I would like to redirect a 5" air pipe to the other side of a 2" x 8", second story, floor joist so that I can have the vent exit a bathroom vanity kick plate. I realize I cannot just drill a 5" hole through the joist, however I was wondering if it would be feasible using the method below. It is my understanding you can only cut 1/3 the height of the joist and you must have at least 2 inches above and below the hole. Would it be generally acceptable to place a side end-boot on the 5" pipe that transitions to 2-1/4" x 10", cut a oval hole just large enough to accommodate the 2-1/4" x 10" and pick it up on the other side of the joist with a 2-1/4" x 10" box vent? This would leave 2.5 inches above and below the hole. I would then sandwich 1/2" ply between a second sister 2x8 joist and glue and screw each layer (and bolt if recommended). As a very general test, I cut this size hole in a 2"x8"x8' joist, supported it at each end and bounced on it. I realize this is not really a sound structural test but I just wanted to get a basic sense as to the remaining strength of the joist. It was surprisingly resistant to deflection. I would be interested in your comments/suggestion. I have attached a couple of photos as well (one of the joist test and the other is the duct work described above).

The other option I was entertaining was to do basically use the same method above but instead of cutting the 2.25" x 10" hole; I'd drill four 2" holes side-by-side with 0.66" between each. Then I'd screw and caulk the boot on one side of the joist and the box vent to the other side (rather than passing the entire 2-1/4" x 10" box vent through the joist). The advantage would be that there would be 3 support "posts" between each of the four holes. The disadvantage would be air flow restriction.

Obviously I would not want to create a structurally unsafe situation, but I was wondering if one or both of these methods would satisfy the rules for joist cutting or is it just wishful thinking.

Thanks for taking the time to read this post!


Duct work

Thank you to those who commented. Below are two images to help further explain the project. The green line indicates the future location of the vanity kick plate. The yellow line indicates the proposed new location for the 5" duct (transitioning to the 2-1/4" x 10" as described in my original post). The pink lines indicate the location for the 2x8 sister joists and the blue would be the plywood sister. All sisters would be glued, screwed and or bolted. The red "X" indicates the location where I'd like the vent to exit the kick plate. (The water supplies are old and will be moved and the new drains have not yet been installed).

The next diagram below shows how I was intending to sister the joist with two 2x4 with plywood in between.

I would also bridge the joist with vertical or horizontal 2x8 pieces where possible.

If the plan above is ill-advised; I am also considering capping the 5" duct and heating the room with under-floor electric radiant heating. However, I'm not sure if Under-floor heating can serve as the sole heat source where temperatures can get as cold as -25 degrees. I am also concerned about the reliability of Under-floor heating. If it failed; I'd be without heat in the bathroom.

Thanks again for any comments or suggestions.

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  • Thank you for your comments and suggestions. I am exploring the possibility of moving the 5" hvac pipe over enough in the wall, from the basement to the second floor bathroom, so that it would come up on the other side of the joist in question. This way I can avoid going through the joist.
    – user62190
    Commented Nov 8, 2016 at 20:05

5 Answers 5


A wooden beam like used for a joist will typically face bending loads. You'll have a great deal of compressive stress on the top of the beam and a great deal of tension stress on the bottom beam. The center serves primarily to keep the top and bottom of the beam aligned so they act as a single beam, and is under shear stress parallel to the grain.

As such, a hole like you pictured (in the center vertically and horizontally) will probably be fine.

I'd be leery of doing that closer to the end of the joist because if the shorter remaining center portion of the beam (hole to end of joist) was too short it could fail in shear and the top and bottom of the 2x10 would act as independent 4" beams (weak).

I'm not a structural or mechanical engineer, who would obviously be more qualified to determine this, but I am an engineer.

  • I wonder if the added sister joists would reduce the sheer enough?
    – user62190
    Commented Nov 4, 2016 at 22:17
  • With the hole near the center as it would be in your application, I'd be comfortable that it would be plenty strong. If you want to be extra-sure, a single sister would be plenty. Note that I'm speaking from a structural standpoint, which is not entirely coincident with code requirements. Electric radiant would be pricey to operate, especially as the sole heat source. If you floor with tile you might want to put in the vent and electric radiant; it makes the floor very comfortable on winter mornings as supplemental heat. Commented Nov 7, 2016 at 14:18

Thanks for your comments. I ended moving the forced air duct over 10 inches (from the starting point in the basement and in the main ground floor wall) so that it comes up in the bathroom on the other side of the joist. It was extra work but at least I did not have to compromise the joist. In addition, I also strengthened the floor further by running four sister joists, which were glued and screwed the full span of the bathroom - (from supporting wall to supporting wall). I decided to add the extra joists since the sub floor was already up and I could do so without too much difficulty. I had also discovered a "Deflecto calculator" on a tiling website which indicated my floor was barely strong enough to support a tile floor even without making any holes in the joist. On this site you enter your joist size, wood type and spacing and it lets you know if the deflection for the joists is within an acceptable range to accept tile. In the end I ended up with a strong floor than I started with so I will sleep better now :). Thanks again too everyone who shared their opinion.


I cannot find a definition of "hole" in the 2015 IRC, but my guess is that they are referring to circular holes only in the language in sections like R502.8.1 listed below:

Relevant excerpt from 2015 International Residential Code, Section R502.8.1 ... 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.

For the four 2" holes side-by-side with 0.66": Holes must be spaced 2" apart. This proposal is in clear violation of the above clause and would require structural engineering approval to determine if it is acceptable in your application.

For the 2.25" x 10" hole option: If only circular holes are implied by the IRC as I strongly suspect, then you would again be violating R502.8.1 because you are effectively creating a bunch of circular holes right next to each other (again, violating the 2" spacing rule) in order to create a single elliptical hole. You would again need structural engineering approval to determine if it is acceptable in your application. This answer explains some of the logic of why circular holes are allowed to be bored in structural members without compromising structural integrity.

  • Thank for you comments. I will post what I ended up doing...
    – user62190
    Commented Dec 12, 2016 at 16:06

Other than the obvious answer of "you should go see an structural engineer about this"...

For minimal impact on the joist strength I would drill the 2.25 inch holes spaced 2 inches apart, and run 2 ducts If i need the airflow. The 0.66 inches between the holes seems too weak.

The other option I think would be safe with the existing oval hole is to sandwich 2.5"x0.375" steel bars onto the joist to reinforce the top and bottom of the beam.

  • With regards to your suggestion to reinforce the joist with steel bars; do you mean in addition to adding sister joists or without?
    – user62190
    Commented Nov 4, 2016 at 22:21
  • With at least one sister joist to create something like a flitch beam where there is metal plates being sandwiched by the wood joists
    – Netduke
    Commented Nov 7, 2016 at 14:50

Note: I didn't read your whole explanation because, per 2006 IRC 502.8.1, "bored holes min. 2" from top, bottom, or other holes, max. size 1/3 depth". Note: this would preclude several holes 0.66" apart.

I know, you figure if it's less than "1/3 depth" in height one can, maybe, make it greater than "1/3 depth" in width. Nope. Because this max. size for holes also pertains to notches of any height. I know, notches are different than holes. However, the spirit of these limitations is in regards to the stresses on the wood framing members and "1/3 depth" is the universal length restriction for holes and notches.

If you posted a sketch of the vanity kick plate, in relationship to the air vent feed and floor joist, I might be able to tell if a 5" to 4" reducer would work with the box vent.

I can't visualize your explanation regarding the plywood and "second sister joist".


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