This paper presents an experimental study of the fretting crack nucleation threshold, expressed in terms of loading conditions, with a cylinder/plane contact. The studied material is a damage tolerant aluminium alloy widely used in the aerospace application. Since in industrial problems, the surface quality is often variable, the impact of a unidirectional roughness is investigated via varying the roughness of the counter body in the fretting experiments. As expected, experimental results show a large effect of the contact roughness on the crack nucleation conditions. Rationalisation of the crack nucleation boundary independently of the studied roughnesses was successfully obtained by introducing the concept of effective contact area. This does show that the fretting crack nucleation of the studied material can be efficiently described by the local effective loadings inside the contact. Analytical prediction of the crack nucleation is presented with the Smith-Watson-Topper (SWT) parameter and size effect is also studied and discussed.