I need to install a bathroom ceiling fan for a new bathroom I'm adding to my house. The only issue is that the floor joists are running perpendicular to where I need to run the 4'' aluminum duct pipe.

My question is do I just bore a 4'' hole through the joists?

Based on this answer from StackOverflow and this one it seems I can bore a hole 1/3 the depth of my joists (which are 11'') as long it's a joist (and not a beam) and the hole is no closer than 2inch from each edge... is that correct?

Thanks in advance!

Follow-up Note: This room is on the first floor, and there is a room above it (so venting through the attic isn't a possibility - as far as I know)


  • 4 x 3 =12 so 1/3 of 11 is less then 4"
    – Alaska Man
    Commented Jan 16, 2021 at 21:02
  • @AlaskaMan - Yea, I was wondering about that too. 1/3 of 11 = ~3.667inches. Being a diy n00b, I didn't know if that was close-enough.. or I was being too precise. Looks like the consensus though is DONT DO IT haha Commented Jan 17, 2021 at 0:03
  • Isn’t the 1/3 guidance only for I-joists? I don’t think you can cut anything nearly that large in standard dimensional lumber. It doesn’t mention which type of floor joists they are.
    – daneb
    Commented Jan 17, 2021 at 4:59
  • it depends where you are some places don't even allow 2" holes.
    – Jasen
    Commented Jan 17, 2021 at 5:32
  • A 4" duct takes a 4 1/4" hole to get it to pass through. It may possibly with a 4 1/8" hole, but it will be really tight.
    – Jack
    Commented Jan 17, 2021 at 7:44

1 Answer 1


Looks like you have your specs and they don’t allow it. But no I would never drill that big a hole in joists. It’s common to do so on rim joists however as they are supported underneath by the foundation.

For any vent the best choice is to have it as direct and short as possible with as few as corners as possible. The best choice is usually through straight up through the roof.

I have seen some interesting choice for exhaust pipe too. Try to avoid flex hose as that limits air flow.

  • Other thing to consider is how close the exterior vent is (if you choose to go through outer wall) to your soffits. You don’t want to vent the moisture out of the house only to have the soffit “suck” the moist air into your attic where you definitely don’t want it.
    – Ethan
    Commented Jan 16, 2021 at 22:32
  • Thanks for that info... yea, I was equally worried about boring a hole in joists, which is why I stopped and made to research before proceeding. Thank you for taking the time to respond Commented Jan 17, 2021 at 0:05
  • Oh... and there is a room above this room, so venting through the attic doesn't seem possible as far as I know Commented Jan 17, 2021 at 0:05
  • How many feet is it to run out parallel with the floor joists to closest outside wall? But I assume you are working with a finished ceiling along that route so you aren’t inclined to redo drywall?
    – Ethan
    Commented Jan 17, 2021 at 0:10

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