If you look at enough houses, you will see some with polybutylene pipes, a type of plumbing that has a history of leaks.

As a home inspector, I see perhaps three such houses a week -- as many as a tenth of the properties I inspect.

An inspector cannot tell whether the pipes in a particular house will leak someday. For prospective buyers, the presence of polybutylene raises a big question: Should they even consider buying such houses?

To decide, it helps to understand what polybutylene piping is and what the options for dealing with it are. Polybutylene has been the subject of lots of litigation, so you also need to understand something about the lawsuits and what their results could mean for you.

As many as 6 million houses and mobile homes built from 1978 to 1996 used some type of polybutylene piping, according to industry estimates, and many still have it. In the Washington area, there are an estimated 40,000 such homes. Some, but not all, of the houses have had leaks.

Polybutylene pipes were developed as a less expensive alternative to copper pipes. You can usually tell if plastic pipes are polybutylene just by looking: Almost all of them are a solid, uniform gray color, although a small number were made in white, black or silver-gray.

Not all plastic pipes are polybutylene. One common type of plastic pipe is polyvinyl chloride. PVC pipes are white. Those systems use a large, external PVC joint and are run through the house much like copper. PVC has no history of leaking and is the choice of many builders and remodelers.

A new, translucent white piping recently has shown up in new construction. This product, made of a flexible cross-linked polyethylene joined with brass fittings, is found in houses built in the past three years. There don't seem to be problems with leaks.

During the years of its use, the ways polybutylene was installed in houses changed.

Early on, from 1978 to 1983, rigid pipe was joined to acetal, or plastic, couplings and fitted with crimped aluminum bands. This has been the most common method to fail in houses.

The second method used the same pipe and coupling, but with a copper crimp band. This method was used from about 1983 to 1989.

The third method, used from 1989 to 1996, combined the same rigid pipe, joined with a copper coupling and a copper band.

Problems with leaks occur mostly at the joints. The reason appears to be that chemicals, including chlorine commonly added to the water supply, can erode the plastic, which causes the joints to leak. The erosion and failure can take years to happen, which means that there are polybutylene systems that haven't failed yet but might.

There is yet another, very different, type of polybutylene installation, found in houses built from 1989 to 1996. It's called a manifold system. In such a system, there's a central station for all the beginning points of every plumbing endpoint. In other words, wherever water is used in the house, its plumbing lines start at the central distribution center.

Rather than the rigid pipe used in other installations, the manifold system uses flexible piping. The central station is usually found near the water heater and mounted on a wall. From there, a flexible line runs all the way through the house to the supply point, such as a bathroom vanity. These systems often used polybutylene, though that ended with the suspension of all polybutylene use. Manifold systems can be used with other types of plastic piping.

Because manifold systems use flexible pipes, there are almost no joints along the way -- and the joints are where polybutylene commonly leaks. The junctions of the piping also are visible at either the supply end or the manifold and can be monitored for problems. This is a major improvement over the old system, but beware: Joints have been found in this system where lines cannot bend.

You don't fix polybutylene, you replace it, with copper or PVC pipes. If your house or the house you are considering buying has polybutylene, there are two class-action lawsuit settlements that could cover some or all of the cost of refitting the house, and in some cases reimburse you for damages from leaks.

The settlements in Spencer v. Dupont and Cox v. Shell have compliance rules that are fairly strict and different from each other. In both, payments are available only to homeowners whose pipes have leaked; there are no payments simply because the pipes might leak in the future.

The big difference between the two is that the Cox v. Shell settlement pays up to 100 percent of the re-piping costs.

Spencer v. Dupont covers 10 percent of the cost of re-piping homes with eligible systems -- polybutylene pipes with acetal plastic fasteners. The payments cover only systems replaced within 15 years of installation, which is usually during construction.

The Cox settlement covers houses built from Jan. 1, 1978, to July 31, 1995. For systems with plastic fittings, the leak must occur within 13 years of construction and the claim must be filed within 14 years of construction. For systems with copper fittings, the leak must occur within 16 years and the claim must be filed within 17 years. So if your house was built in early 1985, has copper fittings and hasn't leaked yet, the coverage has expired.

Mobile homes are also covered under the Cox settlement, although the leak must occur within 10 years of installation and the claim must be filed within 11 years of installation.

As you might guess, removing all the polybutylene pipes in a house and replacing them with copper or PVC is a big job. It entails cutting out sections of the plasterboard covering the wall framing, removing the old plumbing, installing new pipe and returning the walls to their original condition. There are companies that will do the job, but cost is a major consideration.

Like most inspectors, I rarely give out prices for repairs or replacement items. However, the cost to replace the plumbing in a three-bedroom, 2 1/2-bath town house would be from $4,000 to $6,000. For a four-bedroom, 3 1/2-bath single-family house, it would cost $6,000 to $8,000, said Dan Johnston of Plumbing Express of Alexandria. Those numbers certainly need to be considered when looking at a potential property.

When I'm inspecting a home and find polybutylene, I know that my finding might cause the buyers to seriously reconsider a purchase. We're looking at a big potential problem that may need to be corrected, but we are also looking at a plumbing system that may never leak.

As always, it is better to know what you're getting into and be prepared with solutions. The presence of polybutylene is too much for some buyers to contend with; they will keep looking.

Others, perhaps those who have found the otherwise perfect house in a perfect location, will hear my explanation, then just shrug and say, "We'll take care of it if we need to."

Alan D. Gould is a home inspector who works with U.S. Inspect in Chantilly.Three plumbing methods were used before polybutylene pipes were abandoned. Left: Pipes were joined by plastic couplings, fitted with crimped aluminum bands. Center: The bands were copper. Right: Both the couplings and fittings were copper.Polybutylene pipes also were used in a system that employed flexible pipes and a wall-mounted central distribution point, above. The advantage was that most -- but not all -- joints were eliminated.