8 Major Defects Home Inspectors Routinely Miss

I run All Seasons as an owner-operator in Wyncote and I've been inspecting homes across the Philadelphia region since 2003. In April 2026 I spent a couple of hours with a spreadsheet of 18 recent one- and two-star Google reviews of competitor home-inspection companies serving this market. I wanted to know, concretely, what homeowners were complaining about.

The single most common theme — matched only by complaints about inspectors working for the agent instead of the buyer — was this: the inspector missed something major. A third of the reviews. “Missed active knob-and-tube.” “Missed a lead service line.” “Missed a roof that needed replacement within six months.” “Missed attic mold you could smell from the driveway.”

The good news is, these aren't mystery defects. They're the same eight or ten things I find on most pre-1978 Philadelphia-area homes. They're findable. They're photographable. And they get missed mostly because the inspector didn't open the right panel, didn't walk the right part of the roof, or didn't take the extra 15 minutes to confirm what a visual guess suggested.

Here's the list, in the order I encounter them most often in the field.

1. Active Knob-and-Tube Wiring Behind a “Modern” Panel

Philadelphia-region homes built before 1940 were wired with knob-and-tube — porcelain insulators and cloth-wrapped single conductors. Most of these homes have had their electrical service upgraded at some point: a new 100- or 200-amp panel, new meter base, new service entrance. A lot of buyers see the shiny new panel and assume the wiring has been replaced.

It often hasn't. The upgrade was the panel and the service drop, not the branch circuits. The old knob-and-tube conductors were simply reconnected to the new breakers. The result: a home that looks modern from the panel but runs 80-year-old insulation behind the plaster. Most homeowners insurance carriers now refuse to write policies on homes with active knob-and-tube — which directly affects whether you can even close.

What to ask: “Did you open the panel cover and confirm every breaker is connected to modern cable (romex/NM)?” A yes-or-no answer. If your inspector didn't pull the panel cover, they didn't check.

2. Does the house have lead water service lines?

Philadelphia has one of the largest remaining inventories of lead water service lines in the United States. The Philadelphia Water Department has an active replacement program, but confirming what's actually coming into a specific house still requires someone looking at the pipe at the main inlet in the basement.

Lead lines are identifiable by sight and touch: a dull gray color, softer than copper or galvanized when gently scratched, often with a distinct bulge or bend where the city-side pipe transitions to the house-side plumbing. This is a five-minute check with a phone camera.

What to ask: “Did you photograph the main water inlet, and is the service line lead, copper, or galvanized?” If you don't see a photograph of the inlet in your report, it wasn't checked.

3. Is the cast-iron drain stack developing pinholes?

Nearly every Philly home built before 1960 runs cast-iron drain piping for waste and vent stacks. Cast iron begins pinholing from the inside out at roughly 60-80 years of age. The failure is usually silent — water seeps through invisible pinholes into the wall cavity and stains the kitchen ceiling or bathroom wall months later.

A thorough inspection evaluates every accessible run of cast-iron stack: in the basement, in the kitchen sink cabinet, in any mechanical closet. Look for visible corrosion scale, wet-looking deposits, and the characteristic rust halo that forms around a weeping pinhole.

What to ask: “Did you evaluate the cast-iron stacks at every accessible point, and is there any visible pinholing or corrosion scale?”

4. Are there roof flashing failures at chimneys and valleys?

A roof can have perfectly intact shingles and still leak — the shingle field is rarely the failure point. Flashing is. Step flashing at chimneys, roof-to-wall transitions, valley flashing, and plumbing boot flashing all age faster than the shingle body. In 20+ years I've found that 70% of active roof leaks I trace on pre-1990 homes are flashing failures, not shingle failures.

An inspector who looks at the roof from the ground (or worse, from Google Street View) cannot see flashing details. Walking the roof — or at minimum getting close with a drone or ladder to the eave — is how flashing gets inspected, and it's explicitly addressed in InterNACHI Standards of Practice.

What to ask: “Did you walk the roof? If not, how did you evaluate the chimney flashing, valley flashing, and roof-to-wall step flashing?”

5. Are bathroom exhaust fans venting into the attic instead of outside?

This is one of the most frequent causes of attic mold I find on Philadelphia-area homes, and it's invisible from inside the house. The bathroom fan does its job — pulls warm, humid air out of the bathroom — but instead of that air going to an exterior cap, the duct terminates loose in the attic. Every shower adds moisture. Every winter, that moisture condenses on the cold underside of the roof sheathing and grows mold on the plywood.

Detection happens in the attic: trace every exhaust duct from the fan housing to its termination. If the duct ends inside the attic (or if you can find no duct at all beyond the fan body), the fan is venting into the attic. Fix is simple — extend the duct to an exterior cap — but only if somebody notices the problem.

What to ask: “Did you verify in the attic that every bathroom exhaust fan vents to the exterior?”

6. Are deck ledger boards flashed properly to prevent rot and collapse?

The deck ledger is the horizontal board that bolts the deck to the house. When the ledger isn't flashed correctly with Z-flashing or a continuous membrane, water runs down the siding, behind the ledger, and into the band joist. Over years it rots out the connection between the deck and the house — which is exactly how decks collapse under party loads.

This one is easy to miss because the failure is behind the siding. A competent inspector pulls up a corner of the siding above the ledger (or uses a borescope through a siding gap) to visually confirm flashing is present, or uses a moisture meter to detect elevated readings in the band joist.

What to ask: “How did you evaluate the deck ledger flashing?” “I looked at it from underneath” is not a sufficient answer. Ledger flashing installs behind the siding, above the ledger — you can't see it from below.

7. Does the Philadelphia home have a backwater valve to prevent sewer backup?

Much of Philadelphia and the inner suburbs run on combined sewer systems, meaning storm water and sanitary waste share the same pipe leaving your house. During heavy rain, city mains surcharge — and without a properly installed backwater (check) valve on your building drain, that surcharge can push raw sewage back into your basement floor drain, laundry standpipe, or basement toilet.

The inspection check: locate the main sanitary cleanout in the basement, confirm whether there's a backwater valve installed, and photograph either the valve or the lack of one. Homes in Point Breeze, Fishtown, West Philly, Grays Ferry, and the lower Northeast are especially susceptible.

What to ask: “Is there a backwater valve installed on the main building drain, and if not, is the home in a combined-sewer flood zone?”

8. Are the HVAC and boiler systems well past their expected service life?

This one sounds obvious but gets missed constantly — because the equipment is running. An oil-to-gas converted boiler from 1995 that still fires is still worth flagging. Manufacturer life expectancy on most residential boilers and furnaces is 15-25 years. I regularly inspect systems that are 30-40 years old and the report simply says “operational at time of inspection.”

Operational at time of inspection is a legal statement; it's not a useful one for a buyer trying to budget the next two years. The useful finding is the age of the equipment, benchmarked against its expected service life, with a note about whether replacement should be budgeted in the near term. Name plates and service tags make this a 10-minute check.

What to ask: “What's the age of the HVAC equipment, the water heater, and the boiler — and where are we on the expected service-life curve for each?”

What do all eight commonly missed defects have in common?

Notice what ties these together: every one of them requires the inspector to do the extra thing. Open the panel. Photograph the water inlet. Trace the bathroom fan duct in the attic. Walk the roof. Pull the siding corner. Check the cleanout. Read the name plate.

An inspector who's under volume pressure — moving to the next job in 60 minutes, generating a report from a template, trying not to irritate the agent — skips these checks. An inspector who has the time and the incentive to be thorough does them. That's the real delta between the reports buyers praise and the ones buyers complain about.

If you're buying in Montgomery, Philadelphia, Bucks, Chester, or Delaware Counties — or in Camden, Burlington, or Gloucester Counties on the Jersey side — I'd be glad to take your call. I do every inspection personally, I spend 2-4 hours at the house, and I confirm all eight of the items above on every report. If something in that list isn't present, I'll tell you why.

Bob Oberholtzer
Owner, All Seasons Home Inspections
610-348-6728  |  Free Estimate