Why a plumber uses a laser tape for underground rough-in

Plumbing
Rough-In
Site Prep

Under-slab rough-in is set out from the finished wall line before the pour, and a closet flange landing an inch off is a jackhammer and a re-inspection. Dropping a plumb bob into a trench needs a spare hand, still air and something to hang from. The T1 Tomahawk's green laser puts the point directly in the ditch from where you stand, so the stub-up lands on the mark first time and the dimension is saved for the as-built.

What replaces the plumb bob

Green laser drops the point into the trench
No plumb bob, no string, no spare hand
Sets closet flange and stub-up locations
Dimension saved for the as-built record
Works with backfill and dirt around you

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Underground rough-in means transferring a point on the ground to a point in the trench, which normally means a plumb bob and a free hand you do not have. Shooting the laser down to the opening gives the same transfer with one hand and no string. In this plumber's words, that laser was the reason he bought the tape.