They have to pay you for all the time you are on the job--which includes waiting for materials--during the normal work week (e.g. M-F 9-5), the same as if you were waiting around the office or shop for a delivery. They do not need to pay you for time outside normal work hours (e.g. weekends, evenings), just as they would not pay you for that time at home. If they don't pay you for time that they should, you can try filing a wage and hour complaint with your state department of labor, and/or file a lawsuit for unpaid wages.