How inspectors estimate remaining life
Under both national standards of practice, they are not asked to. A home inspection identifies systems and components and reports the condition observed on the day — what a roof or a wall covering is doing now, not a forecast of when it will stop. That distinction is written into the standards themselves.
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What the standards of practice say
InterNACHI's Home Inspection Standards of Practice, last revised October 2022, defines a home inspection as a non-invasive, visual examination of a property's accessible areas, identifying defects both observed and deemed material by the inspector. Its limitations section states that an inspection “does not determine the life expectancy of the property or any components or systems therein,” and its exclusions put the service life expectancy of any component or system among the things an inspector is not required to determine. The roofing section repeats it: the inspector is not required to walk on any roof surface, perform a water test, warrant or certify the roof, or “predict the service life expectancy.”
The ASHI Standard of Practice draws the same line in its general exclusions, where “the remaining life expectancy of systems and components” sits alongside the causes of conditions and deficiencies, and future conditions including failure. Inspections to that standard are, in its own words, “not technically exhaustive,” and their stated purpose is to give the client information about the condition of the inspected systems and components at the time of the inspection.
Inspectors do talk about wear, and many will say a covering looks early or late in its span — but that is commentary, not a determination the standard obliges.
InterNACHI Home Inspection Standards of Practice and the ASHI Standard of Practice, both checked 2026-08-27.
What they do observe, component by component
Both standards are built the same way: for each part of the house, what the inspector inspects, describes, and reports as needing correction.
- Roof. InterNACHI has the inspector look, from ground level or the eaves, at the roof-covering materials, gutters and downspouts, vents, flashing, skylights and other penetrations; describe the type of roof-covering material; and report observed indications of active leaks. ASHI asks the inspector to describe the methods used — which matters, because a roof read from the ground is a different observation from one walked.
- Exterior. Wall-covering materials, eaves, soffits and fascia, a representative number of windows, all exterior doors, flashing and trim, walkways and driveways, decks and porches, railings and guards, and the grading and drainage where they could affect the structure through moisture. Judging the integrity of multiple-pane glazing or thermal seals is outside the standard.
- Foundation and structure. Reported as in need of correction: wood in contact with or near soil, active water penetration, and observed indications of possible foundation movement — the standard's own examples are cracks in sheetrock and brick, out-of-square door frames, unlevel floors.
Absent from every one of those lists is an install date. The inspector is looking at granule loss, curling, rust bloom at cut edges, sealant gone hard, staining that tracks from a lap — what the material is doing. Age is not an observation.
Why age alone is context, not condition
InterNACHI's definitions make the point sharply: that a component is near, at, or beyond the end of its normal, useful life “is not, in itself, a material defect.”
The material pages here are full of the reasons why. A 3-tab shingle roof carrying a 20-year baseline can be further along in year 10 — south-facing, high UV, poor attic ventilation cooking it from beneath, a hail season behind it — than a comparable roof in year 18 in a mild climate with balanced ventilation. A tile roof's tiles may be barely aged at 40 years while the underlayment beneath them, on its own 20-to-40-year clock, is what ends the roof's first service life.
Where the numbers on this site fit
A published service-life range is context to carry into an inspection conversation, not a replacement for one. It is useful in three ways, and no others.
- It says what the trade literature expects of the material, so a reader knows whether a roof is early or late in a documented span — and which variables move it.
- It names the source and its strength: a manufacturer's document, trade literature, and a dated baseline chart are not equally strong, and every number here carries a tier badge saying which it is.
- It sets the warranty term beside the range, so the two get read as the different things they are — the subject of warranty term vs. service life.
What this site will not do is take an installation year and hand back a verdict. There is no remaining-life tool here, for the same reason the standards exclude the determination: it would be a prediction about one building, made from a population average, by something that has never seen it.
Anything involving structure, gas, or electrical service is an inspection question, not a web-page question. The standards agree: ASHI's structural section says the inspector is not required to provide engineering services or to offer an opinion on the adequacy of structural components.