A structural module which is suitable for combining with like structural modules to form an area suitable for walking on and/or for travelling directly over by a vehicle, comprises a flat top wall and a bottom wall spaced therefrom by side walls so as to define a volume between the top and bottom walls. The top wall is provided with a plurality of apertures to permit the flow of liquid into the volume, and the side walls and / or the bottom wall are provided with apertures to permit the flow of liquid out of the volume. The size and shape of each aperture in the top wall is such that the maximum diameter of sphere that the aperture would let through is in a range up to 10 mm.