If the basement of a house smells of mildew and is six inches deep in mud and frogs, there's obviously a problem. If the house is brand new, problems may not yet have developed but may be equally serious in the future.

As with new and used cars, the only difference between problems with new and used houses may be the feeling of outrage and betrayal born of an expectation that for all that money and a fresh new start, there should be no problems. Often that is the case, but there still are lemons. Houses, like cars, should be approached with good-natured distrust and as much information as possible.

The first step in avoiding water and structural problems is recognizing the problems that may arise on a given site, their causes and their symptoms. Will it flood? It will if it is on a flood plain. Will it slide? It will if it is underlain by unstable soils and the building is improperly prepared. But where are the flood plains and where are the unstable soils? This information is available on land-use maps.

Geology and land-use maps for local counties are available from the U.S. Geological Survey, Map Sales Office, National Center, Reston, Va. 22091 (860-7000), or 1200 South Eads St., Arlington, Va. 22202 (557-2781). The USGS maps of Fairfax County also may be obtained from the Office of Planning, Massey Building, 4100 Chain Bridge Rd., Fairfax, Va. 22030 (691-4253). USGS Circular 806 lists maps and reports available for the Washington-Baltimore area.

Rocks and soils have different characteristics that must be considered for successful building, site by site. The Bright Seat Formation in Prince George's County is a well-drained greensand, which makes excellent foundation material, yet right next to it are clays of the Potomac Group, the most troublesome material in the entire Washington area, subject to swelling, settling and sliding.

When these symptoms appear on slopes, they are commonly interpreted as "erosion" but actually are slope failure -- the inability of a slope to maintain its angle under a load. Watch for trees, especially saplings, that are standing at any angle except straight up. Tilted trunks show that the slope is moving. In clay areas especially, watch for slip scarps or small landslides. These are crescent- or horseshoe-shaped bare spots; below the crescent will be a bulging area of jumbled, uneven soil. They are seen in over-steepened hills and cuts.

What is "over-steepened?" Anything above its "angle of repose," that is, anything that is asked to stand at an angle higher than its physical characteristics allow. The angle of repose of a mass of ball bearings or of water is zero degrees. If you try to dig a hole or make a cut in a pail of water, the water will slide back to its stable angle -- flat. The angle of repose of the Potomac clays is 3 to 5 degrees, not much higher than water's. Cuts for roads, houses, or the mass and height of material behind retaining walls should be considered accordingly.

Cracks should be evaluated carefully and never dismissed as "just settling." They are extremely useful warning signs because they show how a force is acting against a structure.

Because mortar joints are the weakest part of a wall, movement may show up there first, as stepwise or horizontal cracks. Vertical cracks, breaking through mortar and block alike, are extremely serious and indicate severe pressures and rapid movement. They demand immediate remedial action to avoid loss of the wall and possibly the house.

What if the walls are paneled? If paneling is warped, water-stained or very new, you might want to look behind it; if that is not possible, examine the outside walls with extreme care. Also, check for doors or windows that do not close properly, especially if they show signs of recent planing. While sticky windows or doors may be due to other causes (such as a Washington summer), the possibility that they have been caused by shifting of the foundations and walls should be examined.

Time also must be considered; a brand-new house with half-inch-wide vertical cracks is not "just settling" -- it is on the move.

After evaluating the geology and what effect it may have on the house, one still must determine the exact construction methods that were used, regardless of location.

In one bizarre case, a house was built over Wissahickon Schist, a kind of rock that weathers to a sandy, well-drained material. These soils are quite stable, yet a slump scarp appeared on the very gentle slope above the house, and the basement wall bulged and cracked.

Such behavior is expected in the Potomac Group clays, but in the Wissahickon, it is physically impossible. Nonetheless, the hill was sliding so badly that the house had to be evacuated immediately for fear of imminent collapse.

The builder, determined to guarantee a dry basement but not understanding geology or soils, had believed that a mass of clay would form a waterproof barrier to ground water. He had, therefore, actually cut away part of the slope and refilled it with a particularly nasty swelling clay imported from Prince George's County.

To avoid some water problems, a good general rule is to avoid areas of high water. This is not quite as obvious as it might seem. Flood plains are really part of the river, but this may be less than obvious or hard to remember in the dead of winter or a July drought. Native vegetation is a clue; watch for willows and the papery yellow river birch, which grows in areas of very high or fluctuating water table. Here one builds at one's peril. And many do.

The words of our grandfathers, "if God's willing and the creeks don't rise," have little meaning to many of their grandchildren. The reaction many of us have after having violated the Law of Inevitable Consequences is annoyance or outrage that such impertinances of nature are allowed to incur. They do and they will again, so check the geology maps.

Those who deal better with fact than theory should go house hunting on a rainy day with followup visits after a rainy period. Does the property flood? Is all the grass lying down and pointing one way? Are walls damp? Are there cracks in the walls that signal a break in the waterproofing compound and an entry point for water? Do downspouts dump uphill or alongside the house? Is water able to enter basement-window wells or doors? If there is a sump pump, does it run constantly? Or is one needed where there is none at all?

On inside underground walls, look carefully for "flowers" -- white, powdery residue made up of tiny crystals of gypsum or calcium carbonate. Minerals leached out of the masonry by moving water are deposited by evaporation on the inside wall. You will find them under blistered and peeling "waterproof paint" or directly on unpainted walls. They are a sensitive indicator and valuable for detecting the extent and location of problems that, being less than cataclysmic or too new to have produced mildew or cracks, might be otherwise overlooked.

Many excellent examples of bad foundation construction and very silly attempts at remedial engineering are best appreciated during a rainstorm. In one instance, the basement, cut into a slope, intersected a layer of hard clay. Ground water sliding down this "confining layer" on its way to the creek below the house was dammed behind the basement wall. Water table rose to the surface, producing a constantly soggy backyard.

The division between ground water and surface water is a shaky one and, given the opportunity, changes constantly. If surface water cannot enter the ground because water table is already at the surface or an impermeable soil or a parking lot is in the way, then additional water will remain as surface water and pool or run off.

In the example above, because the water table was already at the surface, all rain water flooded through the property as runoff. To alleviate the flooding, a series of catchments were built. These were wide, impressive and extremely expensive, but only six inches deep. In this case, even a six-inch dent in the ground-water level would have been an improvement, but the catchments were built with solid masonry sides so that they were entirely limited to intercepting surface runoff (with leaves, brush and mud). The connecting drainpipe was not sloped uniformly, and the joints between pipes laid at different slopes were not sealed. On a rainy day, enthusiasm for the lovely architecture, excellent floor plan and delightful landscaping was tempered somewhat by the sight of a six-foot-high spurting fountain of water where no fountain was ever intended to be. A rainy day might give a buyer the information he needed not to buy that house. On the other hand, understanding the principles behind the problem -- and therefore how easily the problem could be solved if approached properly -- might make the house a good buy.

It is possible to ruin a perfectly good building site by poor construction techniques. It is also possible to build a perfectly satisfactory structure on a less than ideal foundation and to resolve a seemingly impossible problem -- if the causes are understood and the related physical properties involved are considered and respected. They must not be ignored or confronted head-on, but be turned gently and with understanding toward one's purpose. "Nature to be commanded," said Roger Bacon, "must be obeyed."

NEXT: Fixing wet-basement problems.