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The paper describes the use of two fundamental design parameters, the void porosity and layer thickness in rock armour constructions. These design parameters are very sensible for factors such as the boundary definition of a rock layer, rock production properties, intrinsic properties and construction properties. Differences in the value of the design parameter cause a considerable financial risk. This risk contains two directions, the first is the effect on the hydraulic performance of the structure and the second is its relation to materials procurement. This paper describes and investigates the second risk for the contractor: the large margin in the calculation of the void porosity, which influences the amount of rock (in weight). Often this risk is on account of the contractor, so it is necessary to have better insight into these values in order to reduce the financial risk. Focus is on the variation of the porosity at the bottom, at the top and at the transition between two layers of graded material. This has resulted in correction coefficients for the layer thickness, as well as for the computation of the void porosity as a basis for payment by the client.