Which pipe to use for sewage underground: cast iron or plastic

6 min

In this article, we will help you figure out which pipe is better for underground sewage, consider the advantages and disadvantages of various materials, the required diameter and connection options.

Which pipe to use for underground sewage

external sewer pipesThe market offers our attention pipes for underground sewerage of different diameters, materials and connection options.

To make the right choice, you need to consider:

  • temperature and chemical aggressiveness of effluents;
  • volumes of discharges during periods of maximum load;
  • minimum temperatures of the climatic zone;
  • the depth and angle of the sewerage;
  • type of soil.

External sewer pipes must meet numerous requirements: to be strong, reliable, wear and frost resistant. Sewer pipes, correctly selected for specific conditions, will ensure efficient drainage of waste water for many years.

Pipe material

Sewerage can be laid underground using the following materials:

  • cast iron;
  • polyvinyl chloride (PVC);
  • polypropylene (PP);

  • polyethylene (PE);
  • asbestos cement;
  • ceramics.

The last two options are used very rarely, and more and more users are opting for pipes made of cast iron and plastic.

Cast iron pipes

Gray cast iron is inferior in its characteristics to malleable cast iron. In the modern version, sml cast-iron socketless pipes are recommended (you can learn more about them and choose on this page).

external cast iron sewer pipes


  • durable;
  • service life up to 85 years;
  • withstand heavy loads;
  • resistance to temperature extremes.


  • not suitable for saline soil;
  • are heavy, increasing the cost of delivery and complicating the installation process;
  • a rough surface reduces the flow rate of wastewater and provokes blockages;
  • high price.

Polyvinyl chloride pipes

PVC pipes for outdoor use are orange. The gray ones have a wall thickness of 2.7 mm and are not able to withstand the pressure of the soil, therefore they are used for installing exclusively internal sewage systems. For laying in the ground without boxes to a depth of more than two meters, corrugated pipes are intended.


  • affordable price;
  • resistance to temperature extremes;
  • strength, wear resistance;
  • do not collapse under the influence of aggressive chemicals;
  • the inner walls are smooth, therefore no growths form on them;
  • light weight, which facilitates installation;
  • corrosion resistance.

PVC external sewer pipes


  • if the temperature of the drains is above 40 C °, they quickly become unusable due to the temperature elongation of the material;
  • suitable only for gravity drainage;
  • used for small private houses.

It should also be noted that the characteristics of PVC pipes may differ depending on the manufacturer. In addition, such pipes can differ in stiffness, and depending on it, they can be used at different depths. The stiffest S-class pipes can be used at depths of up to 8 m, medium-stiff N-class pipes at depths up to 6 m, and the lightest L-class pipes at a depth of 2 m.

Polypropylene pipes


  • smooth inner surface;
  • service life up to 100 years;
  • resistant to temperatures up to 100 C °;
  • do not decompose under the influence of acids and alkalis;
  • light weight, which simplifies transportation and installation;
  • installation is quite simple and quick, does not require the use of special equipment.


  • soft, can deform under high pressure;
  • pipes with increased wall thickness, intended for burial at greater depths, are more expensive;
  • deteriorate under the influence of ultraviolet radiation, which complicates storage.

external sewer pipes polypropylene 22

Polyethylene pipes

On sale there are smooth and corrugated, which have greater strength.


  • resistance to loads allows PP pipes to be laid to a depth of 15 m;
  • smooth surface with high flow capacity;
  • a light weight;
  • on sale a wide range of connectors, tees and branches that facilitate the installation process;
  • service life exceeds 50 years.


  • cannot be used for drains with temperatures above 65 C °;
  • deteriorate from UV radiation;
  • can only be laid in the ground, the exit of a part of the system over the ground is unacceptable.

With a huge number of advantages, plastic has a low strength, therefore, the outer surface of the pipes is made corrugated, increasing their rigidity. At the same time, the inner surface remains smooth so as not to trap drains.

external sewer pipes polyethylene

Asbestos cement

For the production of such pipes, a mixture of Portland cement and asbestos fiber is used.


  • excellent resistance to aggressive media, so virtually any waste can be drained through such pipes;
  • durability, and in this regard, asbestos-cement pipes are among the leaders, since they can be operated for up to 100 years;
  • ease of installation, since it is quite simple to dock the individual parts of the pipes;
  • light weight, which facilitates installation;
  • such pipes are not prone to overgrowth, so they will serve as they should for many years.


  • high fragility, since even minor mechanical damage can compromise the integrity of the material. Even new pipes at the ends can have chips and cracks, so when buying you need to carefully look at them;
  • the complexity of delivery due to high fragility.

These disadvantages negate all the advantages of asbestos-cement pipes, so they are practically not used today.

outdoor sewer pipes asbestos cement


Ceramics have been used for the manufacture of sewer pipes since antiquity, and today the material is still used for this purpose.


  • high resistance to temperature extremes, extremely low and high temperatures;
  • complete inertness to any substances, even strong alkalis, even acids. Such pipes can withstand the influence of absolutely any effluent;
  • ease of installation;
  • the inner surface is characterized by low roughness, which means that it will not overgrow and clog.


  • too high fragility, which complicates the transportation of pipes and their installation. New pipes may already have cracks, and their presence is indicated by a slightly rattling sound when tapping on the pipe;
  • ceramic pipes are not very long, which leads to the need to make a large number of joints, and this negatively affects the integrity of the system;

It is because of their high fragility that it is very difficult to use ceramic pipes, and they are almost ousted from the modern market.

outdoor ceramics sewer pipes


When choosing the diameter of the sewer pipe, it is necessary to take into account the external and internal dimensions, since the wall thickness can vary significantly.

For domestic outdoor sewerage, the 110 mm standard is applicable. This diameter is enough for servicing a country house. When several houses or a whole village are combined into one sewer network, the diameter should increase in accordance with the calculations for the project.

All the time we were talking about pipes with a circular cross-section, which are used in the vast majority of cases, because drains move along them as quickly as possible. However, this is far from the only option. Sometimes elliptical pipes are used: they are necessary when the soil creates strong pressure on the pipes or when the load of the drains on them is too high. When the sewage system is not too deep (up to 1 m), pipes with a semicircular and rectangular section can be used. It is also worth noting that if the sewer system is not located deeply enough, then it is necessary to isolate it in order to avoid pipe rupture during the cold season.

sewer pipes outer diameter

Connection format

  • Welding is the most difficult one, requiring special knowledge and the use of equipment. Welding is carried out on metal and plastic pipes. Such connections are extremely inconvenient in cases of need for repair.

  • Flare – a cut of one pipe is inserted into the flare of the other. The tightness of the joints is achieved through the use of rubber seals and sealants. When using this method, when installing cast-iron pipes, tow is additionally made.

  • Socketless (flanged, sleeveless) – pipes that are even at both ends are butt-to-joint, a rubber cuff is applied to the connection and tightened with a clamp. The socketless method of connections is most convenient for repairs and replacements of a sewer section.

sewer pipes external connection

Having analyzed which pipes for underground sewage are most suitable in a particular case, you will get rid of many problems associated with the difficulties of cleaning and repairing.

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Passionate about DIY for over 10 years and all the new tools, I give you through this blog all my advice, tips, tests in all areas: carpentry, electricity, masonry, gardening, home automation ...


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