I have about 900 sq. ft. of attic space. Currently, only a small area around the entrance is covered by loose planks for storage. The rest is uncovered(the joists are exposed). I do not want to spend a lot of cash of this project, so ideally, I would like to take my time(years if necessary) to buy and collect cheap and donated hardwood to finish the floor. I understand by taking my time that most of the wood will not match in size or color which is not a concern to me, at least not cosmetically. I plan to plane down the pieces that do not match in thickness. If anyone out there has any ideas about why this would not be a good idea please let me know. Also, is there anything wrong with just nailing the wood straight in from top into the underlying plywood? As you can tell I'm not too concerned about appearance. I just don't want the floor to fall apart. Any thoughts on the subject would be greatly appreciated. Thanks.

The first question would be, WHY? 2nd ?, Someday that house WILL be one the market with a for sale sign and no buyers! Whatever you do with your home should be to make it better in all ways. It's usually a sizable investment. 3rd. You say mostly joists but then write about nailing into plywood. Is it plywood or joists? 4th? You don't want the floor to fall apart then install PLYWOOD and forget about it! Plywood would be more appropriate for this application, Stronger too, Per/lb. If you plan on using this area for storage, as it sounds like you do then, I would keep in mind the weight you think you would be adding. Especially, if your planning on staying there for a good long time as it sound like that's the plan. You could, over the years, overweight the trusses, unless it's a real attic, and that could present structural consequesces down the road. Best of luck!