I'd get myself a stack of $5 and $10 dollar bills as well as bottles of water and see how long the job takes and what the weather is like and how pleasant they are! Yes, that counts with me.
I'm visualizing a garage, so they don't have to deal with steps -that's already an easier job than your dad's basement. If they have to make decisions about what to toss, that will add to their time, but if all they have to do is move things and toss them onto a truck, they should be able to be very efficient.
All that said, I wouldn't tip less than a total of $20, but I also wouldn't add another $20 too fast which is why I said I'd get me a stack of 5's and 10's. I know they should share, but I do not count on that - I put my tips into the hands of each worker.