Your home’s roof is pretty important, but many home inspectors will essentially do a drive-by visual inspection. This will catch a roof in very poor condition—one with warped or very worn shingles, or a roof slathered in tar for quick, temporary leak repairs—but it won’t catch any substantial problems.

If your roof is easy to access, your inspector might get up there for a closer look, but if it’s difficult to access (or if the weather makes it unsafe to walk on the roof) they will likely skip it. In many cases, you’re going to have to hire a specific roof inspector if you want to make sure the roof is in good condition.

Home inspectors will generally perform only a cursory check on the major appliances of the home—including the hot water heater and heating system. This might involve turning them on and checking basic functions. As long as the old dishwasher doesn’t explode when they turn it on, they’re going to give it a passing grade and move on. When my wife and I bought our house, everything seemed to be in working condition. A week after we moved in, the washing machine went to the Big Junkyard in the sky. You can hire an appliance repair professional to come in and take a closer look, or you can just hope the appliances work and be ready to replace them if necessary.

A lot of older homes are slathered in lead paint, which is pretty toxic and can be expensive to remove. But most home inspectors do not test for it, because you need to be specifically licensed and insured to do so. At best, your inspector might note some flaking paint and suggest you hire a lead paint specialist to check, but they won’t do much beyond that and may not pay attention to the paint on your walls at all.

The guiding principle for most home inspectors is to check things visually, and most will not even consider moving something heavy—or delicate—in order to see behind or under them. This is in part because it’s not their job to be movers, and in part because you don’t own the house yet, so they can’t be too invasive and must avoid damaging the property at all costs, or it will be a huge mess for everyone.

If you’re on well water, a home inspector might offer to test the water quality and will visually inspect the filtration and softening system if present. This might involve an extra fee. On the other hand, they might not do anything at all with your well water, and you’ll have to hire an official well inspector to ensure that you’ll have sufficient (and sufficiently safe) water when you move in.

Most home inspectors will confine their examination of your sewage or septic system to a visual examination of visible pipes and anything else they can see. This will catch major problems like leaking pipes or septic tanks, but won’t catch anything more subtle that could cause you a lot of trouble and expense in the future. If you want to make sure you’re not buying a problem, you can hire a septic inspector to check things out for you.

A few years after buying my house, I decided to change out the ventilation fan in our bathroom, and discovered it vented ... nowhere. A previous owner had installed the fan and simply didn’t vent it outside, so every time we ran it we were blowing humid air into the void above the drywall. This isn’t all that uncommon, actually, but it would have been nice to know—but our home inspector didn’t notice, because home inspectors don’t concern themselves with anything behind walls or ceilings or under the floor. If they can’t see it without much effort, it simply isn’t checked. That applies to wiring and plumbing inside the walls, and HVAC ducts as well.

Hazardous materials are a mixed bag. Depending on where you live, your home inspector might conduct tests for something like radon gas, or tests for various hazmats like radon, methane, or asbestos might be add-on services you can select and pay for. That means you’ll have to know to ask for them (or know that you need them if offered). But often these tests are simply not part of the inspection service at all.

There’s a misconception that anyone who works in a construction-related (or even tangential) field must be versed in every aspect of the trades. But trades like plumbing and electrical work are highly skilled, specific fields that require lots of training, experience, and licensing. In other words, your home inspector is probably not a licensed plumber or electrician, and their inspection of your home’s pipes and wiring will be relatively superficial. Leaking pipes, backed-up drains, and ancient knob-and-tube wiring? They’ll probably notice those things and alert you. But they’re not going to conduct an in-depth inspection.

Some home inspectors will look at your home’s heating and air conditioning systems, and some explicitly will not—and may even include a disclaimer in their contract exempting those systems. Similarly to plumbing and electrical systems, this is usually because the inspector is not an HVAC expert and doesn’t want to risk damaging components. But even if the inspector looks at your HVAC system, it will likely be a cursory inspection, so you might want to consider a separate HVAC inspection anyway.

If you’re buying an old house, part of the charm might be the lack of 90-degree angles. Old homes often have sloping floors and walls that are visually out of square, but your home inspector will almost certainly not consider these aspects of the home. The simple reason they don’t check whether the floors are level is because it’s actually pretty rare—houses settle, floors tilt slightly, walls go out of plumb, and doors stick. It’s just not worth it to check in most homes. If you’re concerned about it (because of cracks in the walls or a really dramatic floor slope, for example) you should probably consult an engineer.

Finally, home inspectors are not exterminators. A visual inspection for obvious signs of termites or carpenter ants is probably the best you can hope for, and the inspector won’t care much if there are roaches or other pests scampering about while they do their walkthrough. As with the other things outside a home inspector’s purview, if you’re worried about bugs in a house you’re thinking of buying, hire an exterminator to take a look before you close the sale.