Despite the remarkable advances in the field of macro-synthetic fibre reinforced concrete, the successful use of plastic macro-fibres as the sole reinforcement in structural elements still requires thorough research. Several questions hindering its wide-spread use have yet to be addressed. Do plastic fibres guarantee a clearly defined structural reliability? Is there any influence of the formwork and casting method on the orientation of this fibres? Does this orientation affects the structural behavior of the slabs? To answer these questions this paper focuses on the study of the structural response of hyperstatic concrete flat suspended slabs with different width/length ratio reinforced only with structural plastic macro-fibres. First, the experimental program is described and then the structural results obtained are presented. Then, a study of the influence of flowability and wall-effects of the formwork in the orientation pattern of macro-plastic fibres is carried out in order to identify the preferential orientation of fibres caused by the geometry of slabs.