When you think of the need for a plumber, most people tend to think of an immediate repair such as a clogged drain which won't clear with typical means available to the homeowner, dripping faucet, or burst pipe. Although such jobs are common for plumbers to conduct, you should be aware plumbers do more than such basic tasks. There's a reason plumbers have to be licensed. They have a practical knowledge of how to approach repairs and they also understand how and why plumbing systems work both as a design and in physical reality.
Systems Replacements for Environmental Considerations
Especially in an older house, as plumbing repairs become more common it may be time to consider replacing the system with a modernized style. Modern plumbing systems do more than just replace an older system with a new one. Modern systems incorporate better design styles for increased efficiency. Today's plumbing systems are designed for fixtures which use less water at higher pressures to provide the same results you've come to expect while lowering your water bill. Such systems also address the environmental concerns of lowering water tables and drought conditions faced by much of the country.
Renovations and Upgrades
A full system replacement may not always be warranted, but renovations and remodels are a perfect time to conduct a partial system replacement, and when upgrading a kitchen or bath you likely need a licensed plumber to disconnect and reconnect the pipes anyway. A great plumber can do more than just the plumbing work but is actually capable of conducting the entire job rather than your need to hire a general contractor who is going to sub contract a plumber to do just a portion of the work involved.
Maintenance and preemptory repairs come up as a time when you also need a plumber. You should never have to jiggle the handle of a faucet or toilet to keep from maintaining a constant drip. Although a plumber can usually make such repairs, if the faucet is older or you've always wished for a different style it might be a good idea to go ahead and replace it. A professional plumber will be able to advise as to the best solution and work toward your needs of budget and convenience toward assuring your appliances are in the best working order they can possibly be so it's not something you have to worry about.