This question was triggered by the following line in a home inspection:

Blocking is not installed for I joists above beams in basement.

I have found pictures of blocking at http://www.diychatroom.com/f14/sister-studs-ceiling-easy-question-11349/ (linked from Floor Joist Blocking). I have also found that cross bracing is an alternative to solid blocking. According to the National Wood Council's Details for Conventional Wood Frame Construction:

Adequately nailed subflooring will maintain the upper edges of floor joists in proper alignment. Nailing the ends of joists to band joists or headers provides additional joist support that, under normal conditions, eliminates the need for intermediate bridging.

I feel like I understand the term, and what must be done, but my wife understood the inspector to mention load distribution in relation to the supporting steel beam when he commented to her during this portion of the inspection.

Is my understanding of blocking missing something? Does the possibility of compromising the positioning or structural integrity of the steel support beam exist because there is no blocking?

  • 1
    Have you tried contacting the inspector, and asking for clarification? Most home inspectors are happy to answer any questions you might have.
    – Tester101
    May 23, 2012 at 16:30
  • @Tester101 in process. I am hoping to verify my own understanding of blocking and its importance, however, as I may be installing it or adding it to my list of items to eyeball, should I need to look at additional houses. May 23, 2012 at 16:36
  • 2
    This pdf might provide a bit of extra information.
    – Tester101
    May 23, 2012 at 16:46

1 Answer 1


Blocking typically refers to the structure that prevents the joists from twisting, which would be a concern over a load bearing beam. A solid piece of wood, often the same material as the joists themselves, is installed perpendicular to the joists without any gaps (e.g. I joists would use a piece of I joist for blocking). Two 2x4's installed in an X could also be done, and I've seen that with engineered joists, but check with your local building codes to be sure it complies.

I could also see blocking refer to additional vertical members (2x4's) to carry the load from the beam up to another load bearing object above the floor (e.g. a column). The last type of blocking I can think of would be fire blocking to prevent fire from traveling through the ceiling between parts of the home, though it's unlikely you'd see this requirement in a single family structure outside of a garage.

Like Tester101 says, call the inspector and ask for clarification, they should be happy to provide it.

  • So essentially my understanding is correct in that the blocking is in relation to the floor joists themselves and has little to nothing to do with the steel support beam under the joists? May 23, 2012 at 18:00
  • 1
    The steel support beam is what would cause the joists to twist (due to pressure pushing up on the beam from below), so the inspector wants you to install the blocking directly over the beam. You are correct that the blocking is installed on the joists themselves.
    – BMitch
    May 23, 2012 at 18:03
  • The house has stood for twelve years with no signs of twisting: loose, squeaky, or warped flooring; off center or damaged joists; cracked drywall, etc. Assuming that there has been no twisting already corrected, then most likely the steel support beam would not be adversely effected, as there is no extra weight pushing down on it, nor is it being pushed off center due to apparent twisting of the joists? May 23, 2012 at 18:11
  • 3
    @JoshuaDrake If this is a home inspection as part of buying a house, some of the inspectors comments may simply be suggestions. They make note of things that may not be done correctly according to current codes, but may have been acceptable when the home was built. So blocking may or may not be required, the inspector is simply saying "if the house was built today, there should be blocking here.".
    – Tester101
    May 23, 2012 at 18:26
  • 3
    @JoshuaDrake, The steel support isn't being effected, it's affecting the joists above it. The blocking is the prevent the joists from twisting because of the pressure of the beam. Take a sheet of paper/thin ruler/etc, hold it vertically between your hands, and have someone push up in the middle. It will twist and bend where that force is applied. Just because it hasn't happened already doesn't mean it cannot happen in the future.
    – BMitch
    May 23, 2012 at 18:51

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