Tips on Foundation Soil Grade Around Houses
New homes and room additions are frequently the victims of poor planning with regard to grading. I can't tell you how many houses I have seen that were put too deeply into the ground. This condition causes marshy ground, wet basements or flooded slabs. In virtually every case, a simple drawing or calculation would have solved the problem.
Many modern building codes have responded to this problem. They require that the top of foundations or slabs be at least 6 inches above the highest point of soil at any location around the house. Furthermore, the ground must fall away from the foundation at least 6 inches within the first 10 feet around the perimeter of the house. Note that this is a minimum requirement. The more slope the better.
Using these calculations, this means that when laying out a new house, you need to pay attention to the existing grade before you dig. In fact, you need to site the house and see just what the highest elevation of the ground is at any point within 10 feet of where the foundation will be. Knowing this, you can now begin to dig the foundation.
I always made a point to put the top of my foundations 18 inches above this highest point. You would be surprised how quickly the dirt from the hole disappears when spread out around the house. Although the foundation would look high prior to backfilling, the ground had a very gentle slope once all grading was completed.
In my opinion, you can never have too much slope away from your house. My guess is that anyone who has a wet basement or ground that slopes back towards their foundation will agree.
Those of us with existing houses that have grade problems face different challenges. Landscaping, sidewalks and other improvements must be dealt with in trying to correct grade problems.
If you are lucky enough to have a sloped lot, your task of establishing grade can be accomplished. It may take a small piece of earth moving equipment like a Bobcat or skid-steer loader, but it will be worth it.
If you have a situation where ground is slopping towards your house (houses built on hillsides), the trick is to slope the ground gently by creating a swale. This swale, or ditch, allows you to do two things. It gets water away from the house and at the same time collects the water which runs downhill towards your house. You direct this swale around a corner of the house and continue until the natural slope of the ground is falling away from your structure.
Those of you who live in houses on flat ground face a more serious challenge. Sometimes the ground is so flat that there is no way to easily create a swale or sloping condition. In these cases, you need to pipe roof water as far away as possible. Downspouts that dump water onto the ground near the house can cause serious problems. You would be surprised at the volume of water a 1 inch rainfall can produce.
You can also consider surrounding your house with a moat, something like the old castles used to have. This moat is simply a ditch that is dug around the problem areas of your house.
You dig this ditch as wide and as deep as you can handle. A 2 foot wide by 2 foot deep trench can be very effective. Once this trench is excavated, fill it to within 1 inch of the top with large 1 inch washed gravel. This trench acts as a collection area for surface water. As long as your soil can absorb water (even at a slow rate), you will have improved drainage conditions around your house.
Only in very wet seasons, when the water table rises around your house, will you experience problems.