Abstract: An underfloor electrical distribution system is comprised of a bottomless trench duct transversely overlying a cellular subfloor deck assembly. The bottomless trench duct has spaced-apart, parallel L-shaped side rails, the vertical legs of which terminate in an inverted U-shaped channel and form the sidewalls of the trench. Preshaped void closure plates, which are attached to the side rails, transversely extend into the troughs of the cellular subfloor deck assembly to prevent the ingress of concrete into the trench duct while the concrete floor is being constructed. The preshaped void closure plates may be held in place within the inverted U-shaped channel by means of rivets fastened to the vertical leg of the side rail. Alternatively, horizontal slots in the inverted U-shaped channel may be cut to receive the void closure plates, which are frictionally held within the U-shaped channel.