I cut out the subfloor in my half bath on 2nd story above garage. I didn't expect to see 2x4s proving the most support, and just one 2x6 joist. Can someone explain to this newbie what's going on? Thanks.enter image description hereenter image description here

1 Answer 1


Those are modern engineered floor trusses, or truss joists. They often have flat 2x3 or 2x4 top and bottom chords and a web of diagonals and verticals between.

The 2x you see is a stiffener, or "stiffback", which ties the trusses together to spread load and minimize movement and vibration.

It looks a bit like someone cut out some of the stiffener, maybe to get a duct in the space below. The builder may have added additional stiffeners nearby.

  • Thanks! Would it be inadvisable to block those truss joists with flat 2x4s? So I can install new subfloor?
    – Chris
    Jan 15, 2017 at 12:56
  • Go ahead and add joint blocking wherever you like. Subfloors usually have tongue-and-groove connections, so it's a good idea. I'd use construction adhesive or screws or both.
    – isherwood
    Jan 15, 2017 at 15:26

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