Tank cover system with substantial gas seal

A substantially gas tight cover in combination with an open-top structure comprising a tank or channel having a cover-supporting rim or edge including a pair of generally parallel rims at opposite sides of the tank or channel, comprising:

• a gutter extrusion at each side of the tank or channel, engaging each of the parallel rims, each gutter extrusion being substantially continuous throughout the length of the parallel rim and including a gutter for catching water, a rim top engagement flange for engaging a horizontal surface of the rim, a rim wall engaging flange positioned to engage against an inside wall of the rim, and a beam connection extending over the tank for securement to an end of a structural beam,
• a series of spaced apart, generally parallel structural beams, each extending generally from one of said parallel rims to the other and having ends in engagement with the gutter extrusions and supported by the gutter extrusions,
• a series of decking panels extending between structural spanning beams and resting on the structural spanning beams, the decking panels having flanges engaging the beams in substantially gas sealed relationship, with an elastomeric gasket positioned between the panel and the beam,
• a flashing member extending substantially continuously along the length of each gutter extrusion, and sealingly connected to the gutter extrusion and extending upwardly to close spaces between ends of the spanning beam, the flashing being in substantially sealed relationship with the decking panels,
• the gutter extrusion having a sealing gasket creating a substantial gas seal between the gutter extrusion and the tank rim, and
• a series of spaced clips retaining the gutter extrusion to the tank rim, each clip being secured to the rim top engaging flange of the gutter extrusion and extending generally horizontally outwardly, away from the tank and being secured to the tank rim near an outer end of the clip.