If it is a primary residence and the roof/assessment is specific to your unit then the amount for the roof is applicable to apply to lower your capital gains when you sell your unit (Keep your receipts/payment stubs/invoices).
If the assessment is for a roof that is shared by your entire condo association that is much more complex and you will have to talk to your accountant to gauge what amount is applicable against your capital gains.
We are not accontants so don't take our advice till you check with a professional that is licensed to give advice to you on your situation.
If the condo is rented and the special assessment is for upkeep or improvements it is a business expense and can be amortized.
If the condo is your residence keep the receipts as it can possible add to the basis of any capital gain offset when you sell and realize the gain.
If it's a primary residence, the tax reporting is different than if it is an investment property. For a primary residence it's a repair (which is not a write off) and for an investment, it's an expense (which may be a right off).
In any case, only an accountant will know the correct answer for you. No matter the answer, keep records of what you spend so that when you sell, you or your accountant will be able to itemize what things may be considered on your taxes or not.
It depends on whether you're living there or not. Just as if you were living in a single family home and needed the roof repaired, no--that wouldn't be deductible. That's a repair.
If it's an investment property and you're renting it out, then yes. It's an expense. Just as if you had a single family home that needed a roof repair. It's an expense and you can deduct it.
That's pretty much in line with Keith's point--if your other condo fees or HOA fees are deductible, then a special assessment would likely be, too. Otherwise, no.
I'm guessing that you yourself live in the condo and got hit with an unexpected assessment. (It happens. I've lived in condos.) In that case, the special assessment is just like the condo fee itself, making it (in all cases I'm aware of) not deductible.
But certainly check with an accountant to be sure.
Hope that helps.