Trapezoidal riblets are experimentally investigated in both their drag-reducing and drag-increasing regimes in an air turbulent channel flow facility. The nondimensional riblet spacing s +
and height h +
in viscous units are varied by changing the bulk Reynolds number in the range 5×10 3 <Re b <8.5×10 4
. The change in skin-friction drag is accurately measured by comparing the pressure drop across the test section with smooth and ribbed walls at given values of Re b
. The results show that riblets in the drag-increasing regime behave like typical “k”-type roughness in the fully rough regime only up to h + ∼30
, at odds with present understanding. Further increase in h +
yields a constant relative change of wall shear stress and thus a clear deviation from the fully rough behaviour. The present results are accessible online at the following digital object identifier: 10.5445/IR/1000089978.