Rules of thumb, with their limits
Repair or replace: what the published numbers say and leave out
No rule settles the question for every roof. A few published rules of thumb frame it, and a contractor's look at your roof answers it. Here are the numbers, where they come from and what each one cannot tell you.
The 10 percent line
The Energy Department's repair brief passes along a rule from a 2003 field guide: once more than a tenth of the old roof needs repairing, put on a new one. That is one trade publication's guideline, printed in a short homeowner brief. The brief never says whether the tenth is counted by area or by individual shingles, so a contractor has to read it with judgment.
The 20 percent line for slate
For historic slate, the National Park Service says repair is preferred over replacement when possible, and that when roughly a fifth or more of the slates are damaged or displaced, replacement is often the more economical choice. It also advises repairing damaged slates promptly to prevent water damage and handing slate work to experienced slaters. The number applies to slate, and the two sources use different thresholds because the materials differ.
Where the metal fits in
Flashing can fail on a roof whose covering is still sound. The Park Service notes that metal flashings often wear out before slate does, and the roofing manufacturers' association calls flashings the weakest points on a roof. Building America shows chimney step flashing layered with the shingles under counterflashing set into the mortar. A flashing repair can be its own job, and chimney flashing repair describes it.
What the attic adds to the decision
New shingles do not change how much heat leaks into the attic. The UMass guide calls poor insulation and air sealing the root of ice dam trouble and recommends at least R-38 of ceiling insulation in the northern United States. The Building America guide calls self-adhering membrane around ice-prone areas good practice and says builders may cover the whole roof. If winter leaks are the reason for the question, the attic belongs in the answer.
What happens to the paperwork either way
Under the state's ordinary maintenance notice, roof covering work on a house standing alone with one or two dwellings, in any amount, needs no permit and no inspection. Verona's FAQ says replacing shingles alone needs no permit while sheathing work does. So the choice between repair and replacement does not by itself change the permit answer in Verona. The paperwork page has the details.
Have this ready
Write down the address, what is visible from the ground and the roof's age if known, then call (862) 396-4866. Tenants start with the landlord. Towns covered: Verona, Cedar Grove, North Caldwell, West Orange.