Can You Run PEX through Floor Joists? (Explained)

PEX is the latest technology in the plumbing industry. Many property owners are switching to these high-density plastic tubes when installing water supply lines because it’s more affordable, sustainable, and acid resistant.

Being a new plumbing product, not many people are familiar with its installation process, especially if your project has floor joists.

Because of that, today, we will be focusing on whether it’s okay to run PEX through floor joists and everything else you may need to know.

Floor joists made of lumber on construction site

Can You Run PEX through Floor Joists?

One of the advantages of using PEX is that you can snake them through floor joists. However, it’s not as easy as it sounds that’s why most people avoid this installation method.

floor joist is very different from the typical floor setup. The main purpose of a floor joist is to distribute the load of a structure.

The beams at the top squeeze when exposed to compression, whereas those at the bottom stretch due to tension. However, there is a neutral area in the middle where bending or stretching doesn’t occur.

You need to understand the working mechanism of a floor joist because the best place for you to drill a hole for PEX to run through is at the neutral area. Ideally, the hole where the PEX will be installed should be 2 inches from the bottom and the top of the beam.

The other thing you must remember when installing PEX through floor joists is that the hole you will drill shouldn’t be more than 1/3 of the joist’s depth. It will weaken the joists and interfere with their load-bearing abilities if it’s bigger.

Finally, you also need to know where to drill holes for PEX on floor joists. Holes can be drilled anywhere but not near rim joists and walls.

Do Building Codes Approve the Installation of PEX on Floor Joists?

You can never be too sure unless you refer to your local building codes. As you may already know, various areas have different building codes. Therefore, local construction regulations may recommend or advise against installing PEX on floor joists.

If they approve of running PEX through the floor joists, you need to keep the above guidelines in mind.

Will the Expansion of PEX Be a Problem When Installed through Floor Joists?

PEX can contract and expand by 1 to 2.5 inches per every 100ft. These changes can be observed with as little as 10°F changes in temperature. As you install PEX through your floor joists, you need to be aware of this factor.

If the PEX plumbing isn’t ferrying hot water, you have nothing to worry about. But if it’s going to be carrying hot water, the holes on your floor joists may not accommodate an expansion of up to 2.5 inches unless the beams are pretty big and thick.

Before proceeding with this project, refer to what I discussed above. The hole you are going to drill on your floor joists shouldn’t be more than a 1/3 of the joist’s depth.

If the joists are too small, you don’t have to risk running PEX through them because there won’t be room for expansion.

Can PEX Weaken Floor Joists?

Moisture damage and rot are usually the main things that weaken floor joists. But PEX installation can also bring a similar result.

When installing PEX plumbing, you have to drill holes in the floor joists. If you drill holes in the wrong places, such as areas that experience severe bending or stretching, you will weaken the floor. And it may eventually start to sag in that particular area.

The other way PEX can weaken floor joists is if you dig holes that are too wide. Even though you may be doing this to ensure the PEX fits snugly, it can impact the floor joist’s load-bearing abilities.

Because PEX has the potential to weaken floor joists, you need to be careful when installing this modern plumbing system.

Should PEX Run Through Floor Joists or Under Them?

That will depend on where the floor joists are located. If they are in a story building, you will need to run the PEX through the floor joists. Otherwise, if you put them under, they will look disorganized and hideous.

However, if the floor joists are in a basement or crawl space, you don’t have to go through all the trouble of running PEX through them. In such a scenario, you can run the PEX plumbing under them, and everything will work and look just fine.

Whether you should run PEX through or under floor joists depends on the location. The bottom line is that as long as no one will be seeing the PEX plumbing, there is no need to go through the rigorous installation method of running these pipes through floor joists.

Water pipes PEX and mounting tools on the table. Plumbing

Can You Hang PEX Tubes Under Floor Joists?

If drilling holes through floor joists seems like a lot of work, there is an easier alternative. You can make the tubes seem more organized by hanging them under the floor joists.

They will still be visible and can eat up some headroom. But with such a setup, you don’t have to worry about weakening floor joists or expansion of the PEX tubes.

You have to weigh the pros and cons of each method of PEX installation and determine which works best for you.

Should You Hire an Expert to Install PEX Through Floor Joists?

Even though you can run PEX through floor joists by yourself, this project requires intermediary DIY skills. First, you have to identify the neutral axis on your floor joists, and then you need to drill a hole that will fit PEX tubes without interfering with the load-bearing strength of the beams.

If you doubt your skills, you can always hire an expert contractor to install them for you. They will follow the procedure laid out above.

The only difference is that you will spend more on the project due to labor fees. But it will be 100% worth it than doing something wrong that may damage your floor.

Therefore, there is nothing wrong with bringing in an expert to install PEX on your floor joists. You may even hire them to work on replacing the copper pipes in your entire house apart from the floor joists.

Do You Have to Be Worried about Nails on Floor Joists When Installing PEX?

PEX plastic tubes are strong and durable, but they are not ‘nail-proof’. Therefore, if you have decided to install PEX through the floor joist, remove any protruding nails or bend them flat with a hammer. Exposed nails can puncture through the PEX tubes resulting in damage and leaks.

Do You Have to Insulate PEX Running Through Floor Joists?

Most area codes require all heating pipes installed in an unheated area to be insulated. Therefore, if your local codes have such a requirement, you will need to insulate PEX after running it through floor joists.

To go about this process, drill holes in the joists, as advised earlier. Run the PEX through these holes and then add a layer of insulation beneath the floor joists. You will have insulated the PEX tubes and hidden them from the ceiling beneath.

Even though it’s easier to attach PEX under floor joists, such a setup is extremely difficult to insulate. Also, when the PEX expands, it will rub against the insulation.

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