Assuming you are self-employed as a handyman or something and you report your business income/expenses on SCH C as a part of your personal 1040 tax return, it really doesn't matter if you call them "supplies" or "expenses" the deduction is the same. But since you're talking about something that you can physically touch and hold, I would list your materials as supplies since that is "in fact" what they are. (You can't complete the job without them, right?)