Start with clean contact surfaces
Wipe grit from the level's reference edge and the surface underneath. A single crumb can tilt a short level enough to change what the bubble shows. Look for dents on the working edge, loose vials or a twisted body. Set the instrument on a firm surface rather than a padded mat. Check the manufacturer's instructions if you are unsure which edge is intended to rest against the work.
Use the reversal check
Lay the level on one position and note the bubble's relationship to the lines. Mark the two ends lightly on removable tape. Rotate the level end for end and put it back between the same marks, using the same contact edge. On a consistent surface, a sound instrument should give a corresponding reading. A noticeably different result is a reason to investigate the level rather than adjust the furniture to suit it.
Check the work in more than one place
A level reports the section beneath it, not the entire room. On a long shelf, take readings at several positions and distinguish a sloping shelf from a bowed board. For a freestanding unit, check side to side and front to back without loading or leaning on it. Do not assume that a floor must be altered simply because the bubble is off centre. Record the observation first, then use the furniture maker's intended levelling method if adjustment is needed.
Before you call it done
- Remove grit from both contact surfaces.
- Reverse the level in the same position.
- Check lengthwise and across the item.
Know the limit. A spirit level is not a structural assessment. Stop if furniture is unstable or a floor has changed shape unexpectedly, and have the underlying issue assessed.
