Home buyers hire inspectors to learn about needed or soon-to-be-needed expensive repairs. But how thorough and helpful are the inspections?

Undercover shoppers from Consumers' Checkbook, an independent nonprofit consumer advocacy group, rented a typical three-bedroom, three-bath, two-story, single-family house in the suburbs of Washington, D.C., and — posing as prospective buyers — scheduled home inspections with 12 companies to put inspectors to the test.

Checkbook's undercover shoppers were astonished by how often many of the inspectors they hired missed obvious defects. Before the inspections, Checkbook staff identified or created 28 problems they thought any inspector should catch — from a big leak under the kitchen sink to inactive electrical outlets, roof damage and signs of a rodent infestation. As a group, inspectors caught these problems only half the time.

But what really surprised Checkbook's staff was how little work many inspectors bothered to do for their average fee of $540. Few performed up-close inspections of the roof; several didn't test all the windows, outlets, appliances, or fixtures; and the reports supplied by some were meager. For example:

• Only three of the 12 inspectors raised ladders to inspect the roof, which was significantly damaged. Many of the nonclimbers failed to report its broken shingles and missing drip edges.

• Only about half opened and shut all of the windows.

• Several did only cursory inspections of the furnace and water heater.

• Five didn't inspect all the window A/C units.

• Seven didn't check every light fixture.

• Four didn't test every indoor electrical outlet.

• Four inspectors failed to record obvious water damage to the living-room ceiling. There was discoloration and peeling paint.

Three inspectors were in and out in 90 minutes, compared to 2.5 hours or longer for a few others.

The written reports supplied by several inspectors were very short. One handed off a 10-pager with no pictures.

A major reason for superficial inspections is many inspectors explicitly deny responsibility for checking lots of major home components. Many companies refuse to check chimneys, climb ladders or enter crawl spaces. Citing "industry standards," some firms test only some electrical outlets, light fixtures and windows. Some inspectors won't run HVAC equipment, remove panels on circuit-breaker boxes or test water heaters.

Before you hire an inspector, ask what exactly they will do and how long it will take them to do it. You can often determine the thoroughness of inspectors' work by looking at sample reports they should readily supply, if requested. Already have a concern about the home? Make sure your inspector will check it.

Often, real estate agents recommend home inspectors. But the thorough reports of the best (pickiest) home inspectors work against the interests of even trustworthy real estate agents, who want to avoid trouble and close sales.

Worse, inspectors who receive a lot of referrals from your real estate agent might shy away from pointing out lots of problems or major flaws for fear of losing that business.

Ask prospective inspectors about certifications they have and inquire about their backgrounds. This is a field where experience matters. And because Checkbook found big price differences among companies, and little correlation between work quality and fees, make sure you don't overpay for an inspection.

If you are buying a newly built home, definitely get an inspection. Inspectors and real estate agents we spoke with repeatedly warned that builders (and DIY remodelers) frequently create lots of defects.

Before your inspection, carefully review the seller's disclosures and thoroughly check the property on your own.

Inspect some things yourself — switches, operation of window treatments, doors, etc. — and speak up if you see something that doesn't look right. Act as an extra set of eyes, but don't disrupt your inspector's workflow. Make sure all problems found are recorded in your report with pictures and descriptions. If you later find something was omitted from the report, ask for an amendment, especially if you want the sellers to help pay for the fix.

Most general home inspectors don't check for many problems, including some that might generate major expense — such as asbestos, urea formaldehyde foam insulation, radon gas, mold, termites and defective drywall or stucco. If you or your inspector suspects a problem with any of these — or a major issue with roofing, plumbing, electrical, or drainage — bring in a specialist for a second opinion.

Twin Cities Consumers' Checkbook magazine and Checkbook.org are arms of a nonprofit organization with a mission to help consumers get the best service and lowest prices. They are supported by consumers and take no money from the service providers evaluated. You can access all of Checkbook's ratings, including those for area home inspectors, free of charge until July 3 at www.checkbook.org/StarTribune/HomeInspectors