Pro-Form DVF8-WT 5" Inside 8" Fire Stop For Flex used with Pro-Form Direct Vent Systems. This fire stop is used for situations where a pipe must be ran vertical to the ceiling or floor to prevent combustion. The framing distances are for a 1" clearance - 11" x 11", for 2" clearance - 11" x 12", for 3" clearance - 11" x 13".