IBC 2308.8.2 states a rule of not notching more than 1/6 the depth of a wooden joist (emphasis added):

Framing details. Joists shall be supported laterally at the ends and at each support by solid blocking except where the ends of the joists are nailed to a header, band or rim joist or to an adjoining stud or by other means. Solid blocking shall not be less than 2 inches (51 mm) in thickness and the full depth of the joist. Notches on the ends of joists shall not exceed one-fourth the joist depth. Holes bored in joists shall not be within 2 inches (51 mm) of the top or bottom of the joist, and the diameter of any such hole shall not exceed one-third the depth of the joist. Notches in the top or bottom of joists shall not exceed one-sixth the depth and shall not be located in the middle third of the span.

Would it be acceptable to notch a joist more than 1/6th of the depth if adding a steel plate or sistered wood beam (going the full length) or some other method (short of putting a column in the middle)? I need to cut about 2.5" out of a 9.5"-deep beam to get an HVAC duct through.

The joist is between the basement and first floor. Above is, I believe, the living room.

The joist spans 10'.

  • $\begingroup$ I'm voting to close. As your Question is written, not knowing what kind of loading you have, what type of wood, and other considerations for the structure, it's impossible to say whether or not notching your beam is acceptable. Also, where did you get this 1/6 rule? $\endgroup$ – grfrazee Aug 24 '15 at 21:58
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    $\begingroup$ You probably want to add some more information to your question. As it stands now, the question is, "Is it possible?" Anything is possible. If you would like more of an answer, you will need to add additional information about the location of the cut, loading, ect. Even then there may not be enough information to actually answer the question. $\endgroup$ – hazzey Aug 25 '15 at 1:05
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    $\begingroup$ 1/6 rule: co.routt.co.us/building/handouts/notching.pdf $\endgroup$ – Octavian Aug 25 '15 at 15:41
  • $\begingroup$ @AndyT DIY often falls back on "follow your local building code." IMO the question works on either site. If the author wants it migrated, the way to indicate that is to flag "in need of moderator intervention" and write a message explaining where and why it should be migrated. $\endgroup$ – Air Aug 25 '15 at 15:41
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    $\begingroup$ We can't tell you whether it's okay to make this particular modification to this particular notch. If you're interested in answers that address the implications of deeper notches and/or methods for accommodating them in floor joists, that seems like a more reasonable expectation. $\endgroup$ – Air Aug 25 '15 at 15:56

The 1/6th rule

IRC allows a notch 1/6th of the depth of a wooden joist (but not within the middle third) on the basis that the joist is sized sufficiently for the stress at midspan, and a 5/6th depth beam in the end thirds is hence sufficient for the lower stress experienced there. Hence a 1/6th depth notch will not affect the design capacity of the joist.


In engineering terms it is eminently possible to reinforce a beam such that a deeper notch will not cause the beam to fail. A simple rule applicable in this case is that the maximum stress is proportional to the width of the beam times the depth cubed.

Hence, if you wanted a beam that was 1/2 of the depth of the original beam, then in order to provide the same stress as the original beam would have experienced, you'd need a beam that was 8 times the width!

Looking at your numbers, and trying to minimise the width as much as possible, I would say:

  • The IRC rules suggest you're allowed a beam depth of 5/6 of 9.5" = 7.92".
  • You want a beam that has a depth of 9.5" - 2.5" = 7"
  • Hence your depth is reducing by a factor of 7 / 7.92 = 0.884
  • Hence your breadth needs to increase by a factor of 1 / (0.884^3) = 1.45

So attaching a wooden joist to the side which is half as wide as your original beam should be sufficient. Alternatively, by using a steel plate (as you suggested) you would need much less material.

However, there are of course key points that I've missed here. You need to transfer the stress from the original beam into the attachment. You would hence need to design connections to transfer the stress, and over an appropriate length.

The real world

In most jurisdictions it should be possible for engineering calculations to be carried out, proving that the reinforcement/modification is sufficient. Once the change is then approved by the local authority (on the basis of the calculations), work will then be able to go ahead. I would advise you to either contact a local engineer or contact your local building inspector.

Joist width vs IRC width

You mentioned in one of your edits (since superseded I believe) that the joist was wider than the minimum required by IRC. Possibly this is because the joist is wider and/or longer than suitable for the minimum width. I would hence advise against thinking "the joist is wider than the minimum width, therefore I can notch deeper".


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