my roofer is asking for most of the money ($6,000.00)when materials are dropped. I thought that it was paid on completion. I have had work done

When to pay for work is part of your negotiation.  There is no law on that.  What does the written agreement you have with your contractor say about when payment should be made? As a part of any payment to the contractor, you must also receive a conditional release from any subcontractors or material suppliers to make sure that every gets paid... or at least if the general contractor doesn't pay them, then they can't go after you for payment.  Here's the forms: http://www.cslb.ca.gov/Resources/GuidesAndPublications/LienReleaseForms.pdf