How deep should I lay water pipes? Will 18 inches be enough?

Depends on how cold it gets where you live. Must be below the frost line (the level where the ground freezes).

Arrawn:

As Harold suggested outside water pipes always get set below the frost line... inside water pipes underslab are a different story. Let us know more about your application and we'll see what we can tell you.

Zippit...

When it comes to water pipes and frost lines you don't gauge anything... you always call your local building department and ask the inspector what the frost line is in your particular area and you install your water pipes below that. This is the only way to guarantee no frozen pipes!

MARK

Listen to the experts. Call your Building Department. In Wisconsin we ran our sewer and water at least six feet below grade. Here in Florida we just put them in at two or three feet so it's important to know your frost line. Good luck, Tom