"And so I began. Before I could start on the real cool stuff though, I had to fix this. Apparently when we had a leak in our roof ~15 years ago, the contractors we paid to fix everything replaced this section of floor with MDF instead of OSB, so the first thing I noticed when I walked in the room was that it felt like I was about to fall through the floor. To get the old piece off I put 2" screws in various spots around the edges to pry on since there was no way I was getting a pry bar under an edge. Once I had it up I discovered the big reason, on top of the fact that it was the wrong material, that it was flexing so much. All four edges were completely unsupported, no floor joists within 6" of any of them. So before I put the new sheet down I screwed 2x4 blocks along all the edges to the floor joists and the surrounding subfloor so that all the edges would be firm. I secured the new sheet with liquid nails underneath and 2" deck screws every 12" on top."