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Older homes offer a host of opportunities for customization, blended with character and history, which is why many people love them. But the flip side is that the older they are, the more likely they are to come bundled with unpleasant surprises -- including some you might not even have heard of.

Fixing them is sure to be expensive, but knowing what to expect can help you prepare your budget. Here's what to look for in an old home, depending on when it was built.

1900s

A home that is more than 100 years old might seem like a sweet deal, especially if you're fond of classic architecture and "This Old House," and don't mind a healthy dose of down-and-dirty DIY. But buyer beware: These gorgeous antiques can make "The Money Pit" seem like a documentary of your life.

"It is somewhat inevitable that many of these older homes will have some type of settling, so you will see floors that look like they are leaning slightly," says Kevin Lawton, a Realtor in Bordentown, NJ. Often, tilting is nothing more than a cosmetic annoyance -- but it could indicate expensive-to-fix foundation issues.

Unrenovated homes might also have knob-and-tube wiring, the early electrical systems that were common until the 1930s. Ceramic knobs and tubes run through the floor joists or wall studs, carrying electricity throughout the home. However, the rubber insulation can degrade and create a fire hazard. And without GFCI outlets (ground-fault circuit interrupters, which trip the circuit if there is a surge in current), the system isn't grounded.

Fixing it is pricey, so get ready -- and start working on getting estimates from some good electricians.

1920s

Balloon framing was invented as an inexpensive alternative to traditional timber framing, and was used frequently from the late 19th century through the 1920s and early '30s. Instead of complicated joinery to keep post and beams well-connected, balloon framing utilizes… really, really long boards. And nails.

Sketchy as it all sounds ("balloon" was originally a derogatory term indicating that the frame might just blow off in the wind), the structure itself is relatively sound. But the framing can be a tremendous fire risk. Unless the gaps between the planks are filled or well-insulated, basement or crawl-space fires can rocket upstairs.

1930s

Homes built in the 1930s and earlier -- and the occasional '40s home -- typically used clay sewer lines. Tree roots can invade every type of sewer line, but the clay kind are particularly vulnerable because terra-cotta, the material they are made of, is brittle and liable to break. And because plumbers typically cobbled together shorter pipes to form the line, they might separate over time, causing leakage and soil contamination.

Philadelphia, PA, interior designer Larina Kase had a '40s house with clay pipes, and encountered this problem.

"Roots grew into it, and we had a sewage flood," she says. "Newer homes use PVC pipes, which is what we replaced it with."

Clay isn't your only potential sewer problem: Your water main and sewage line may be installed too close together to meet modern standards. Most municipalities require at least 10 feet of separation. While no one will make you dig up your existing lines, the regulation can greatly increase the cost of a replacement if either line breaks. During repairs, many cities will require moving the line to meet code.

1950s

Clay lines aren't the only troublesome sewage system. Cast-iron sewer lines, popular from the mid-20th century to the '80s, can corrode or crack, with a replacement cost in the many thousands.

Buyers might also find ungrounded outlets, especially if the home hasn't been sold since its construction. Expect an expensive update to ground your system and mitigate your fire risk. At the very least, GFCI outlets will need to be installed.

1960s

Feeling a little chilly in your Mid-Century bungalow? Check inside the walls.

"1960s homes were not built with an emphasis on insulation," says Kershan Bulsara, a manager with Roofmaster in Ontario. "Energy prices were much lower, so preventing heat loss was not a huge concern for builders or homeowners."

Luckily, unlike replacing a sewer system, improving insulation is an easy DIY. Rent an insulation blower from your favorite hardware store -- expect to spend about $50 per day -- and your home will be toasty long before wintertime.

1970s

Expect to find fewer outlets per room and fewer circuits -- which means all your electronics could trip a fuse. And if you find single-strand aluminum wiring -- common during the copper shortage of the late '60s and early '70s -- be prepared to replace the system. Faulty connections overheat the wiring, creating (surprise!) a fire hazard.

Of course, you can't ignore the No. 1 problem of homes built in the 1970s: the decor. Shag carpeting? Avocado walls? Fake wall paneling? Ugh. Fortunately, you don't need a home inspector to suss those out.

1980s

By the '80s, builders had figured out how to build a mostly modern home. But there is one quirk to look out for: PB (polybutylene) piping. Prone to rapid degradation and subsequent bursting, these gray-colored water pipes cost homeowners millions of dollars in water damage.

Most PB has long been replaced, but if you're unlucky enough to score a home that still has the faulty piping, budget for a replacement. Even if you don't care, your homeowners' insurance company may decline to cover you until a change is made.

Count yourself lucky if your old home isn't rife with problems -- but not too lucky. One of the joys of home ownership is knowing that something will always go wrong. But even when it inevitably does, don't panic. Everything is fixable.