  We study the thermally activated oscillations, or capillary waves, of a
neutral metal cluster within the liquid drop model. These deformations
correspond to a surface roughness which we characterize by a single parameter
$\Delta$. We derive a simple analytic approximate expression determining
$\Delta$ as a function of temperature and cluster size. We then estimate the
induced effects on shell structure by means of a periodic orbit analysis and
compare with recent data for shell energy of sodium clusters in the size range
$50 < N < 250$. A small surface roughness $\Delta\simeq 0.6$ \AA~ is seen to
give a reasonable account of the decrease of amplitude of the shell structure
observed in experiment. Moreover -- contrary to usual Jahn-Teller type of
deformations -- roughness correctly reproduces the shape of the shell energy in
the domain of sizes considered in experiment.
