This research presents the development and application of a constructability design model, which determines the print speed and filament layer height combination that yields the fastest vertical building rate, whilst ensuring for the successful construction of an object. A 3D concrete printed structural wall element is used to validate the model. High variation in material rheological properties lead to an over prediction by the model if mean model parameter values are used. Consequently, a probabilistic design model is developed to reduce the impact of high variation in material properties on the accuracy of the deterministic design model. The first-order reliability method (FORM) is applied and material partial factors derived.