Temporary rain water collection assembly for use on a roof of a building

A rain water collection system for a blue roof system that includes a tray assembly comprising a container, a water absorbing layer located in said interior region of the container, ballast members and a cover plate. The water absorbing layer is supported on a first set of support surfaces to locate the water absorbing layer above the bottom wall of the container to provide a water collecting space into which will flow excess water that exceeds the water absorbing capacity of the water absorbing layer. The sidewalls of the container have openings therethrough to facilitate the passage of over flow water from the container and the introduction of air therethrough into the interior region. The upstanding columns have openings to facilitate the passage of air therethrough into the interior region of the container.