Verbatim from Google's People Also Ask, answered for Long Branch's historic slate roofs.
Q.01
Is it worth repairing a slate roof?
Almost always, yes. A natural slate roof lasts 75 to 150 years, far longer than the slates' fasteners or flashing, so most slate problems are repairs rather than replacements. On a Long Branch Elberon Victorian, restoring the original slate preserves both the roof and the home's historic value.
Q.02
What is the 25% rule in roofing?
The 25 percent rule means New Jersey code requires a roof section to meet current standards once more than a quarter of it is replaced in a year. Slate roofs rarely hit that line, since slate repair is done a few tiles at a time, which keeps historic Long Branch roofs in original material.
Q.03
Can I repair a slate roof myself?
It is not recommended. Slate cracks under foot traffic, and the repair needs matched slate plus copper hooks set without face-nailing the surrounding tiles. A wrong step or the wrong fastener does more damage than the original leak, which is why slate work on Long Branch Victorians goes to a roofer who handles slate.
Q.04
Do I need a permit to replace my roof in New Jersey?
Yes. New Jersey requires a construction permit for a roof replacement, and the City of Long Branch issues it through the Building Department at 344 Broadway. Slate repairs that replace only a limited area usually fall under repair scope, but a full replacement or structural decking work always needs the permit.