Abstract: A cavity former (10) for forming one or more cavities in a concrete slab, includes a hollow body about which, during use of the former, poured concrete flows and subsequently cures to form a concrete slab in which the hollow body defines a cavity. A plurality of flanges (20, 21) projects from at least two sides of the body to define floors for respective channels adjacent the sides of the body. A plurality of supports (50, 51, 52 and 53) is integrated with and outstanding from the hollow body and/or flanges to support reinforcement mesh above the body and elongated slab reinforcement in the channels. Respective co-operable formations are disposed on the flanges and respective further sides of the body whereby respective pairs of the cavity formers may be located and relatively positioned together by the formations with one cavity former resting on a flange of the other, whereby to define the channels, which channels receive the poured concrete.