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#196140 09/15/10 01:25 PM
Joined: Jan 2005
Posts: 5,316
Cat Servant
Someone refresh my memory, please ... what are the 'rules' regarding drilling holes through floor joists? Placement, size, etc?

These are the 'old fashioned' joists, solid wood, perhaps 2x10 nominal.

(I've been doing so many "wood I-beam" or TJI jobs, I've forgotten what real wood looks like laugh )

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Joined: Mar 2005
Posts: 212
I'm definitely not quoting a code here, but the 'rule of thumb' I was taught is that you could drill a hole of a diameter up to 1/3 the nominal width of the joist within an area 3x the nominal width from a bearing point, in the center of the joist. Off center holes had more restrictive rules as to size, but I can't remember them.

Joined: Jan 2005
Posts: 5,316
Cat Servant
Thanks, Greg!

It turns out that your memory isn't far off! The actual rules do limit your drilling as to both location and size.

Size? The holes can not be larger in diameter than 1/3 the actual depth of the joist. For a 2x10, that would mean a hole no larger than 3-5/32" diameter.

As for location:
1- You are not allowed to drill in the middle third of the span at all;
2- No part of the hole can be within 2" of either edge of the joist; and,
3- Maybe most often overlooked - no part of the hole can be within 2" of another hole (or edge of a notched area).

I want to thank those -I did get some PM's- who provided the information. The source is IRC 502.8.1 ... that is, the 'residential code.'

Please note that these are the rules for "sawn" or dimensional lumber. That is. 'ordinary wood.' The rules are very, very different for LVL's, glulams, TJI's, and any other form of 'manufacturered' wood. It also matters whether the wood is being used as a truss, a joist, a rafter, or a stud.

I'll say it again: electricians need to know more than just the electric code.

Joined: Jul 2002
Posts: 8,412
Likes: 3
Meanwhile, plumbers carry on as usual.

Any hole put in structural timber needs to be put in the center of the joist.
That is where the compression and stress are less with a given size of timber.
This also depends on the length of span of the joist, it's cross-sectional area and what it is holding up.

Last edited by Trumpy; 09/16/10 01:41 AM.
Joined: Jan 2005
Posts: 5,316
Cat Servant
The trouble with 'thinking' is that, when there is a rule, opinions are of little value. No matter how smart or logical you are.

Many are taught to NEVER drill a joist. Period. Yet, as Trumpy points out, we see the most extreme holes made by plumbers all the time, and the building never seems to fall down as a result.

The IRC (International Residential Code) pretty much agrees with Trumpy's assessment, though in a more specific manner. Let me illustrate, using an imaginary 2x10 with a 9ft. span.

Over that 9ft. span, NO drilling is allowed in the middle 3ft. (In many web discussions, folks confuse the "center of the span" with the "center of the width," which really confuses the issue).

In the end sections, no drilling is allowed within 2" of the edges. This means you have a 'permitted to drill' section (remember that a 2x10 is only 8-1/2" wide) that is 4-1/2" wide, in the central section, and 3ft. long.

Your largest allowed hole is 1/3 of that 8-1/2", or 3-5/32" diameter. There needs to be 2" of solid wood between the edges of every hole.

Let's assume that you goof, and make a pair of 3/4" holes with only 1" of wood between them. You would have to consider them as a single 2-1/2" (3/4 + 1 + 3/4) hole.

Just for comparison, let's look at the same situation if a TJI ("wood I-Beam") is used instead:

For a TJI, small holes like 3/4" are completely ignored, and can be placed anywhere in the web. Nor is there any requirement for spacing between such holes. You are NOT required to use the factory pre-punched sections.

A few final notes: I have found my cordless impact driver, when used with Irwin's "Superbor Max" bits, to make a much shorter assembly than the usual drill/chuck/bit arrangement. With a 'head length' of about 11", it easily fits between most studs and joists.

"Round" Romex can use a 5/8" hole. "Flat" Romex needs 3/4" minimum, and 7/8" is reasonable. Smaller holes are much easier to bore, and the drill will make many more between charges.

Likewise, I have found that it pays to be 'anal' in lining up your holes; it makes the wire pulls much, much easier.

Joined: Apr 2002
Posts: 7,297
Likes: 5

You say..."Likewise, I have found that it pays to be 'anal' in lining up your holes; it makes the wire pulls much, much easier."

I once had a 'helper' drill 1" holes thru really old 2x10s that were really 2" thick..and 12" on centers, that were all 'up' at about 30 degrees. I used the term 'anal' in a really different context.

Joined: Oct 2000
Posts: 4,054
Likes: 3
Warning ... shameless plug coming... smile

Regarding the lining up of holes in joists, I think the following is a pretty nifty thing - mostly valuable for conduit or piping runs:


Joined: Jul 2002
Posts: 8,412
Likes: 3
Now THAT is a good idea, Bill!

Joined: Jul 2004
Posts: 9,688
Likes: 10
That looks like an interesting tool but I doubt the result is any better than a chalk line and a drilling jig made from a little scrap of wood.

Greg Fretwell
Joined: Aug 2007
Posts: 853
Never gave it a thought. never been called on anything to do with holes.
Not even fire stop. That falls on the GC.

I just center best as the eye can do. then run with it.

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