Ok folks. I'm glad to be in a forum with experienced people that take the speculation out of hardwood flooring. I just stripped 4 layers of linoleum and wood floor and found particle board. I took up the 3/4 particle board and found it was also under the cabinets too, so I used a rotor zip and a chisel and I scribed around the cabinets leaving particle where it belongs- man! I found some rot in subfloor near the door and will have to replace that, but I noticed a little give (flex) in other areas of floor so I realized that this house is sitting on 1/2 plywood. I think joists are no more than 12 to 16" apart but can verify later time. Guy at Lowes says to use Fir plywood as it is stiffer for flooring needs. Then I start reading 3/4 is th eway to go to alleviate the problems. My problem is my living room has hardwood and its most likely 1/2 inch. House was built around 61 in New England and the other parts of house have red oak hardood and few areas with small gaps and slight curling in hallway and some creaks - typical stuff. I have to keep 1/2 to make kitchen and living room flush. Is there anything I need to consider here? Do I just replace all the 1/2 inch I can or have to add to the floor before doing this? My friend who is a plumber and handy with flooring is willing to help me install only if I have all the prep work done because he hates doing that kind of work and is busy. Thanks a Million Guys!