How to Check if a Shower Opening Is Plumb

Check the levelness of the top of the rough cut opening. If this part is not level, the sides of the opening will not be plumb. Hold a spirit level across the bottom of the drywall or wall studs and check the bubble floating in the liquid in the center of the level. The opening is only level if the bubble is in the center of the marked lines.

Hold the level against both sides of the opening to check for plumb. Use a spirit level that features a fluid chamber that is horizontal when the level is held vertically. Move the level down the entire length of each side to ensure the entire edge is plumb.

Hang a plumb bob from the drywall or stud at the top of the opening. If the sides of the opening do not line up with the line of the plumb bob, the opening is not plumb. "Framing Basics" by Rick Peters says that the point of the plumb bob must point directly to the point where the base of the wall opening should be. Cut the opening to meet the line created by the line of the plumb bob if needed.