Vent stacks extend from the inside to the outside, exposing them to significant temperature differences, especially when extremely hot or cold water flows through the pipe. Under these conditions, vents tend to expand and contract. Because of this movement, a flexible gasket is the best way to seal a vent.

To makes a gasket, cut small sheets of EPDM (inner tubes from old tires work well, too). Make a circular hole that's smaller than the vent. Because the gasket needs to slipped over the vent as it is installed, you’ll have to provide them for the plumber. Bring them to the site in advance and staple them to the top plate near each vent location. The plumber slips the gasket over the pipe so that it's above the top plate. After the plumber is finished, run a generous bead of sealant around the hole on top of the plate. Gently, press the gasket into the sealant. Staple around the edge to hold the gasket in contact with the sealant.