There are other skills plumbers need aside from being able to work with water. One of the most common things that a reliable on is fixing water lines.
The wax ring is used to seal the drain outlet on the bottom of the toilet to the drain opening in the floor. When a toilet is installed, the ring is placed onto the drain... the toilet is dropped onto the ring... and the toilet bolts (also known as closet bolts) are tightened enough to lock the toilet in place. If that wax gasket ring starts to fail or isn't seated properly? Sewer gasses can sneak out from under your toilet causing your bathroom to have that suspicious smell. It can also cause water line repair to escape, sometimes invisibly, with each flush potentially damaging the floor hidden under the toilet.
Plumbers are at a high risk for health problems. This is because they are regularly exposed to many different illnesses and infections. They can contract psittacosis and histoplasmosis from the regular contact they have with animal droppings. Plumbers also see regular exposure to harmful chemicals and elements like lead and the other chemicals that can be found in the plumbing materials they work with on a regular basis.
Blisters in asphalt or felt roofs only need to be treated if they contain water or have caused the roof covering to leak. Press the blister to check if moisture is expelled. If so, it must be sealed with a patch.
When the drain line stops up it is just a matter of calling an air conditioning contractor to come out and clear the drain line. While he is there he will add an algae preventative to the system to prevent future occurrences. Proper air conditioner maintenance is the key to preventing water line repair leaks.
And the way to know what's sufficient? Well, until the work is done an the bill paid you won't know for certain, but this is the best that can be suggested and normally works very well.
A large plumbing job, such as re-piping or installing additional fixtures, may require a permit. Check with your city's department of building and safety before asking for estimates, and avoid any plumbers who are reluctant to comply with regulations.