Question: I just installed a new Kohler toilet. It flushes fine but after the flush the level of water in the bowl slowly drains. Why would this be and is it anything to worry about?

Answer: 
Get the dimensions you need first, then judge by looks. My guess is that the roofing boards would be lower grade (ie, more knotty) than floor boards.
I would expect floorboards to be planed to tight thickness and width tolerances and to be relatively smooth on one side. Roof boards can be sawn rougher with more tolerance for variation in thickness and width.

Frankly though, it could very well be that the only difference is where they were pulled from!