Corrugated, galvanized steel roofing panels make nearly seamless shower walls. They’re held in place with trim pieces so there are no nails. Before the panels were installed, the walls were completely waterproofed with cement tile backer board and an acrylic sealer. The panels are cheap — $12 to $15 for a 2-foot-by-8-foot panel — and recyclable, making them a green choice.