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I have old 3x9 ceiling/floor joists (actual dimensions), 48 inches on center. I needed to run additional wiring so I made a new 1" hole above and slightly offset from an existing hole with 2+ inches between the closest edges of the holes, based on the following:

R502.1.11.1 [IRC 502.8.1] Sawn lumber. Notches in solid lumber joists, rafters and beams shall not exceed one-sixth of the depth of the member, shall not be longer than one-third of the depth of the member and shall not be located in the middle one-third of the span. Notches at the ends of the member shall not exceed one-fourth the depth of the member. The tension side of members 4 inches (102 mm) or greater in nominal thickness shall not be notched except at the ends of the members. The diameter of holes bored or cut into members shall not exceed one-third the depth of the member. Holes shall not be closer than 2 inches (51 mm) to the top or bottom of the member, or to any other hole located in the member. Where the member is also notched, the hole shall not be closer than 2 inches (51 mm) to the notch.

joist penetrations

However, I'm reading some conflicting guidance saying the holes need to be horizontally 2 inches apart instead of the distance being measured in 2 dimensions, although I couldn't find specific code references that states this is indeed the case. Does anyone have such references, or know engineering principles that would explain why this would be fine or not?

Also, I noticed an existing penetration on the left is slightly shy of 2 inches from the bottom. It is located ~7 1/2 inches from the beam. Should this concern me enough to hire an engineer?

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    the weakest part of that joist in the knot at the left margin.
    – Jasen
    Commented Aug 24, 2023 at 11:04

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None of this concerns me. Ideally you'd offset in both directions to avoid creating straight lines of holes, which could result in some weakening. A series of horizontal holes could result in splitting along the grain, for example. For these three holes it's not an issue.

Keep in mind that the rules are written to prevent egregious damage to structural members. Occasional, slight violations will not bring the house down. Also keep in mind that natural flaws in a joist can occur at close intervals and near the edge of the joist.

As long as you don't have a whole cluster of penetrations and flaws in a series of joists, you have nothing to worry about. Just about any 1970s or older home I've ever worked on has situations just like yours.

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  • The really concerning thing might be that kink in the (C?)PVC pipe (unless that's some kind of conduit, or not actually a pipe) through the left hole.
    – Huesmann
    Commented Aug 24, 2023 at 13:48
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    That looks like /3 cable to me, possibly for a range or dryer.
    – isherwood
    Commented Aug 24, 2023 at 14:37

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