I have an Exterior BPO on a (attached) condo. The builidng that the subject condo is in is in average condition except for the roof. The roof is in a deteriorated, poor condition. Of course the owner of one condo is not responsible for the entire cost of repairing the roof; the association would handle the repair. But, how do I account for this repair when doing the BPO on the single unit? Divide the est. cost of repair by total units?

As former treasurer of a condo association, most units have a % of ownership, which is how overhead cost are allocated. Most often it relates to square foot of the unit as a percentage of the total square footage of the complex. (Some associations charge more the higher up in a building or if some units have much better views(lake view vs non-lake view). However, since you may not be able to get your hands on the association data, I would make a rough estimate.

If the Association is allowing the Building to deteriorate to the point that damage is visible to a Passer-By like you, then that sounds like more of a broken Association problem than a Roof Issue.

Condominium Owners are typically entitled to have their buildings always appear to be in a "well-maintained condition" . . . . and your observation brings into question the solvency of that Association. (And that diminishes the value of the entire complex.)