Happy Thursday! Welcome to another email from Back of the Envelope, where I share what I learned recently.

Today I am going to talk about notching and drilling holes through sawn lumber joists (I-joist is a different story).

This came up because an electrician recently told me that he often notches the bottom of joists to run Romex wires because he was taught to avoid drilling holes…

Something tells me that’s not quite right, so I had to look it up.

Below is what I gathered, so you don’t have to 😉 – it could be helpful on your next wood project (or when you remodel your home…)

(Estimated reading time: 2 minutes and 10 seconds)

Notches and holes prescriptive requirements

IRC, IBC, and NDS all say pretty much the same things, which I’ll summarize below. But here are the sections in case you want to dig in:

• IRC R502.8 Cutting, Drilling and Notching

• IBC 2308.4.2.4 Notches and Holes

• NDS 3.2.3 Notches

• NDS 4.4.3 Notches

Notch requirements:

1/ Do not notch 2x4s

2/ Do not notch within the middle third of the span

3/ Maximum notch length = D/3 (D is the joist depth)

4/ Maximum notch depth = D/6

For example, for a 2x10, D = 9.25”, so the maximum notch length would be 3 inches, and the maximum notch depth would be 1.5”.

(Side note: Interestingly, a tidbit from NDS 3.2.3 says that if you follow the notch guideline above, “the stiffness of a bending member, as determined from its cross-section, is practically unaffected by a notch.”)

Hole requirements:

1/ Do not drill holes in 2x4s

2/ No limitation on where you can drill along the span

3/ Maximum hole diameter = D/3 (same as notch length)

4/ Keep at least 2” edge distance (edge of the hole to top edge or bottom edge of joist)

Here is a nice detail from Woodwork that demonstrates the requirements:

(Note that I didn’t mention the D/4 “end cuts” -- that would be a topic for another day)

(source: Woodworks)

And here is a quick tabulated table, so you don’t have to pull out your calculator while walking the site:

Protection of wire

Now, another thing to watch out for is that the contractor might accidentally kill the wires in the notch while installing the nails/screws for the ceiling drywall.

In fact, the electrical code specifically says that if the wires are installed in notches, they shall be protected by 1/16” thick steel plates (link to the electrical code here – you know you want to read it).

Simpson actually has products for these, and they are called “NS” (aka nail stoppers, link here). They are non-structural, so it’s not typically specified by structural engineers (I assume it’s the electrician’s responsibility?)

Regardless, if you are walking the site or remodeling your home, it’s a good idea to look around to ensure they are provided.

Alright, and that’s all I got for now. Hopefully you are now a little more equipped to talk to an electrician (or plumber) about notching and drilling.

Have a good one!

Andy Lin
Back of the Envelope/Structural Engineer HQ