In this experimental study the Manning’s n roughness coefficient is investigated in uniform-turbulent-steady water flows, within sloped (Jo) rectangular open channels of composite roughness. The channel’s bed always remains smooth, but both walls’ roughness is varying, through adhesive rubber vertical strips (κxκ, where, κ=4mm) at various distances, λ. Thus, a number of ratios λ=12.5-25-50-100 and ∞ (entirely smooth channel, κ=0) are examined in an effort to fully understand the flow’s hydraulic behavior in such open channels of composite roughness. A large number of experimental measurements were performed in the hydraulics laboratory and the results appear to be systematic. The present study is aiming at helping the hydraulic engineer in his practical water flow problems.