PEX, also known as cross-linked polyethylene pipe, is a flexible, non-corrosive polymer tubing primarily used for residential water supply. However, it does have numerous other uses too. A commonly asked question is, can you bury PEX underground like you can metal or PVC pipes?
Among the three types of PEX, two types, PEX A and PEX B, are suitable for burying underground. These two PEX tubing options have enough flexibility to allow them to withstand the fluctuations of water freezing and thawing, but should be buried below the frost line. PEX B is the preferred choice for burying underground.
In this article, we’ll look at burying PEX, including the different types, the depth for placement, and the advantages of PEX over PVC. We’ll also discuss the types of fittings recommended for PEX tubing, advantages of PEX tubing, connecting it with PVC, and burying it in concrete. Our goal is to provide you with the information you need for burying PEX pipe in the ground.
Contents (Jump to Topic)
Can You Bury PEX Pipe in the Ground?
PEX tubing has many uses. It is mainly used for residential water lines but also as transport for heating and cooling components, chemicals, and even sewage. PEX can be used in most instances where you’d use pipes made of copper, CPVC, or PVC.
PEX is a popular choice of piping for direct burial underground due to its ability to expand. Thus reducing the chances of freezing which is a major risk of hard piping. However, water can still freeze in PEX in low temperatures, which can weaken the pipe, blow connections or fittings, or cause the pipes to leak or burst.
To prevent freezing of the PEX tubing, it’s recommended to bury the piping below the frost line. You can check with your local or municipal water company to find out the appropriate depth. When running PEX pipe underground, pouring sand around the tubing can prevent rocks and stone chips in the dirt from damaging the pipes.
Another option is to run the PEX through PVC piping, which acts as a sleeve to protect or insulate and shield the tubing from damage and even freezing. It can also make it easier to do repairs if the PEX does get damaged. Remember to check with your local building code before installing to make sure there are no restrictions or limitations, and your local utilities to ensure there is nothing buried where you plan to dig..
Is Burying PEX Pipe Underground Up To Code?
PEX piping meets all major building code requirements, including for the use of cold and hot water distribution applications, radiant floor heating, and water service lines. However, you should check to see what codes (local, state, federal) meet your situation to ensure you’re in compliance. Codes have been updated constantly since the adoption of PEX systems in 1993.
Which codes do you need to meet? For questions, you can consult two organizations – Plastic Pipe and Fittings Association (PPFA) or Plastics Pipe Institute (PPI). You can get training aids or answers to technical questions from either organization.
Using PEX for underground applications must meet C904-06 American Waterworks Association (ANSI/AWWA C904-06). This standard defines the designation code (PEX 1006 – ASTM F 876) of a standard PEX pressure pipe material. It states that underground water service lines must be between ½” and 3” while conforming to SDR9 standard dimension ratio.
All PEX tubings and fittings are suitable for direct burial applications, given you’re meeting the allowances set by the manufacturer’s written installation instructions. Fittings allowed by this standard include crimp (ASTM F1807), cold expansion (ASTM F1960 or F2080), mechanical compression, and press for both cold and hot water distribution systems. It also includes PEX systems meeting ASTM F876 (Standard for PEX tubing) and ASTM F877 (Standard for PEX hot and cold water systems).
What Type of PEX Is Best for Underground Use?
PEX – cross-linked polyethylene (sometimes called XLPE) – comprises ethylene molecule chains, or polymers. These non-connecting polymers create the flexibility of PEX in two density types – high (HDPE) or low (LDPE). HDPE has tighter packed polymers, giving it more impact resistance than LDPE. When HDPE goes through a process called cross-linking, the unconnected polymers become connected.
Cross-linking of HDPE gives the material more durability and resilience, which allows it to work better for radiant and potable applications. There are three cross-linking processes – A, B, or C. Only PEX A and PEX B are suitable for burial, with B being the best.
PEX-A
PEX-A is HDPE cross-linked through a process called the Engel. With this method, peroxides (oxygen) mix inside the HDPE before forming, which allows for a stronger bond. This type of PEX tubing has the highest cross-linking, therefore, the most softness and flexibility. In terms of which type of PEX is best for burying underground, A is second-best.
The extreme flexibility allows the PEX tubing to have better expansion against freezing water, which provides a high degree of resistance to cracking against frigid temperatures. But unfortunately, PEX-A also has a lower bursting pressure, meaning it won’t survive lower temperatures as well as PEX-B without bursting. For this reason, it’s rarely recommended to choose PEX-A for underground burial.
PEX-B
PEX-B is the first material of choice for burying underground due to its harder rigidity compared to PEX-A. It’s also the most affordable type of piping, with all the same benefits as PEX-A. This product goes through a silane process (also called moisture or steam cross-link) after it’s extruded into a tube. Then the tubing goes through steam formed within a silane catalyst that causes the polymers to create cross-links.
The production process causes PEX-B to have more rigidity, making it more difficult to manipulate during installation. But it also makes it more durable against bursting under freezing temperatures. The higher bursting pressure is one of the main reasons why PEX-B is the best tubing for burial. Not only will there be less chance of pipes bursting, but there are also fewer issues with leached chemicals. The reduced flexibility of PEX (due to a lower ratio of cross-links) means that if the tubing kinks underground, you’ll have to dig it up and cut out the kinked area, and replace it with new tubing and fixtures.
PEX-C
PEX-C is the stiffest type of PEX and is not suitable for burying in the ground. This type of tubing has cross-links formed due to the exposure of the HDPE tube to gamma radiation or an electron beam. The rigidity of PEX-C means it’s more prone to kinking, cracking, and bursting during freezing conditions. When used for burial, this type of piping will require frequent repairs due to damage. The best use for PEX-C is when you need a sturdy, straight pipe, or need to make short repairs. However, it isn’t suitable for running continuous lengths of piping underground.
| PEX | A | B | C |
| Flexibility | Most | Medium | Little |
| Burst pressure | Medium | High | Low |
| Cross-link process | Engel (oxygen) | Silane steam | Gamma radiation |
| ASTM Standard Approved | Yes | Yes | Yes |
| Burial Underground | Yes (Possible) | Yes (Best) | No |
Can PEX Fittings Be Buried?
A big selling feature of PEX tubing is that you can run it for long distances without using a lot of fittings to seal joints. When there is the need to connect PEX with fittings, it must be done with mechanical fasteners (transitional or directional) but never with solvent cement. When deciding on the proper fittings for your PEX pipe running underground, start with the manufacturer’s instructions. Many times, there will be brand-specific fitting systems.
All fittings have regulations that define the standards for material and performance. You should see a certification mark from a third-party agency, including (among others) –
- CSA
- NSF
- UL
- IAPMO
- IGC
It’s also important to check with local building codes. There may be limitations on what types of fittings may be used in direct burial applications since these areas are typically inaccessible, making repairs more difficult.
Cold Expansion Fittings – PEX Reinforced Ring
Cold expansion is one of the most common types of fittings. This method involves placing a reinforcing PEX ring over the end of a PEX piping and then expanding the tubing before inserting the fitting. The end of the pipe then forms a seal over the fitting as it retracts back into its normal shape. To perform the expansion of the PEX ring and tube, you will need an expander tool. Fittings using a PEX reinforcing ring must meet safety standard ASTM F 1960.
Cold Expansion Fittings – Metal Compression Sleeve
You can also use a cold expansion method for metal compression sleeves, which requires expansion of the PEX piping before it’s pushed over an oversized fitting. A metal compression sleeve goes over the connection once the piping returns to normal size to keep the fitting in the pipe. You also need a tool to expand the tubing and for slipping the sleeve into place. Cold expansion fittings using metal compression sleeves should be ASTM F 2080 certified.
Insert Fittings – Plastic or Metal
Metal crimp rings fit over the PEX tubing and keep it held onto the fitting. Crimp rings are copper or stainless steel, while the fittings can be bronze, plastic, brass, or copper with a ribbed or barbed annular end. To create a seal with insert fittings and PEX piping, the crimp ring goes over the tubing (pushed back from the end) before the hose goes over the fitting. After the fitting goes into the tube, you slide the crimp ring down over the ribs (or barbs). Finally, you’ll compress the ring with your crimp tool.
Copper crimp rings are not recommended for burial due to the propensity for expansion when exposed to frozen water. Over time, this issue can cause leaks. Stainless steel crimp rings are corrosion resistant, making them better for underground use.
Push Type Fittings
Before you choose to use push-type fittings, you must verify that installation does not go against local codes or manufacturer instructions. Not all types of push fitting are appropriate for use underground. You will need to look for IAPMO-IGC 188 and ASSE 1061 standards.
Push-type fittings are often called quick connect due to having an interlocking piece that links the fitting and the pipe. The pipe pushes into the fitting and secures as the interlocking piece creates an anchor that secures the tube to the fitting. Many quick connect fittings feature an o-ring (or gasket) that gives the PEX pipe its seal. To make this fitting permanent, run a support liner through the pipe and add a twist collar or snap ring (or another locking component).
Another possibility is Sharkbite fittings – a brand of brass (lead-free) push-to-connect fittings with EPDM O-rings with stainless steel 316 grab rings. When using Sharkbites for burial, the fittings require a protective coating with an impermeable material (try silicone wrap) that will block ground contaminants.
How Deep to Bury PEX Water Line?
PEX pipes and all underground piping should be buried at least 12” to 18” below the frost line to protect the pipes from freezing. The frost line refers to the furthest depth underground groundwater begins to freeze, also called the frost depth. In colder zones, the frost line can be three to four feet down. You can check specific data from the US Department of Commerce or refer to this map of US frost lines. Values range from 0” up to 100” underground.
Can PEX Be Used Outside?
Although PEX pipe is excellent for underground use, it’s not rated for outdoor use unless properly protected. For example, PEX piping should not run along the ground without wrapping it or burying it. PEX piping used outdoors must have an insulation wrap to protect the tube from freezing. The wrap can also prevent the tubing from damage due to the friction of expansion and contraction. PEX can develop holes if there’s friction on a pipe that isn’t insulated.
PEX tubes will also degrade faster due to UV ray exposure, resulting in a shorter lifespan. One way to prevent sun damage is to use a UV-blocking material to wrap pipes that run above ground. You can also apply a coat of latex paint over the tubing. Abrasion clips (suspension clips) should brace pipes that run through wood joists, studs, or are encased in concrete.
Do PEX Pipes Burst or Freeze?
The flexibility of PEX tubing means that the pipes experience expansion if water freezes in the lines. Once the water thaws, the tube will contract back to its original shape. This ability makes PEX pipe freeze-damage resistant. However, that’s not the same thing as freeze-proof. For example, if the temperature drops below 20℉, water will freeze in the pipes.
The high-pressure potential of PEX means that there’s less risk of the pipe bursting due to the water turning to ice. When ice forms in hard lines, there’s a higher risk of the pipes cracking or bursting. Remember though, the repeated expansion and contraction action can cause the tubing to weaken, leading to the pipes leaking or failing. In addition, if there are cracks present when water freezes in the lines, it could cause the tubing to rupture.
Is PEX or PVC Better for Underground?
PEX and PVC are both types of plastic. PVC though, is a hard rigid pipe (think of the plumbing under your sink) with no flexibility, while PEX is extremely flexible (except for PEX-C). PVC (and CPVC) was around years before PEX, making it a more common choice. Many experts will still recommend PVC over PEX.
PVC does offer higher resistance to corrosion, a low price point, and an extended lifespan. Plus, PVC can also stand up better to tree roots than PEX or other piping. Unfortunately, the rigidity of PVC makes it less reliable against freezing compared to PEX pipe. This reason is why PEX is a preferred choice for underground piping.
Advantages of PEX Tubing
- Flexible
- Temperature resistant
- It doesn’t require as many fittings due to coming in a coil – although PEX is usually an unbroken line that doesn’t need connectors. If you have to use fittings, choose plastic PEX or dezincification-resistant brass PEX in areas with aggressive soil or water. Stainless steel clamps rings are more corrosive resistant than copper crimps
- Less expensive than copper, PVC or CPVC
Can PEX Be Buried in Concrete?
PEX is an HDPE (high-density polyethylene) material which makes it strong enough to handle going inside concrete. This tolerance and PEX tubing’s flexibility make it common for radiant heating embedded in concrete floors.
Connecting PEX to PVC Underground
When trying to connect PEX to existing PVC underground, the best method is to use a pair of threaded adapters and some work. Use a PEX fitting featuring female threads and, for the PVC, a socket solvent fitting that has male threads. When connecting metal to plastic threads, it’s best to have plastic threads on the male end and metal for the female fittings. It may be easier to fit the PEX coupling to the PVC coupling before cementing it to the PVC pipe, especially if twisting is needed to thread them together. You can also use push-to-connect adapters like SharkBites, which connect PEX with CPVC, PEX, or copper pipes.
Conclusion
PEX tubing has multiple uses, with the most common being for residential water supplies – as evidenced by PEX pipe coming in red (hot) or blue (cold). In addition, the flexibility of PEX (A & B) makes it an ideal piping system for burial because the pipes can expand in response to water freezing inside.
While both PEX-A and PEX-B are suitable for use underground, PEX-B offers everything you could want. It has flexibility without sacrificing durability, a high burst pressure, freeze-damage resistance, and the best price – making it the best type of PEX pipe to bury in the ground.