Abstract: A roofing system is provided for large-area wide-span building structures such as natatoria, comprising an interlocking gridwork of purlins supported on beams. A portion of the structure can be provided with a track, and another portion of the structure mounted on wheels movable along the track, so that one portion of the roof may be slid open or closed by movement along the track, so as to open the interior of the building structure to the atmosphere.