This requires knowing where the water is coming from. Often basements are built below the water table when the water table rises during hard rains or wet springs. Sometimes the water is coming from poorly directed roof drains or puddles forming along the foundation. If your basement has a water problem or even if it hasn't, but you intend to finish the basement and you don't want your work ruined, it is well worth the effort to ensure the basement stays dry.
- Proper grade around the foundation
Surface water from rain or snow melting should not collect against the house. If it does, it may work its way down the foundation looking for a path in. Always make sure the dirt is graded in a slope, even a gentle slope, away from the house, if possible. If your house does not have gutters and downspouts, you can ensure the water drains away from the house by using 6-mil black plastic and landscaping rock. Slope the ground about 6" in 10' and cover the ground with an 8ft width of plastic. Wrap it up on the foundation 2 inches and then cover the plastic with a 2-inch layer of rock. This will ensure the water drains away from the house before soaking in.
- Roof runoff and downspouts
The best option would be to have gutters collect the water and downspout that drain away from the house. I run the downspouts into PVC pipe and then run that underground away from the house. It helps of course to have a house on a slight hill so that the pipes can surface away from the house. If your house is on a flat lot with no place to drain to, you can consider putting in a drywell or a bubbler. A drywell is a drum or tank in the ground, full of holes (and now bottom) that can accept your drains and then let them seep into the ground away from your foundation. The bubbler is similar to the drywell, but has a grate at ground level so water can bubble out the top if it isn't leaching away fast enough. At the very least, make sure the downspouts direct the water on to splash blocks that direct the water away from the foundation.
- Footer drains
These are the least easy to retrofit, yet best protection. If you are building a new home, be sure to include them. A footer drain runs all the way around the foundation's footer BELOW the level of the basement floor. On a lot with a grade, the footer drain pipe can be piped to daylight downhill from the house. In this case, the footer drain pipe should be run outside the footer. All though it means a longer trench, the further from the house the better. Not because of the water, but the cold. The open pipe allows cold outside air to run up against the footer, but a long run of pipe minimizes the effects. On a level lot, the footer drain pipe will run inside the footer and should run into a sump where a sump pump can be installed to pump out any water which collects in the sump. For very wet locations, the ground under the basement's floor slab should be filled with crushed stone, and that area connected to the footer drain pipe, by running a pipe through the footer for outer footer drains, if necessary.
To correct the problem on an existing house, you, or a hired contractor, should install the footer drain inside or outside the house depending on your lot and relative ease (neither is easy) of the work. Digging all the dirt away from your foundation is not easy and may ruin your foundation plantings. But, likewise, cutting into the basement floor to install the drain pipe inside the footer is a difficult chore as well. There are contractors who specialize in this work.
By installing a footer drain, you will keep a rising water table from rising into your basement. On bad days, I have seen, on some floors, water coming up through cracks in the floor making small fountains a couple inches high, as well as water coming in all along the floor to wall seam and every crack in the wall. When a footer drain was installed it eliminated the water problem entirely.
After you are sure no water will be coming in, it is a good idea to seal out as much moisture as you can. Concrete and cement block foundations do allow a significant amount of moisture through. And moisture is constantly trying to make its way from areas of high concentration to areas of lower concentration.
Before finishing the walls inside the basement, I recommend using a concrete sealer. I have used and recommended a product called Thoro-Seal. It is a portland cement product that mixes up sort of like cement and it painted on with a big heavy brush. It is fairly inexpensive and goes on easy. You have to mix it up, hose down the wall, and slop it on. It leaves the wall with a rough texture. If you don't use that, you may opt for one of the epoxy paints also made for this purpose. Remember though, while these products will seal out moisture and some water pressure, they will NOT keep a basement dry if there is any significant water pressure. In other words, don't count on these to be the cure for a wet basement. In most cases, you will still need to run a dehumidifier in your basement in the warm months. It will be more humid in your basement since the relative humidity goes up when the temperature drops. So those humid summer days will cause the basement to be even more humid and damp. Also, some moisture will come up through the floor and should be eliminated.