To me, it would be dangerous to have a fire underneath a tarp, but I see people do it all the time. Native Americans generally had a fire in the center of their tepees, with a hole in the top acting as a chimney. I think as long as you have something over the fire, like a boiling pot of water, you'd be OK. A bare fire creates a narrow column of heat above it for quite a distance, you'll have to use your hand to figure out how high that can get.
There is so much information on this forum. I support the Tdickenson`s thought process on this topic that canvas will not be affected by sparks if you do not mind a heavy tarp.
Fire retardant additives
Moderators: Looks like michaeljj is a spam bot. Might want to nuke that account.
For a fire I would do more of a lean to style... and as somebody said earlier, leave a little gap at the bottom to help move the smoke out the front.
Any Idea on how much the Swack Shack tarp weighs?
Your going about this all wrong. There is no reason to need a tarp over the fire. If your concerned about your woodpile, keep under your tarp with you, or a space blanket draped over it will do just fine. For the fire itself, there is no reason to need a tarp, just make the fire bigger. If you make a fire big enough no normal amount of rain will put it out. So, next tme it starts to rain, just build your fire up more by adding more wood, that's al your need to do. If your looking to shelter a small cooking fire, build it under the tree canopy.
Rather than having a fire resistant tarp, carry a square piece of welding cloth about 3'. Twist small rocks into the corners and tie around them so you can suspend the cloth between the fire and the tarp.