I had a new shower pan installed and the drain plate looks like it is not seated flat. It appears that there is a rubber gasket right below the drain plate. I would normally call the installer back but he did such a crappy job remodeling our bathroom, I never want to see him again. Found out he was on meth. Anyhow, does this drain plate unscrew? If so, should I use a sealer besides the rubber gasket?