I want to lay either hardwood or engineered hardwood flooring in a 22x18 bdrm. The floor support consists of old chestnut barn beams instead of conventional joists, followed by a layer of 1x4 boards( old barn siding?) run perpendicular to the beams. There are 1in spaces between each 1x4. On top of these boards is a layer of 3/4 in particleboard which appears to still be in good shape. Other than a bit of sagging, the floor seems solid. I hesitate in tearing out all this particleboard as I'm worried I'll disrupt the boards underneath. Could I leave it, cover it with 1/2 in plywood or OSB, and nail in the flooring? If I used 2 in nails I think I can anchor them into the 1x4 boards underneath the particleboard. What do you think?

If you are going to lay flooring over the particle board I would recommend engineered. The nails should hold engineered even if you might miss the 1X4. The particle board will not hold when it gets wet. If it does get wet, you may have coverage from your homeowners insurance for a covered disaster. Fire insurance will usually cover a major water spill. Check with your insurance agent.

Fastening into particle board is not a recommended practice. Plus you will not hit the subfloor enough to hold in the floor and you may get squeaks and pull out.