Someone made me an offer to perform work on my roof in the amount of $5000.00. I asked when he could start, and he advised me he could start the following Monday. My response to him was "that seems fine" I never actually told him to do the work. Is this a contract?

No not considered a contract. Can you tell us more and why you ask?

Down here a GC can't install a roof, has to be a Roofing Contractor. And Permits take time.
A contract should have been drawn up.
I still can sometimes conduct Business by Verbal or a Handshake, and I honor what I say.
If he showed up on Monday with Materials in hand, and is qualified, I would have him do it. But still write up a contract to keep everything on the up and up.