Transferring a line from ceiling to floor

Learn to mark a point on the floor that sits exactly below a point on the ceiling, using a plumb bob.

TL-01Understand what a plumb line isA plumb line is a straight vertical line between two points, one directly above the other. You need this skill when a mark on the ceiling (say, where a joist runs, or where you want a pendant light) has to line up with a mark on the floor below it, for example for a stud wall or a socket above a worktop. Gravity gives you a perfectly straight vertical line for free, if you use the right tool.

TL-02Pick a plumb bob or a long spirit levelA plumb bob is a small pointed weight on a string. When it hangs freely, the string is always perfectly vertical, that's just gravity. A spirit level is a straight metal or plastic tool with a liquid-filled bubble; when the bubble sits dead centre between two lines, the level is vertical. Use a plumb bob for drops over about 1m, and a spirit level (ideally 1m or longer) for shorter drops or where there's no room to hang a string.

TL-03Mark your starting point clearlyMake a small, sharp pencil dot on the ceiling, not a fat smudge. A vague mark gives you a vague line, and any error here gets carried straight down to the floor. If you can, mark the exact centre of a joist or fixing point, not just 'roughly there'.

TL-04Hang the plumb bob and let it settle completelyAttach the string to your ceiling mark so the weight hangs freely, well clear of any wall, pipe or furniture. Wait until it stops swinging entirely, this can take 30 seconds or more. The mistake almost everyone makes is marking the floor while the bob is still gently swaying; even a tiny wobble throws your line out by centimetres over a long drop.

TL-05Mark the floor under the still pointOnce the bob is completely motionless, get down at floor level and mark exactly where the point hovers. If it's hard to see, slide a piece of card or paper behind it so you can judge the position against a plain background. Mark a small dot, then double-check it before you lift your head away, it's easy to misjudge the position from an angle.

TL-06Confirm the line with a straight edgeHold a long spirit level or a straight batten between your two marks. The bubble should sit dead centre, not just close. This is your proof: a plumb bob that was disturbed by a draught, or a mark you misjudged, will show up here as an out-of-true bubble.

TL-07Know what 'right' looks likeYou've done it correctly when the bob hangs dead still with no visible sway, your floor mark is a crisp dot rather than a smudged guess, and a spirit level placed between the two marks reads perfectly vertical. As a final check, measure from a fixed reference, like a skirting board, to both marks; the distance should match exactly.

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