Experimental results of vertical oscillations of dust grains in the driven electrode sheath of a capacitive RF plasma are presented. Below a certain pressure, waves suddenly start to propagate in the lower planes of a plasma crystal. Further decreasing the pressure causes the waves to spread in the entire dust cloud. The waves are found to propagate at constant frequency but with a higher phase velocity as the pressure is reduced. A mechanism based on the ion–dust two-stream instability is proposed to explain the observations. From the numerical solution the wave frequency is found to be highly dependent on the value chosen for the dust charge and so gives a diagnostic method for determining the charge on the dust.