How do you find the centreline of a pipe overhead?

Plumbing
Rough-In
Layout

Hangers, sleeves and penetrations are all dimensioned from a pipe's centreline, and a round surface is exactly what a hook cannot find the middle of. Running the T1 Tomahawk's laser line across the pipe shows where the centre sits against the ceiling before the reading is taken. What goes on the layout is the centreline, not whichever side you could reach.

Why the centre is the hard part

Hangers dimension from the centreline
A hook cannot find the middle of a curve
T1 laser line crosses the pipe
Centre located against the ceiling
The layout gets the centre, not a side

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A round pipe has no edge to measure to, and the middle of it is exactly the point a hanger or a penetration is dimensioned from. Running the laser line across the pipe shows where the centre sits against the ceiling, so the reading you take is to the centreline rather than to whichever side you could reach.