Doesn't it always seem that as soon as you get one thing fixed in your house, something else falls apart?

Order a new toilet because the plumber says they just don't make those parts anymore, then find out the tile floor underneath is shot. Replace a stove, then the refrigerator conks out. Then, when you're hoping to get a breather, the roof gives up the ghost.

Maybe the bigger problem is that some of us just do not have a good idea how long the parts of our houses last, and what we should do to maintain them.

The National Association of Home Builders (NAHB) says Americans spend $65 billion to $75 billion annually on maintenance, repair and replacement, often because of the premature failure of a material exposed to weather.

But information on durability is hard to come by. Some manufacturers offer estimates for particular products, and some trade associations provide life expectancies for building materials or appliances. Consumer Reports magazine provides special reports on appliances and a "Fix It or Forget It" survey of consumers, updated last in 2001. The NAHB published the most comprehensive guide on structural materials and appliances in 1993. That also drew on consumer surveys and manufacturers' data rather than an independent evaluation.

Is it its last legs?

The Association of Home Appliance Manufacturers has another list of life expectancies for a variety of items, based on manufacturers' data. It came out in 2001, and the next is scheduled for 2006.

How long should it last? Here is a list of the expected life spans (in years) of major building materials, interior components and appliances: SIDING • Brick, 100 • Steel, 90 • Aluminum, 80 • Fiber cement, 50 • Wood boards, 35 • Hardboard, 25 • Stucco, 25 • Vinyl, 18 ROOFING • Slate, 45 • Clay tile, 45 • Architectural, 30 • Concrete tile, 25 • Metal, 25 • Asphalt, 20 • Wood, 20 GARAGE DOORS • Steel, 40 years • Fiberglas, 30 • Wood, 20 • Vinyl, 13 WINDOWS • Unclad wood, 39 years • Aluminum, 23 • Aluminum-clad wood, 19 • Vinyl, 19 • Vinyl-clad wood, 16 INTERIOR • Drywall, 30-70 • Insulation, 100 plus • Wood floors, 100 plus • Carpeting, 11 • Sinks, 5-30 • Toilets, 50 • Faucets, 13-20 APPLIANCES • Water heater, 14 • Central air conditioning/heat pump (outdoor unit), 15 • Furnace/heat pump (indoor unit), 18 • Refrigerator, 17 • Dishwasher, 10 • Clothes dryer, 14 • Clothes washer, 13 • Smoke detectors, 12 Sources: NIST compilation of research from industry associations, materials manufacturers, contractors, warranties; Whitestone Research and National Association of Home Builders Research Center; The Washington Post

The problem with these durability guides is that they take time and money to pull together, so they have not been undertaken independently. And none of them offers a picture of your particular house, with your particular weather conditions.

Now, there is a new game in town that could help. The National Institute of Standards and Technology (NIST) has developed a computer software package on durability and cost-effectiveness.

The computer program is part of a decadelong effort on durability by NIST, the research and standard-setting unit of the Commerce Department, and a group that also goes by an acronym, PATH. That is the Partnership for Advancing Technology in Housing, a coalition of government agencies and housing-industry players whose goal is to promote new technologies that would improve the affordability and value of new and existing homes.

Breaking down the costs

The durability software, available on the www.PATHnet.org Web site, not only lets a user see how long his current roof, windows, garage and doors should last but also calculates, by ZIP code, what it should cost to replace them with the same materials or with other options.

NIST software designer and economist Laura Schultz said the approach "comes from the Mr. Potato Head Web site" maintained by toy manufacturer Hasbro. If you click on the Making Faces game for Mr. Potato Head, you can change the way the spud looks by substituting parts. "The idea is the same, of having a frame that resembles your house and then letting you colorize and texturize it," Schultz said.

In the NIST program, you first build a rough picture of what your house looks like, and then you play around with replacements to weigh the costs against the expected service life.

The program also spits out for those four categories of building parts a rundown of the most durable option, the lowest upfront-cost option and the lowest life-cycle or annual-cost options. And you can plug in your own bids for comparison purposes.

Getting a better understanding

It is the first attempt at helping homeowners understand how the choice of materials can play out economically, according to NIST's team of economists and researchers.

It will help homeowners process tough decisions such as what kind of new roof to buy, they said. Sure, slate roofs can last 50 to 100 years, but they cost a bundle. Should you invest in slate to avoid having to replace the roof again in your lifetime; go with the cheapest alternative, asphalt shingles that last 20 years; or look for something in the middle?

"There's no convenient way currently to understand the trade-off between a longer service life and a higher installation cost," said Stephen Weber, one of three NIST economists on the project.

Weber said "the hardest thing to model" in assessing durability is the value of a roof or building product that lasts longer than a homeowner anticipates living in the house. "That's why durable products don't sell," he said, "because of lack of information about their longer service life."

Durability often overlooked

Homeowners "don't have a way to quantify the value of durability, so they don't value it," he said. And, in turn, they do not clamor for it from builders. So builders tend to offer the lowest-cost options.

Builders will take heed, however, if customers want more durability, said Christopher White, a NIST research chemist who is developing independent durability estimates for building products. The estimates will be plugged into the computer program as they are available.

"Builders are very, very responsive to their customers," White said. "As the customers understand the value of durability, then the builders will recognize and offer more durable products."

The NIST team and other researchers say homeowners might choose more durable options if they know they can get the money back in a resale.

"This gets people away from just going with first-cost estimates, and lets them evaluate lifetime costs," White said. "People know almost to a penny what their mortgage costs, but have almost no idea what it costs to live in their house."

Liza Bowles, former head of the NAHB Research Center and now a consultant, said: "The importance of NIST's durability work is really significant for the average person ... because within the building sciences we've never had good answers for how long products will last. ... What NIST is trying to do, both with this project and other research, is to create some standards. ... Once you have measures, none of the product manufacturers will want to be on the low end of the scale."

Computers aren't real life

Mark Richardson, head of Case Design/Remodeling in Bethesda, Md., is skeptical about using computers to replicate life experiences. "Much as I'd love to say 'yes, it's great,' I think it raises more questions than answers" because of the variables the program does not consider, he said. "And it's potentially kind of dangerous ... because it doesn't take into account the subtleties, say the orientation of a house, or of a window or a skylight or siding. Or ... proper installation."

As an example of a subtlety, Richardson said the paint on the south side of a house typically "lasts much less than on the north side because of the sun breaking down the finish." But, he said, "the flip side is that the south side usually has less mold."

Richardson recommends homeowners do an annual checkup on their houses and then consult with a home inspector or remodeler every few years. "Using a computer program," he said, "is like trying to diagnose what's wrong with your health instead of going to a doctor. ... For a couple hundred dollars, you could get a whole house assessment from a home inspector every four to five years and they will tell you those things and explain the subtleties."

A big hurdle for homeowners today, said Kevin Powell, a research analyst at the NAHB research center, is that they "are a lot less cognizant of how to take care of homes" than in the past. As the nation has changed from a can-do agrarian society to a less hands-on one, "a lot of the mechanical aptitude has been lost," he said.

And, while a debate rages over whether today's houses are uniformly less durable than houses of the past, Richardson said, "What is consistent is that houses today need to be taken care of more than houses in the past."

More maintenance friendly

The typical house in the 1960s, for instance, had aluminum siding, mill-finished windows, 20-year asphalt or Fiberglas shingles and few details on the exterior or interior, "so it virtually didn't need a lot to maintain it," he said.

"Now there is a lot more detail on houses — more exterior trims and soffits and fascia, for instance, that require a tremendous amount of attention," said Richardson. "And if you don't pay attention, the materials aren't going to last a third to a half as long as what the manufacturers say."

Houses today are not intrinsically less sturdy or more poorly built than houses in the past, said Powell. But, "there are a whole lot of differences" dictated by technological and lifestyle changes.

"Lifestyles have changed a lot," Powell said. "In the past, houses didn't have anywhere near the number of plumbing fixtures that people have now. ... Houses used to have one bathroom. Now there are two or three. And houses have Jacuzzis and steam saunas and wet bars." The potential for leaks and for problems with moisture and mold is much higher as a result, he said.