Is replacing a rental property's roof a current repair or a capital improvement for tax purposes?

It depends on what you actually did, not just the fact that a roof was involved. Patching or replacing damaged shingles to fix a leak, using materials similar to what was there, is generally a current expense - you're restoring the roof to the condition it was already in. Replacing the entire roof outright, especially with a better or more durable material than the original, is more likely to be treated as a capital expense, because it provides a lasting benefit and goes beyond simply maintaining what already existed.

CRA looks at the scope of the work and whether it's really a repair to part of the roof or a wholesale replacement of the whole thing. A full roof replacement is usually capital even if the roof was genuinely at the end of its life, since the test isn't about need - it's about whether the result is a repair or an improved asset. If capital, the cost gets added to the building's capital cost and depreciated through CCA over time instead of being deducted all at once, which changes both the timing and the amount you can claim in the year of the work.