The 25% rule comes from the International Residential Code, which most jurisdictions in Pennsylvania, New Jersey and Maryland build on. In broad terms, if more than a quarter of a roof section is repaired, replaced or recovered within any twelve-month period, that whole section has to be brought up to current code rather than simply patched back to how it was. In practice that usually means the work triggers current requirements for underlayment, ice barrier at the eaves, and flashing details. It matters because it can make a large repair a false economy: if you are already close to the threshold, replacing the section outright may cost little more and leaves you with a roof built to today's standard. Local adoption and amendments vary, so we confirm the requirement with the authority having jurisdiction before scoping work.
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