vent-plumbing
along with
sub-rough plumbing and
rough plumbing
are done at the same time
as HVAC.
Electrical wiring follows.

vent-plumbing is a subject unto itself.

Every building you see has little pipes sticking out of the roof all over it. They may represent chimneys, or toilets and sinks, or any plumbing that requires a vent for proper drainage.

Venting pipes have to be installed before the shingles, insulation and drywall because they have to go through the roof.

Vents can be cut through the shingles, but that would be a pain in the 'you know what' for you, and probably more pain for the sub who has to install the vents. Consequently, if the vents are installed after the shingles, it would probably cost you much more.

In order for your drain-waste-vent (sewer line) to flow freely, it is necessary for all your sinks, showers and tubs to have vent lines to exhaust sewer gasses and provide necessary air pressure for good sewage drainage.

All your drain lines collect water from any water source; sinks, showers, etc., and they slope gradually downward allowing gravity to facilitate their drainage.

All the vent pipes are from 1 ¼ inches to 4 inches in diameter to avoid possible blockages. (toilets use a 4-inch pipe, sinks, lavatories, bathtubs, and laundry tubs usually have 1/4-inch to 2-inch pipes, and showers use a 2-inch pipe drain.)

Each drain must be served by a vent line that carries sewer gasses out through the roof in order to operate properly.

Sometimes plumbers have to clean-out drains and so all drains/vents need clean-outs in locations that are easy to access. A clean-out is simply a capped off Y-shaped fitting in the line. A plumber can use these clean-outs to snake out the line if and when it becomes necessary.

In 16 years “in a previous life”, we never needed our clean-out, and then one day “it” happened, and that clean out was necessary. I won't tell you about it, other than we were glad for the clean-out; not happy to have to clean it out, but....

TRAPS