Proper drainage of your property can reduce the risk of moisture related damage/issues. A wet crawlspace, basement or slab can result in fungus related damage, mold growth, excessive condensation on windows and other issues. Saturated soil and erosion can also damage the foundation of the building.
One of the most important things that you can do to reduce these risks is to have proper drainage of the property, especially the area around the building perimeter. Ideally you should have a few inches of clearance between the wood building materials and the soil. Then the ground should slope downward away from the building at a rate of 1/2" to 1" per foot for the first 5 to 10 feet.
Another important factor is dealing with roof water run-off. The house should have gutters and downspouts with extensions that discharge the water at least 3 feet away from the foundation. There are many factors that contribute to moisture related problems such as lack of eaves, permeability of the soil, overall surrounding landscape resulting in a high water table, etc. Some houses seem to have no issues even without proper drainage.
If there is standing water against your foundation or if you have standing water in your crawlspace for anything other than for brief periods during heavy rainfalls, or if your crawlspace is very damp, then you should address the drainage around the building.
Swales After providing proper drainage around the perimeter of the building, swales can be used to drain the water out of the property or further away from the building. A swale is just a shallow ditch. A swale can have a very gradual slope and be covered with grass which can be cut with the rest of the lawn or it can be made to look like a creek. Sometimes a swale will lead to a retention basin to allow the water to percolate into the ground.
French Drain Another method of drainage is a french drain. A french drain collects water before it gets to the basement or foundation of the house. Perforated tubing is placed in a gravel-lined trench in the area to be drained. Water collects in the tube and is directed away from the house. One note of caution though is that this type of drain can eventually become clogged. Before you let anyone talk you into installing a French drain, get a couple of expert opinions from licensed contractors. Re-grading the soil around your house may be all that's necessary and can be a more permanent solution. The purpose of a french drainage system is to carry unwanted free standing water away from a building, such as your home.. French drains are commonly installed near the perimeter of a building at the lowest point or anywhere where standing water is found. Additionally, a french drain may commence in the basement or crawlspace. The system may terminate at any point where the water will not drain back toward the house.
Installing a french drain is a relatively simple, but labor intensive project. Obstacles, however, can make the project costly and time consuming. Some of the obstacles you may encounter are concrete walkways, driveways, tree roots, boulders, and underground utilities. French drains are generally shallow drainage systems. However, as with your plumbing waste drainage, french drain operate on the principle of gravity. The drain must slope downward. The minimum recommended slope is ¼ inch per foot. If the landscape grade runs upward along french drain path you'll will have dig deeper to maintain a downward slope.
French Drain Instructions:
Barring any obstacles, a 4 inch diameter, 20' french drain can be created in a day.
From the beginning point of the drain dig a trench about 10 inches deep and about 6 inches wide. (If your yard slopes upward you'll need to dig deeper as you move away from the starting point to maintain a downward slope.)
Next, pour in about 2 inches of rock. Lay the pipe over the rock.
Keep the perforations, or hole turned downward.
Pour in another 2 inches of rock to cover the pipe.
Back fill with soil (about 2 inches).
There should be a slight slope on the pipe (about one quarter inch fall per foot of pipe), which "engineer type people" describe as a 2% slope. This allows the water to flow down the pipe to the outlet. This slope is important because the actual flow area of a four inch perforated pipe is less than the bottom inch of the pipe. If the water gets deeper than this, then it is possible for water to flow backwards out of the pipe and back into the gravel filter, where it may travel back out of the drain trench, defeating the purpose of all that hard work! -- Incidentally, this is why it is not a good idea to connect drain pipes from the roof gutters or surface drains to the subsurface drain pipe. The small amount of time and money you save by not installing a second pipe for the surface water may cause the drainage system to fail. If that happens, then the system must be re-excavated and installed properly, which is a much larger expense than installing it correctly the first time. Once the drain is installed past the point where we are interested in intercepting and diverting the subsurface water, then what? Where do we go from here? The best outlet is to continue the pipe downhill (using non-perforated pipe) at it's proper slope until it comes out of the ground. To the "engineer types" this is called "daylighting the pipe". If there isn't enough slope to the ground to do this, then the pipe must empty into a sump and be pumped up to the gutter at the curb or into the storm sewer pipe - NOT INTO THE SANITARY SEWER!!!