A good thought, but possibly more trouble than it's worth.
Any wood that you have on your property is probably a bit wet. If it's not completely dry, then it will twist, warp, and crack over time as it dries out. You could put it in a low-humidity environment for a year or so, but that may be too long of a wait.
The other problem is that you want a dead-flat surface. If you rough saw them, there is no guarentee that they'll all be the same. You need to face joint them then put them through a planer so that they're all of equal thickness. Also when you glue the boards together, you will want to use biscuits or dowels to ensure that the alignment is correct.