Real estate surveys show that most homebuyers decide to buy a house within the first 15 minutes of seeing it. This kind of impulsive buying can have disastrous repercussions if the house has a major flaw or defect that's not immediately detectable. While first impressions are important, they should be buttressed by a careful inspection.

When you drive up to a new house, take a few minutes to look at it and the surrounding grounds before going inside. Study the lay of the land. The ground should slope away from the house - ideally six inches for every three feet - otherwise rain water will collect around the foundation and eventually seep into the basement. Patios and driveways should also slant away from the house for the same reason. If possible visit the house after a rainstorm. Then, if you see a number of puddles, you'll know that the house has a drainage problem.Are there any trees around the house? Big trees on the property can provide shade during the summer and keep cooling bills down. If they are closer than 10 feet, however, their roots might interfere with the plumbing or the foundation. Evergreen trees can protect your house from harsh winter winds. Willows and poplars are picturesque, but they can be a nuisance if they are too close to the house because they develop shallow root systems that spread over a large areas.

You'll want to inspect the roof and gutters. Obviously, you can't get up to the roof without a ladder. You can, however, get some idea of the condition of the roof by looking at it with a pair of binoculars. See if the roof sags anywhere. A sagging roof indicates weak or damaged framing members and a major repair job.

Look at the shingles. They should be flat, not curled, and of uniform color. Shingles of contrasting color indicate that spot repairs have been made. A few spots here and there are acceptable, but a number of wide patches suggests that the roofing should be replaced.

At some point you'll want to ask the owner how old the roofing material is. This will give you some clue as to how much longer it will last. A roof with asphalt shingles will last about 25 years; one with wood shakes, 40 years; and a slate tile roof 30 to 40 years.

Walk up to the house to examine the walls. They may be of masonry (brick or stone) or sided with wood, aluminum or vinyl. Masonry is more durable than the other materials, but over time the mortar between the bricks or stones will deteriorate. You can test the quality of the mortar by poking it with a pen knife. If it is soft and crumbles, it may need replacing.

Look for cracks in the mortar between the bricks or in the bricks themselves. A few vertical cracks are bound to develop in any house as it settles. Horizontal cracks, on the other hand, are often the result of structural problems. These are cause for concern, especially if they are wider than a quarter of an inch.

If the house has siding, sight along the wall to see if is straight without any sags or bulges. These defects suggest that the wood under the siding has decayed. Another way to test the soundness of the substructure is by pressing against the siding. You should meet with firm resistance.

Inspect the doors and windows to see if they have been caulked properly. Some small gaps are bound to be present, but wide gaps may mean that moisture has seeped into the walls over the years. Look carefully at the glazing around the windowpanes; it should form a continuous bead.

Remember that when you are buying a house, you are also buying into the neighborhood. Therefore it is important to check out the community. This is so important that many real estate agents feel that the community evaluation should be the house hunter's first priority.