I have CI coming up from the floor, and I'm trying to join the PVC main stack to the CI pipe. I've tried using no hub couplings, but the joint keeps leaking around the CI, and the bands keep slipping down the coupling toward the CI. I'm not sure if the CI is slightly too small for the coupling. Is there something I can use to increase the thickness of the CI, perhaps that silicone tape material? I have considered a rubber fernco, but after reading on here that appears to be against code. Any suggestions?