Well, a few things to consider. You say now that you want only a couple of chairs and a table. Perhaps one night, it's you, your 2 chairs, a table, and about 6 of your friends? So, will the roof support the added weight? Understand that roofs are designed to hold snow loads, some water, and then some. BUT, the weight of a human being is much more concentrated when you're talking about 2 feet in shoes (or no shoes.)
Also, would it be safe in terms of a railing to keep folks (you included) from even accidentally falling off? (Lawsuit ...)
And lastly, in the event of a fire, can you safely get off it? Maybe if there was a year-round pool down below, you'd be OK (assuming you live in AZ or FL or someplace like that.)
So, these are just some things to think about. Now, to answer your question more specifically, yes, there are materials that can be put down to make the roof usable. I've seen it used in HOT climates like the Southwest. It's a white, sprayed material that keeps your feet from burning on the hot roof. The material's name escapes me at the moment. What you can do is continue to check around with Roofing Companies. Look in the Yellow Pages under ROOFING - SUPPLIES and start calling. If they can tell you what the material is, ask them for a few Contractors that install it.
My best to ya and hope this helps.
Jay J -Moderator