I had to do some repairs on my bathroom floor this spring. I used the plywood that has a flat smooth surface (A litte more expensive than regular plywood) so the tiles would look nice on top.

I had an extra piece left over and it makes and excellent building surface. Flat and smooth!

Sure wish I had a basement. My wife won't let my airplanes come inside. I am stuck with the kerosene heater in the winter and I sweat my butt off in the summer in my garage!

But its worth it, a basement would be heaven though!