Installing drainage piping requires some small degree of knowledge about how to determine slopes and grades. Drainage piping in order to function, must run or slope downhill to allow water to flow. If you are able to use a builders level it will make your installation that much easier. You must first determine the size of the pipe you are going to need to carry the amount of water in a storm and then add approximately 50% more capacity for 100 year storms. If you can get by with a four inch pipe, use a six inch pipe to assure the pipe is capable of carrying just about any amount it needs to handle. We will use a six inch diameter pipe for this discussion. If you are starting your pipe at a catch basin or other collection point, you will need to determine the height of the outlet of the basin relative to the height of the point where you want the pipe to exit or end. Using a builder’s level, it is done in this manner. Set up the level and tripod in a position where you can see both the starting point and the opposite end of the pipe.

You want to pick something in the work area that cannot move and set this as your bench mark height. A very large rock with a place you can mark and return to over and over to measure. A side lug on a fire hydrant works as well. Do the use the top nut as these move when the hydrant is opened. Once the level is itself leveled, your helper will place a rod or tape measure on the rock measuring from the rock upwards. Looking through the builders level, you can see where the cross hairs in the level hit the tape measure. Lets say the forty inch mark. Write down the number 40 and that is called the HI or instrument height. Now using the tape measure again, measure from the bottom of the hole in the basin, read the tape again and get a measurement. Lets say this time it is fifty inches. Write that down as well and this is called your front sight. Deduct the HI from the front sight and you will get 10 inches of course. Now using the tape, take another reading or sight at the exit point of the pipe albeit a free air point or a hole in another basin. This measurement is sixty inches. Deduct this measurement from the HI and you get twenty inches. That means there is a ten inch difference in height between the two basin openings.

You must now measure the distance between the two basins or a basin and the outlet point. In this example we will use eighty feet. At one eighth inch per foot of slope over eighty feet you would need one inch of slope for every eight feet. Eight divided into eighty is ten so your ten inches of overall height difference works fine. Now we know we will have to maintain a one inch drop ( or rise) in the pipe elevation every eight feet. Most operators prefer to dig uphill if possible so you will more than likely be deducting an inch from each level shot you take. As your operator digs, every eight feet you will need to take a new shot to assure that he is sloping the trench bottom uphill one inch every eight feet. You know your shot at the lower basin or exit point was sixty inches on the tape. Eight feet away from that basin it should read fifty-nine inches. That means the pipe rose one inch. Continue this method until you reach the upper basin or starting point. If you read your measurements correctly, when you get to the upper basin, you will have used ten inches of slope to get to the upper hole.

Most installations call for some type of stone under and over the pipe. If you are using a perforated pipe, this stone is critical along with filter fabric to assure the holes do not clog with silt and to prevent rocks from crushing or piercing the pipe during back fill operations. Other piping requires this stone to provide a solid foundation against pipe settlement. Once the piping is solidly set in stone, you may back fill the balance of the trench making sure no large rocks are placed over the pipe. Large rocks will over time settle down in the soft earth and can eventually crush the pipe. Remember to add the thickness of the bottom stone to your shot measurements.

In areas where car traffic may cross the pipe trench and depending on how deep the pipe is below the ground, additional protection such as steel plates or concrete may be required to prevent crushing of the pipe.

Pete Ackerson

Pete Ackerson is a 30+ year building inspector with experience in both public and private construction industries. From schools to treatment plants, from private homes and condo projects, to large residential landscaping projects, he has worked both in the building design areas and field construction in the Eastern US. In 2006 he formed along with two other building inspectors, Wagsys LLC which produced software for municipal agencies in the fields of building departments, planning boards and Zoning Boards of Appeals.