction as a correction factor to daylight levels in the atrium usually underestimated the loss of light due to the obstructions. For overcast skies this underestimation was typically 10-15%. It was observed that as skies brightened the effects of roof shape and obstruction loss on atrium daylight levels became larger as the inclined glazed surfaces and structural elements either reflected away bright skylight and sunlight or reflected it into the atrium.