 Optionally, the entire rubber sleeve may be jacketed with thin sheet metal, to provide extra stiffness, durability, and resistance to accidental penetration by a misplaced nail or screw.Although the fittings are not cheap, they are reasonably durable (the rubber is typically neoprene or flexible PVC).An alternative design also allows the selective use of belled fittings made entirely of flexible rubber, including more-complex shapes such as wyes or tee-wyes.