The supply and waste lines which pierce the slab are put in place before the slab is poured - in fact, before it is even set up to be poured. The most important thing to check here is the location of all pipes which the plumber has installed. You can do this by measuring from the foundation walls (if these are used) or the forming for the edge of the slab.
Precise positioning is important so that pipes to be located in walls actually end up in the walls, and toilets are properly located.
If a pipe ends up in the wrong place, after the slab has been poured, it may have to be jack-hammered out and relocated - a time-consuming and expensive process.
Copper water supply pipes will be under pressure once the water is turned on. To prevent the chance of leaks in inaccessible places, there should be no soldered joints in them either in or under the slab.
All pipes passing through the slab should be protected with a rubber sleeve to minimize abrasion and possible damage from natural vibrations and movement.
Floor drains may also required for the washer and the water heater. Check to make sure these are in place where they should be.
The plumbing inspector will check the installation before the slab is poured to make sure it meets the plumbing code.

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