Many spirit-level vials have two sets of lines. When the bubble is centered between the inside pair, it indicates level. But when it touches one of the outer lines, it means the level is pitched at a 2-percent grade (about ¼ inch per foot of run), the slope required for waste lines, sidewalks, and rain gutters to drain properly.
Making Sure A Level Is Level
For a level to do its job, it must be well calibrated. To check a spirit level, simply set it on a flat surface and note where the bubble ends up. Then flip the level end over end. If the tool is accurate, the bubble should fall in exactly the same place. Levels with replaceable vials can be adjusted. Otherwise, throw away the bad level, or at least tape over the offending bubble.