A seal arrangement is provided in a floating roof type tank for slidably sealing between the roof and the tank walls. The seal comprises an expansible tube carrying a lip with the latter extending angularly with respect to the tank walls so that upon inflation of the tube, the lip is moved outwardly and its engagement with the tank walls causes it to be resiliently bent inwardly from its normal unstressed position so that the resiliency of the lip maintains it in contact with the tank walls.