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Drilling Holes in Joist: How Big Can it Be?



V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com


Drilling Holes in Joist:How Big Can It Be?

A common reason for drilling holes in a joist is to run an electrical or plumbing line through it. 

Since joists are structural elements used to support the floor above, you have to be careful while drilling into them. 

Improper drilling of holes in a joist can result in a loss of structural strength, making them susceptible to breaking. 

When drilling holes in joists, limit the maximum hole diameter to one-third the depth of the joist.Accordingly,maximum hole diameters for 2×6, 2×8, 2×10,and 2×12 joists are 1-½,2-⅜, 3, and 3-¾ inches, respectively. 

Drilling Guidelines 

Drilling Guidelines
Drilling Guidelines

IRC 2021 recommends drilling all holes after leaving a space of 2 inches from the top and bottom of the joist. 

Another recommendation of IRC 2021 is that the maximum diameter of any holes drilled cannot be more than a third of the joist’s depth. 

The table shown below displays the maximum allowable hole diameter of different sizes of joists. 

Joist SizeHole Diameter(in)
2×4Not Recommended 
2×10 3
Maximum Hole Diameters for Different Joist Sizes

Considering the size of a 2×4 joist, drilling a hole would likely reduce its strength, so it is not recommended.

What Size Hole Can You Drill in an I Joist?

Holes in an I Joist
Holes in an I Joist

IRC 2021 guidelines state that no holes, notches, or cuts are to be placed on an I-joist unless specified by the manufacturer. 

Renowned I-joist manufacturers like Weyerhaeuser, Boise Cascade, LP Solidstart, and others have their own specific guidelines for drilling holes inside their products. 

Despite the few variations, most guidelines presented by the different manufacturers are pretty similar.

The table below details the maximum allowable hole size according to the depth of the I-Joist.

It also specifies the minimum distance from the hole’s edge to the nearest support’s inner surface. 

Round Hole Diameter (in)23456788-7/8
Any 9-1/2″ JoistSpan (ft)81′-0”1′-1”1′-8”2′-4”2′-11”3′-7′
Any 11-7/8″ JoistSpan (ft)81′-0”1′-1”1′-6”2′-0”2′-5”2′-11”3′-5”3′-10”
Maximum Allowable Hole Size for I Joist

For a further breakdown of permissible hole diameters for I-joists of greater depth, read your preferred manufacturer’s product brochure.

All manufacturers stress the importance of not drilling any holes in the flanges of the section. 

Considering their low depth, all manufacturers advise leaving the flanges on.

Additionally, drilling holes in them may alter the load transfer mechanism of the joist. 

Also, do not drill holes near the bearing points of the joist.

How Far Apart Should Multiple Holes Be Drilled in a Joist?

How Far Apart Should Multiple Holes be Drilled in a Joist?
How Far Apart Should Multiple Holes be Drilled in a Joist?

Under the guidelines in IBC section 2308 and IRC 2021 R502.8, the minimum distance between two holes in any section should be at least two inches when measured horizontally. 

Along with the IRC guidelines, the manufacturers also present certain instructions when dealing with multiple holes in a joist. 

A generalization can be made about the changing guidelines from manufacturer to manufacturer for I-joists.

Most manufacturers recommend a minimum spacing of 2 times the diameter of the largest hole. 

What this means is that, suppose you need to drill two holes closely, one with a diameter of 3 inches and the other 4 inches. 

To ensure the structural integrity of the member is maintained, you must leave a space of 8 inches from the edge of either hole. 

What is The Distance Between a Hole in a Joist and a Wall?

The wall supports the joist when it is connected.

It takes up loads of the joist and transfers it to the footing. 

Maintain a minimum distance of 6 inches between the support and the furthermost hole in a joist.

This is done to ensure the joist’s load-bearing capabilities are utilized to the maximum and for occupants’ safety.

End supports enable the structural system to transfer loads from itself to the next load-bearing element. 

Hence it is imperative to provide a clear load transfer path to ensure maximum efficiency of the structural system.

In the case of joists, multiple codes, including IRC 2021, do not recommend the presence of holes near the end supports of a joist. 

Drilling holes near end conditions may decrease the load-carrying capacity of the joist, in turn rendering the structural system unfit to carry the designed loads. 

How Should a Hole be Drilled in a Ceiling Joist?

How Should a Hole be Drilled in a Ceiling Joist?
How Should a Hole be Drilled in a Ceiling Joist?

All joists, regardless of their end use, must follow the drilling guidelines provided by the IRC.

The best way to drill a hole inside a ceiling joist, or any joist for that matter, is to leave a spacing of 6 inches from supports and 2 inches from the top and bottom of the joist. 

Pitched roofs require special consideration.

Due to the structure’s geometry, the joists, too, would be placed at an angle. 

The drilling guidelines for sloped joists remain the same, the only difference being that one has to account for the slope in their calculations and drilling. 

A good thing about ceiling joists is that they do not have to withstand much load. 

Most of the loads subjected to a ceiling joist are of the roofing system and the joist’s self-weight. 

When a structural element is not heavily loaded, the structural engineer either reduces the member size or increases the spacing between two members. 

Where Should the Joists be Drilled?

Ideally, you should not drill the joists.

Since they’re key structural components, it is advisable not to drill holes of any diameter in a joist. 

The size of the hole plays as big of a role in determining the load-bearing capacity of the joist as the hole’s location. 

However, in most scenarios, service lines, such as electrical and plumbing lines, will need to be run through joists.

The safest option is to drill all holes in the joist’s middle part, as recommended by IRC 2021.

IRC also recommends a spacing of 2 inches from the top and bottom and 6 inches from either end.

What is the Difference Between a Hole and a Notch in a Joist?

Found in joists’ middle span Found on the joist’s top or bottom surface
Require power toolsDo not require power tools
The amount of material removed to create a hole is more.The amount of material removed to create a notch is less.
Difference Between a Hole and a Notch in a Joist

A notch and a hole serve the same purpose.

Both are cavities created onto the joist to accommodate a service line. 

While both have the same function, a significant difference is in their locations. 

Usually, you can find holes in the middle span of the joist.

IRC guidelines specifically restrict the presence of holes on the top or bottom of a joist. 

On the other hand, you can find a notch on the joist’s top or bottom surface. 

You can use power tools to make a hole.

 It is more precise and takes less time, but it consumes more material.

Notches, on the other hand, don’t require power tools.

They can be easily made with precision tools but would take up a lot of time. 

Furthermore, the material removed for creating a notch is typically far less than the material consumed for creating a hole. 

Hole shapes tend to be more functional because they can accommodate plumbing, firefighting, and even electrical lines.

The presence of notches is not uncommon for a residential structure, but it offers limited applications. 

Due to size restrictions, you can accommodate some individual wires or small water supply pipes by a notch. 

Frequently Asked Questions(FAQ)

In a 2×10 Floor Joist, Can I Drill Three Holes?

The short answer is yes; you can drill three or more holes into a 2×10 joist. 

The maximum diameter of the hole would depend upon the depth of the section.

A 2×10 floor joist can’t have a hole diameter greater than 3 inches. 

Can I Drill Through a Joist Vertically?

Drilling a vertical hole inside a joist reduces the surface area through which most of the loads are dissipated. 

This, in turn, results in a tremendous loss of structural strength and makes the joist susceptible to breaking. 

Reconfiguring the design of the service lines would be a much better option than risking structural instability. 

How to Drill a Joist Without Damaging It?

Always follow the IRC and manufacturer’s guidelines and choose appropriate drill bits.

Moreover, avoid excessive force or sudden movements, as they can lead to damage.

V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com

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