How do I transfer a level line right round a room?

A 600mm level has a small angular error. Repeated twenty times round a room, that error compounds, which is why a line that looked perfect at each step arrives back thirty millimetres above where it began.

What you'll need

• self-levelling laser level (optional)
• clear tube water level (optional)
• long straightedge and spirit level
• chalk line
• pencil

Step by step

1. Establish one datum point.Pick a single starting height — a fixed height above the highest point of the floor, or a point set from an existing feature. Everything else references that point, not the floor.
2. Measure from the floor at several places first.Floors are rarely level. Knowing the floor drops 20mm across the room tells you where your datum has to sit for the line to clear everything.
3. Use a self-levelling laser if you have one.It projects a genuinely level line round the whole room simultaneously, with no accumulation of error. It is the reason lasers are worth having for tiling and units.
4. Or use a water level.A clear tube of water finds the same height at both ends regardless of distance, obstructions or corners. It is old, cheap and accurate over long distances.
5. Or use the longest straightedge you can.A good level taped to a straight 2.4 metre batten reduces the number of repositions dramatically, and fewer steps means less accumulated error.
6. Reverse the level to check it.Mark a line, turn the level end for end on the same marks, and check it still reads level. If the bubble moves, the error is the level's, and it is half of what you see.
7. Snap a chalk line between marks.Mark the ends only, then snap a chalk line between them. A line drawn along a straightedge in stages accumulates the same errors as walking a level.
8. Close the loop.Continue right round the room and back to the starting point. If the line does not come back to the datum, the error is measurable and you can split the difference.
9. Mark clearly and permanently enough.Small pencil crosses at the datum height at each corner, so the line can be reconstructed if it gets wiped off during work.

Tips

• Always close the loop back to your starting point. It is the only way to know how much error you accumulated.
• Reverse-check your spirit level before trusting it over a long run — an out-of-true level is a common and invisible problem.
• Hiring a self-levelling laser for a day costs little and transforms tiling, kitchen units and dado lines.

Common mistakes

• Walking a short spirit level round the room — Each reposition adds the level's own error and any error in placement. Twenty repositions compound into a discrepancy you can see across the room.
• Measuring up from the floor at each wall — The floor is not level. A line at a constant height above an uneven floor is an uneven line, and it will be obvious against a ceiling or a worktop.

If it doesn't work

Questions people ask