What structure is inside these stair walls between floors (for running wire)?
It should be a doubled-up (floor joist) header hanging between a doubled-up trimmer and a doubled-up stair header. "1905" - well, it's definitely not an engineered joist. (No light above the sink; that doesn't meet code ;)



– hometips.com



It sounds like you're trying to do this Can I drill vertically through this from the main floor to the upstairs?

"It will not affect it structurally  [citation needed]  . If we go by the 1/3 rule, that's .45 inch. So not even a half inch hole is ok. In my 30+ years of experience, I've never put more than a 1/4" hole through a 1.5" member."

"Any more than that and you've substantially compromised it. A 3/4" hole is effectively half of a member. Common sense should tell you that that's ABSOLUTELY NOT OKAY."

"Just don't go cutting it, OK?"

But what you do need to do is cut open the walls and stop asking moot questions until that's done. But if you wanted me to run some Cat5, the last place I'd be doing it is where I'd have to violate the structural integrity of a staircase and its opening.

As seen in the linked picture, you cut holes like that above and below every penetration you plan to make, so that you can get a drill in there, and make them where they should (or shouldn't) be, and feed the wire. If that's 'not where they should be', you get to destroy more drywall, which when running utilities, is the enemy.

I can't look at a picture of drywall and tell you what to do, nor answer this specific question w/o the actual blueprints, nor would I trust them anyway. Verify in field.