I agree with Sarbar--not generally a good idea.

I've had to put out a several fires that have burned tree roots--sometimes for a considerable distance underground. No, I've never been a fire-fighter; it's just a situation I've encountered several times during my many years of outdoor experience. There are always a few idiots who think it's OK to build a fire under a tree, on top of duff and roots. In all these cases extinguishing the smoldering mess involved a lot of digging to get beyond the burned area both horizontally and vertically, and the hauling of a lot of water.

If you can find an area that is all gravel with no organic matter or nearby trees, and not where the land managers insist on Leave No Trace, then it's probably OK.