How to Design a Septic Tank Drain Field

large open sewage draining field
What You'll Need
Tape measure
Graph paper
Zoning ordinances
Copy of building codes
Boring tool

A septic drain field is a vital part of any septic system. An improperly designed drain field will do nothing but cause huge problems with the entire system. When you design your drain field, there are a few things to keep in mind.

Step 1 - Determine the Size

The size necessary for your drain field will depend on a few factors. The soil should be tested for a percolation rate first. You can run the percolation test on your own, or hire someone to do this. To do this on your own, you will need to bore several holes in the field. You will need to bore holes at least 3 inches in diameter and 3 feet deep. This needs to be done in random areas across the proposed area for the drain field. Once you have soil, mark the general area on a map of the field and have the soil inspected for percolation rates.

The size of the field will be based on the size of the home and the rate of percolation. A typical home with a good percolation rate may require as little as 4,500 square feet of field or as much as 9,000 square feet with a poor percolation rate.

Another consideration should be local zoning regulations. Most localities have ordinances determining how far the field needs to be from property lines, streams, wells, water supplies, or any protected land.

Step 2 - Map Out the Piping

A proper drain field needs to have perforated pipes buried in gravel trenches through out the field. The trenches should have a slight slope, usually less than 1/8th inch per foot. The pipes will need to be placed in gravel and covered with more gravel. When you map out the piping, be sure to create a sketch that reflects the depth of the field accurately. This will allow you to plan for the amount of soil under the gravel, the amount of gravel, the space needed for the pipe, and the amount of gravel needed for the cover. A drain field trench will usually need to be about 30 inches deep.

Step 3 - Plan for Materials

Now that you have your map sketched out, you can start to procure the materials you will need. In most cases, the perforated pipes need to be 4 inches in diameter. Purchase enough pipe to run from the septic system to the field and to be placed in the field as you have sketched. Determine the amount of gravel you will need, and prepare to have it delivered to an area close enough to the trenches for you to easily transport it with a wheelbarrow or front end loader. Purchase the appropriate amount of joints, connections, and any geotextile you are using for the system. You will also need to determine how much backfill you will need. While you will have a lot of this from digging the trenches, you may need to have extra dirt to fill in the gaps.