It is common to assume the wave field to be diffuse in often used simplified calculation procedures in building acoustics, such as Statistical Energy Analysis (SEA) or energy flow methods. It is a well known that such diffuseness assumption is not always valid. This paper presents one such example. The structures under consideration are rib-reinforced plate structures. These may be a lightweight floor/joist structure in a building or hull of a ship. The diffuseness, or rather the lack of diffuseness, in such 'ribbed-plate' structures is examined. The amount of power transported at an angle is calculated by means of the structural intensity in that direction.