Can You Notch the Bottom of a Joist?   

Whether you need to put cables for ceiling lighting or need to run wires through the floor above a ceiling, you’ll have to find a way around the joist. However, since the joist is crucial when it comes to maintaining a building’s structural integrity, it may be dangerous to notch the bottom of a joist.

You can safely notch the bottom of a joist without compromising the ceiling’s strength, but you’ll have to follow the correct procedure for doing so. There are depth, width, and space requirements for notching the bottom of a joist safely. 

This article will discuss where you can notch a joist and how to do it properly. I’ll also explain some safety tips and legal requirements to consider when notching joists. Let’s begin!

Can You Notch the Bottom of a Joist?

How To Notch the Bottom of a Joist

To notch the bottom of a joist, you’ll have to consider the depth of the notches, where to make a hole, and how to maintain the joist’s structural integrity throughout the process. Most contractors recommend finding an alternative to notching if possible, but notching may be your only option in some cases. 

Before you start, always mark the area where you’re going to make notches. You’ll have to follow the correct notching guidelines and avoid going too deep or too close to the middle third of the joist. 

Always consider these factors when notching the bottom of a joist: 

Examine the Joist Thoroughly 

Before you can start notching, you’ll have to inspect the joist thoroughly and check for any structural flaws. You may not be able to notch if the joist is too narrow, as this will cause structural problems. 

The most important tip for notching in joists is that only the middle third cannot be notched. Therefore, you’ll have to measure the joist and divide it into three equal parts. The two parts closest to the supporting pillar or walls can be notched since these parts have more structural support. 

Once you’ve checked the allowable area where you can notch, you’ll be better positioned to decide whether you can get the job done.

Decide the Ideal Notch Depth 

The next step is calculating the notch’s position based on its height from the bottom and top of the joist. The depth requirements for notching vary based on the position of the holes. 

  • When notching further away from the end of a joist, you can notch to a maximum of one-sixth of the joist’s depth since these parts are more likely to bend. However, avoid notching the middle third of the joist, no matter how shallow you plan to notch. 
  • When notching closer towards the end parts of a joist, you can make slightly deeper notches, but ensure that they are within a quarter of the joist depth. 

It’s much safer to notch bigger joists, but since most joists have a minimum timber depth of ⅓ feet, you’ll be able to notch at least half an inch (1.27 cm), which is suitable for small wires or cables. If your joists are slightly bigger than 7 inches (18 cm), you can create a notch about 1 inch (2.54 cm) deep. 

Remember, the important thing is to avoid compromising the joist’s structural integrity, even if this means limiting notch depth. 

Measure the Ideal Notch Diameter 

If you’re notching holes to accommodate wiring, consider that most cable wires have a diameter of 0.3 inches to 0.4 inches (0.76 – 1.02 cm). However, many cables may require more space to go through the notches. So, how wide should the notches be? 

When choosing a notch diameter, avoid looking at the cable and consider the rules for relative hole width. The ideal diameter should be ¼ of the joist depth. However, avoid drilling with a bigger bit just because the notch falls within the guidelines, and try to make the notches just big enough for the cables to fit. 

For example, if the joist depth is 4 inches (10 cm), you can drill a hole with a diameter of 1 inch (2.54 cm). Considering that most cables will be less than half an inch (1.27 cm) and most joists are thicker than 4 inches (10 cm), so you’ll have more than enough depth to accommodate most wire cables. 

Ideal Notch Length 

When notching the bottom of a joist, you’ll have to take particular care to ensure that the notch length is within the guidelines. If you create notches that are too long, you’ll create a rip in the joist and compromise its structural integrity. 

The maximum notch length is one-third of the joist depth. Anything longer than this is a rip and against the 1991 National Design Specifications.

What To Avoid When Notching the Bottom of a Joist 

If you’re big on DIY and have some experience, you can do most home renovations without a professional contractor. However, if you’re going to interfere with anything related to the structure of your house, it’s best to consult a professional first. 

Floor and ceiling joists are integral to the building’s structure, which is why it’s important to follow the guidelines when notching or boring through them. 

Here are some essential safety considerations to note when notching joists: 

  • Never notch both the top and bottom of a joist. Even if you notice that the top of the joist already has notches, it’s best to find another way to get the wires, cables, or piping through.
  • Never make a notch closer than 2 inches (5.08 cm) from another notch or hole, regardless of the joist’s span. 
  • Never make notches or holes in the middle of a joist. This could severely affect its structural integrity and may even cause the joist to snap. If enough joists get broken, the ceiling may collapse. 
  • Always try to go for angled cuts, as this doesn’t weaken the joist. However, if necessary, you can go for square cuts as long as you follow the guidelines related to height, width, and other requirements. 

How To Notch the Top of a Floor Joist

The building codes in many areas allow you to notch the top of a floor joist since it’s not likely to affect the beam’s structural integrity. However, the same requirements for notching the bottom of a joist will apply in such cases. [This Is Why Some Floor Joists Are Doubled]

Never notch the middle third of a joist, even if you’re doing it from the top. Moreover, follow the same depth, width, and length requirements discussed above. 

How To Bore Holes in a Joist 

In some cases, notches won’t be enough to accommodate wires or cables, especially if you’re dealing with thinner joists. An alternate method to create gaps for wiring is to bore holes through the joists.

Making holes in a joist is less risky, and you won’t have to worry about damaging smaller joists if you follow the correct guidelines. In fact, most contractors will prefer boring holes in a joist if this means they avoid notching it. 

When drilling holes in a joist, you’ll have to follow certain guidelines, including: 

  • Never make holes with a diameter of more than a quarter of the joist height. If the joist’s depth is 4 inches (10.16 cm), the hold should have a maximum diameter of 1 inch (2.54 cm). 
  • Only drill holes in the center of the joist. Drilling holes too close to the top or bottom of a joist will compromise its structural integrity. 
  • Unlike with notches, you can drill hotels in the middle third of the joist since notching is riskier than boring. 
  • Measure the joist’s span to determine the hole positions. You shouldn’t make holes closer to 0.25 of the joist span where the joist meets the support. Also, avoid making holes further than 40% of the span length away from the joist end.

However, remember that while holes are better than notches, they aren’t ideal, as too many holes could affect the joist’s strength at some point as well. 

How To Strengthen Weakened Joists 

If you’ve weakened a joist by making notches and holes at the wrong places or making too many holes, you’ll have to strengthen the joist to maintain the building’s structural integrity. Leaving a weakened joist is dangerous and could lead to a ceiling collapse. 

The best way to strengthen a damaged joist is by inserting a piece of plywood in the gap. Choose ¾ inch plywood for optimal strength and use nails or screws to fasten the wood in place. 

You can also use this method for cracked joints, but you’ll need thicker plywood. You’ll also have to strengthen the structure with wood along the length of the joist.

However, if the joist planks have sagged or are broken, you’ll have to replace the whole joist. Remember, joists are crucial to maintaining the structural integrity of any building, so don’t compromise on quality when it comes to them. 


Notching a joist isn’t recommended, but you may not have any option in some cases. When notching in a joist, always follow the right depth, width, and length requirements to avoid damaging the building’s structural integrity. 

Moreover, avoid notching the middle third of a joist, as doing so may cause the bean to crack.

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