Rough-In Plumbing Dimensions for the Bathroom Often the hardest part about plumbing is the rough-in. The last step of connecting the sink, toilet, or tub is almost superfluous. Get the rough-in right, and you are 90 percent of the way there. But it doesn't have to seem so daunting. There are generally accepted plumbing rough-in measurements for sinks, toilets, and tub/showers. Rough-In Plumbing Diagram - Ask the Builder Jan 15, 2017AsktheBuilder: A rough-in plumbing diagram is a sketch for all the plumbing pipes, pipe fittings, drains and vent piping. This plumbing diagram might be required for a building permit. This isometric diagram will help determine if all your plumbing meets code.