How doI collect money paid to a roofer who never did the job he was paid to do?

Assuming he does not pay voluntarily, you would do the following--note that this description is basd on NJ practice, but other states will have similar processes and mechanisms:

1) Serve an "information subpoena" on him, to find out about assets, income, etc.

2) If he doesn't fill out the information subpoena, you may ask the courts to have him arrested, as a way to make him fill it out--this is a slow process, though, so be patient; it can take months and months.

3) Once you have the information as to his bank account(s), property, equipment he owns, etc.--whether you get it from the information subpoena or otherwise--you can look to possibly:

a) Levy on bank accounts, or take money out of them;

b) Execute on non-real estate property, or have it seized and sold;

c) Put a lien on real property;

d) Garnish income.

A court officer (i.e. a constable or sheriff) will do these things for you.

Other ways to get information include looking at real estate title and tax filings; looking at the back of any cancelled checks, to see what bank he used.

If the roofer was a corporation or limited liability company (LLC), you can probably only sue the company; if it was a sole proprietorship, you can sue the owner personally as well.