A one-metre datum line is a horizontal line marked exactly 1000 mm above finished floor level on walls and columns, used throughout construction as the fixed reference for every height measurement. That way the screeder, the carpenter, the plasterer and the electrician all work from the same level. The Dutch term is meterpeil or meterstreep; in German it is the Meterriss.
Why not measure from the floor?
At the structural stage the floor is not finished: insulation, screed and a floor covering still have to go on top, and the slab is rarely perfectly level. Measure from that rough slab and you carry every deviation with you. The datum line, by contrast, is set out with a level instrument and is truly level across the whole storey.
Advantages:
- One reference for every trade, so fewer dimensional errors
- Usable whether the floor is still missing or already laid
- At 1 m height it is comfortable to read: no stooping, no stretching
How to set it out
- Establish the datum — the finished floor level of the ground floor — from the drawings
- Use a rotating laser or a water level to mark a line exactly 1000 mm above it
- Mark the line clearly on all walls and columns, with an arrow or triangle pointing to it
- Write “+1000” or “FFL +1 m” beside it so there is no doubt which line it is
- Set it out afresh on each storey, relative to that storey’s floor level
What it is used for
- Heights of window and door frames, sills and heads
- Marking out the gauge (course height) for brickwork
- Heights of sockets, switches and services
- Stair pitch, ceiling height and hanging doors
- Checking screed thickness: measuring down from the datum tells you exactly how much screed is still needed
Related terms
- Datum
- Levelling
- Setting out
- Survey point
- Gauge
- Ordnance datum
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