When someone damages your property either intentionally or negligently e.g. carelessly, they are obligated to pay the cost to repair it to its previous or prior state. In this case, if they will not voluntarily pay to redo the entire roof, you could sue them for the cost to to comprehensively repair the roof, so that the entire roof matches. Because the roof was old, however, you can expect that the court to give the roofers some break on the cost e.g. they might only pay some percentage of the cost to redo the whole roof--if you assume, for example, that you would get 100% of the cost to replace a new roof which they damaged or destroyed, then clearly a 12-year-old roof, which might have otherwise have to be repaired or replaced anyway in several years, would not be worth 100%. So while the roofers have to pay based on comprehensively replacing the roof, so it matches, it is very likely that the court, if you sue, would reduce what you would get by some amount to reflect that the roof was not new.