Medowlark,

Distance! If you are starting a fire the shelter needs only be a foot or two above the fire. As the fire gets going, it can do without the shelter (sort of). If you set up a tarp 6 feet or so above the fire, you can build a reasonable size fire without melting. Be careful not to get the fire too big, or the tarp will be a single use item. :-( I often use a plastic sheet as do many others.

Unfortunately, aspen is nowhere as good as birch. There must be a lot more pitch and/or a lot less water in the birch bark.

Under a balanced boulder might be a problem, but most overhangs in the rockies would be ok. You should watch out for wet rock, however. Rocks out of the river (or those overhangs that were waterfalls before freezeup) can explode as the entraned water goes to steam.

Respectfully,

Jerry