How do I use a plumb bob, and how do I check a wall is plumb?

A plumb bob is a weight on a string, and it is more accurate than any level you own because it is referenced directly to gravity rather than to a bubble in a tube somebody glued in. It costs almost nothing, it never needs checking, it works over any height, and it is the right tool for transferring a point vertically through a building.

What you'll need

• Plumb bob and line, or a chalk line reel used as one
• Two identical spacer blocks of known thickness (optional)
• A cup of water, or a bucket of sand (optional)
• Long spirit level (optional)

Step by step

1. Hang the line from the point you are transferring.Pin, tape or hold the line so its attachment is exactly over the mark. The line, not the bob, is where the accuracy lives — an offset at the top is transferred straight to the bottom.
2. Set the bob just clear of the floor.A few millimetres of clearance. Touching the floor stops it hanging free; too high and small movements of the line are magnified into an ambiguous position.
3. Damp the swing rather than waiting it out.Let the tip dip into a cup of water, or into a bucket of sand. It stops in seconds instead of a minute. Steadying it with a hand always leaves it slightly displaced.
4. Mark the floor from two directions.Sight down at the point from one side and mark, then from ninety degrees round and mark again. Where the two sightings cross is the point. Sighting from one direction alone hides an error in the other axis.
5. Work out of the draught.An open door, a fan heater or a passing person moves a hanging bob visibly. Close the door and let it settle before marking; on a windy site outdoors a bob is not usable at all and a laser plumb dot is.
6. To check a wall, hang the line clear of the surface.Put a spacer block — say 20mm — between the line and the wall at the top, so the line hangs free of any bulges. Then measure from the line to the wall at the bottom, using the same block for comparison.
7. Read the out-of-plumb as a difference.If the top gap is 20mm and the bottom gap is 32mm, the wall leans out by 12mm over that height. That number, not the fact that it leans, is what tells you whether a cabinet needs packing and by how much.
8. Measure at several points across the wall.A wall can be plumb at one end and 20mm out at the other, or bellied in the middle. Take readings at both ends and in the centre, and write them on the wall. Fitted work is then planned around a shape rather than a surprise.

Tips

• Most chalk line reels are designed to double as a plumb bob — the case is weighted and there is a hole in the top to hang it from. That is why the case is shaped the way it is.
• A long spirit level is quicker than a bob for checking a door lining, and a bob is better for anything over about two metres because the level's error compounds with each reposition.
• For a stud wall, a plumb bob or a laser plumb dot transfers the wall line from floor to ceiling exactly. Marking the ceiling by measuring from a wall assumes the wall is plumb, which it is not.
• Do not use a plumb bob to check a wall you are about to plaster or tile without also checking whether the finished face is meant to follow the wall or be plumb. Sometimes the right answer is to follow the building.

Common mistakes

• Holding the line by hand — A hand moves and a held line is never quite where the mark is. Pin or tape the top and let go.
• Resting the line against the wall — The line then follows any bulge in the wall rather than gravity, which is precisely the thing you were trying to measure. Always hang it clear on a spacer.
• Marking while it is still swinging — The average of a swing looks like the centre and often is not, particularly if there is a draught pushing it one way. Damp it and wait.
• Assuming plumb is always what you want — In an old house, forcing a new element to be plumb against a wall that leans by 40mm creates a wedge-shaped gap that has to go somewhere. Sometimes the right decision is to split the difference.

Questions people ask