Now, about working up there: we've had bats in the past. We could see the evidence of them, in that they leave droppings. bunny_hugger was worried we'd disturb the bats while working, and disturbed bats in an enclosed place like an attic are a sufficient condition for an unhappy emergency room visit. I promised to cover as much of my skin as possible, and to work as swiftly as possible, but of course I still worried. While investigating for weak spots in the roof, though, I looked at the place the bats must have congregated and found ... nothing. In fact, I searched the whole attic several times, during each stage of the roof repair (which took a couple sessions spread over a few weeks) and found nothing. Apparently, the bats had moved on.
So it won't surprise you that that evening, we heard the unmistakable squeaking of bats fluttering around the roof, right outside our bedroom window.
While it spoiled bunny_hugger's night's sleep, we went back to investigate the next day and saw again, no bats in the attic. Nor did we hear any for the next week after that. The best hypothesis I can form is that perhaps a bat saw an entry point into the attic --- there's a spot near the bedroom window where squirrels used to get in, before it was covered up with the wire mesh of hardware cloth --- and was trying to get in for a warm spot for the night. Possibly it didn't understand the mesh and kept trying to get in and got frustrated by not being able to get into this obviously inviting spot; but, that suggests this bat incident was a freak event that just happened to coincide with the nailing of plywood all over the interior of the attic. In any case we're hoping this means the roof needs no further care for the rest of the season and that we don't have to worry about bat abatement for now.
Trivia: In 1837 James O'Connor, a shipper in Pittsburgh, developed a boxcar which could be fitted with wheels for train travel or mounted on a canal barge. Source: A Splendid Exchange: How Trade Shaped The World, William J Bernstein.
Currently Reading: From Eudoxus to Einstein: A History Of Mathematical Astronomy, C M Linton.