Onsite Pipe Installation: What You Need to Know

Handle sewer and supply pipes with care and follow placement guidelines for a trouble-free septic system and happy customers.
Onsite Pipe Installation: What You Need to Know
PVC piping should not be left outdoors, where it can deteriorate over time. Keep pipe supplies inside if possible and well organized.

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When we talk about the pipes used in onsite systems, we like to break the discussion into three parts because the function of the piping determines the pipe needs, including size, the schedule and installation requirements. From our perspective, the three parts are the building sewer, or pipe from the house to the septic tank; supply pipes, or pipes from the tank to other parts of the system; and the pipes used for the final treatment and dispersal part of the system, including discharge pipes and pressure laterals. For this discussion we will focus on building sewer pipes and supply pipes. 

The outside clean-out 

Starting from the edge of the house, we like to see a double sweep clean-out added to the building sewer pipe on the outside. Your state’s rules determine where the household plumbing stops and the septic system installation begins, and therefore which contractor may install the clean-out. 

Regardless of who provides the feature, it is helpful from a system management standpoint. First, it allows access to the sewer line outside the building, which means the service provider does not have to locate and use the clean-out provided inside the house to clear a blockage. This reduces tracking into and out of the house. Often the inside clean-outs are covered or hidden by a finished wall, making it difficult or expensive to access. The outside clean-out allows the work to be performed without having to enter the house at all. With many houses being vacant during the day while the occupants are at work, the service provider is not waiting on access to get the job started. 

Piping to the tank should be laid in a properly bedded trench that has been compacted. The pipe should be Schedule 40 PVC and on a slope sufficient to ensure that both the solids and the liquid make it to the tank. For both building sewer and supply piping the suggested slope to move liquid by gravity is 1/4 inch per foot for 3-inch or less diameter pipe and 1/8 inch per foot if the pipe is 3 to 6 inches in diameter. This should ensure the velocity of movement in the pipe is more than 2 feet per second but less than 10 feet per second. 

This means the movement is fast enough to carry the solids along with the liquid, but not so fast the solids and liquids separate. A couple of things to note here: These numbers come from a time when other piping materials were used, including cast iron; and when the supply piping is only carrying effluent and not solids, the slope is not quite as critical. The bottom line with both building sewer and supply pipes is that between sewage events in the house, only air should be in the piping. 

Compact the beddding 

Schedule 40 pipe is recommended both going into and flowing out of the tank if there is more than one tank running in sequence. The bedding for the tank must be compacted so the tank does not settle, bowing the pipe. The pipe should not be connected into the tank until the space between the tank and the original soil has been backfilled and compacted. The pipe must be well supported at both the inlet and the outlet. This also may mean piping is not connected until everything is laid in place, which will require a joint to connect the piping. 

For supply pipes from a tank to a gravity system – so to the distribution or drop box – the piping should be laid on a slope so the effluent flows to the box without standing water in the pipe. Our recommendation is to provide access to the distribution box or drop boxes in sequence. This is most often accomplished with an inspection port out the top of the box to the ground surface. This provides a way to look in the box without digging it up every time and obviously helps locate the parts of the system for inspection or maintenance.

In cold-weather areas where systems are installed shallow we are seeing an increased use of insulation to protect from freezing. This is a necessity where the pipe runs under a driveway or some other hard surface area. 

Supply pipe running from a pump tank, pumping to a drop box and gravity distribution or to a pressure manifold and pressure laterals must be laid on a slope sufficient to drain back to the tank. Remember to put a weep hole in the piping inside the tank so all of the effluent does not drain back through the pump. It is important this pipe is laid in a well-compacted bedded trench as well, so no bellies – or low spots – develop in the line that could collect water and lead to freezing problems. 

Handle with care

Although PVC is relatively resilient, care should still be taken to prevent denting or scraping the pipe when loading, unloading and storing the stock. Damage can occur if tie-down straps are over-tightened. Although the pipe is light, you should resist the tendency to throw or drag it. Contact with sharp objects should be avoided. All of these actions can cause damage or cracking. Piping is not expensive, but it slows installation if you are spending time cutting off sections of cracked or damaged pipe. 

When storing pipe, try to protect it from direct sunlight, excessive heat and potentially harmful chemicals. Pipe should be stored indoors if possible. If it has to be stored outside, it is best if it is covered with an opaque tarp. This is always an interesting conversation with installers in the Southwest who deal with extreme temperatures. When stacking, it is best to have the pipe with the thickest wall on the bottom of the pile. If long sections of pipes are stored on racks, make sure the pipe is supported along its entire length and not allowed to bow in the middle.


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