AHouses are like people. Even with the best of care, over the years the frame, wiring, pipes or systems will weaken eventually. As time goes by, signs of weakening likely will begin to appear before a total breakdown occurs.

An architect or contractor's experience and some thoughtful pre-construction exploration can go a long way toward predicting hidden conditions before renovationwork begins. A look in the basement and attic can reveal the structure, as well as pipes, wires and ducts running to and from the area slated for renovation. Such an inspection can give an experienced professional a good idea of what lies behind the plaster or sheetrock. But even the most knowledgeable contractor can't see through walls. A homeowner would be wise to budget a little extra money for unexpected surprises.

An older house - say 75 years or more - most likely has undergone numerous changes over time. Walls that are opened up can reveal fire or insect damage to wood studs. Sometimes damage is so extensive that the wood framing must be repaired before moving ahead with a renovation.

You might discover that, because of an earlier renovation, some of the floor framing runs in a direction opposite to what you had expected. This also could indicate that a wall originally not thought to be load bearing-integral to the structure of the house-actually is. Thus, if you wanted to remove that wall, you would need a structural beam to support the load it bears.

Sometimes demolition reveals old-fashioned knob and tube wiring behind the walls, even when the visible and easily accessible parts of an electrical system have been updated. Once the old system is exposed, it might be necessary to replace it with modern wiring.

At times, opening a wall or ceiling can reveal old water pipes or drain lines that have developed tiny leaks. These leaks may not have created any visible damage, but they could have caused rot or mold to occur and most likely will need to be replaced.

In many older houses, pipes were wrapped in asbestos insulation. Asbestos may have been removed from exposed pipes, but left in place on pipes still in a wall. Asbestos causes no harm left undisturbed behind a wall. But once exposed, an asbestos remediation specialist must be called in if the wrapped pipes need to be handled. This can cause delays in the project and add to the costs.

These are a few examples of surprises you might find when renovating an older house. A skilled architect or contractor, like an experienced doctor, is attuned to reading these signs and tracing problems to their sources. Some problems are so deeply hidden that they won't turn up until more in-depth exploration occurs.

But please don't let unforeseen conditions like those mentioned above scare you away from your project. Older houses can be rare treasures and are well worth the trouble to renovate as long as you are prepared for the unexpected.