I purchased a house 2 years ago with a partially unfinished basement but had all the supplies to finish. I am getting around to doing the flooring soon. I have nice oak engineered hardwood flooring and underlay. The basement floor has 1" rigid foam board glued and taped with 5/8 T/G OSB overtop as a subfloor. Now the OSB is not secured to the concrete so I do plan on securing the subfloor to the concrete with tapcons. Have not done so in case I need to add another layer of subfloor. The problem I am having is the concrete is not very level so my subfloor is not level. I have one bad area that has a 1/2" dip when checked with a 6' level. There is other uneven areas throughout the basement but none as bad. The walls and bar cabinets are installed over this subfloor. This low spot is right in front of these cabinets. What are my options for leveling this floor so I can install my flooring?
I would be looking at getting some self leveling mortar/ concrete mix and at the very least, filling in those voids. Your sub floor needs to be securely fastened to the concrete floor which means it will "sink" in where the voids are and that will show when the new wood floor is installed. If you avoid fastening the sub floor into the voids hoping to just go over them, you'll feel the depression and hear a squishing sound every time you walk over it. The smoother and leveler your floor is, the better your new flooring will look and hold up. Good luck.