How do I test poly-tarp to see if it is UV proof? Answered

Yeah, those test strips to see if sunscreen, glass, eyeglasses, etc are uv proof. Easy fix: Spread a bottle of sunscreen all over your tarp :D

Thanks for the answers (and sorry for the lag...I assumed I'd get e-mail about any answers and promptly forgot about the question I posted here!).

To conclude, I ended up buying a packet of the blue "sun prints" paper that elementary school teachers get for their classes to make those shadow prints and used that. I don't know if those are exposed only by UV or by other forms of radiation as well, but it worked well enough for my purpose.

I cut small pieces and placed one in direct sunlight, one in the shade of my canopy, one in the shade of a standard blue tarp and one in the shade created by a piece of plywood.  I had three pieces in each location left out for different lengths of time; 3 min. 5 min. and 10 min., I believe.  I placed a quarter in the center of each strip so that I could compare the exposed area with a pristine area.

The results are in:  The strips under the plywood did show some exposure, but not very much.  This leaves me to believe that either the strips are exposed additionally by radiation other than UV or that the exposure was due to scattered UV (visible light gets bounced into shaded areas, after all, so UV likely does too in a normal environment).

The blue tarp offered barely any UV protection at all, as expected; the strips under there were just as exposed as the strips left in the direct sunlight.

The carport seems to be treated against UV radiation.  The strips underneath the carport were exposed further than those under the plywood, but not nearly to the extent of those in direct sunlight and under the blue tarp.

We ended up taking the carport, along with three new ones from CostCo that specifically mention blocking UV radiation, to burning man and setting up our tent underneath.  The others were made of darker polyethylene and certainly blocked more heat, so we spent most of our time during the day underneath those and set our tents underneath the white one.  All in all it was a wonderful trip and none of us got sunburned.

I discovered this in a search on tarp shelters.

I think "UV-proof" doesn't refer to an ability to block UV - most opaque tarps of sufficient thickness are likely going to block UV.

I'm fairly certain that "UV-proof" means it has been treated so that UV exposure doesn't degrade it.

That was a brilliant idea you had and a very well thought-out test.

Glad you had a good time. 

Best of luck on your next adventure too!