And you find that keeps you warm under the tarp? Say it had been raining and it took you 15mins or more to get to the nearest bit of shoreline in your canoe and then another few mins to set the tarp up, obviously you're going to want to get warm and dry pretty soon...Good question!
Alot of it depends upon the tarp's construction (what it's made of & how heat & spark resistant the material is) and the heat of the fire. As a rule of thumb, the tarp should never get warmer than "warm" to the touch. As tarps heat up, they begin to show what I call "pre-combustion" danger signs.
increasing tackiness (stickiness)
I live in the Canadian north and use a tarp within 1 pace of my fires (2 steps) and have never had a problem...but I almost never put a tarp directly over a fire because I don't want it to catch the direct heat off a fire.
Try experimenting with tarps...the ones you don't mind damaging, that is...
This is true, but what if it's cold?If you and other humans can't take the heat from the fire, neither can a plastic tarp at the same distance.