Direct numerical simulations have been carried out for a fully developed turbulent channel flow with a smooth upper wall and a lower wall consisting of square bars separated by a rectangular cavity. A wide range of $w/k$, the cavity width to roughness height ratio, was considered. For $w/k\,{\ge}\,7$, recirculation zones occur immediately upstream and downstream of each element while mean streamlines and spatial distributions of the skin frictional drag indicate that each element is virtually isolated. The maximum form drag occurs at $w/k\,{=}\,7$ and coincides with the minimum skin frictional drag. The dependence on $w/k$ of the Clauser roughness function reflects that of the form drag.