1-1/2", The Sealing Disc of the V-200 must be located at least 6" above the top of the trap arm. NOTE: An Angle Mounting of no more than 15 degrees from the Vertical is acceptable. Location of the V-200 is no way related to the flood level of the fixture that it's venting. The V-200 functions equally well above or below the flood level of any fixture in the home