We plan to have a contractor install new hardwood over existing heart pine that is not refinishable. An area of one room is unlevel in this 100+ year-old house. At least one flooring contractor suggested folding over and layering roofing paper in the low area, then adding luan over that, before installing the new hardwood. Is this a reasonable solution? We do not want to pull up the old floor and level it from underneath.
There is nothing wrong with using roofing paper or even shingles (my favorite) if there is a floor slight out of level. Floor leveler is an option too but I usually use floor leveler for floating floors and shim traditional hardwood with shingles.
The reason for this is leveler sometimes has issues with staples or nails going through it. I prefer shingles vs any other type of shim because shingles don't break, take nails well, and they don't need to be compressed. There are limits to how much you can shim but that is true with a leveler too. But innately there is nothing wrong with shimming flooring with roofing paper/shingles.
One thing I would add. If you are redoing all the flooring, if you can have the joists checked out below it is a good time. The flooring guys probably don't care about this - as the trades usually don't mix. But the flooring might be able to be leveled with a set of jacks and the new floor could be installed without shims or leveler.