I'm installing four 3/8" threaded hanger rods and need them installed in exact positions. The rods will be holding approx. 50lbs total; they are supports for a concealed ducted air conditioner/heat pump which requires supporting at each corner. Manufacturer specifies they should not use struts, but 3/8 threaded rods connected with washers and nuts.

I'm planning on using ceiling flanges to acheive this, but want to install them onto the underside of a subfloor. The subfloor is 3/4" OSB. My ceiling flanges require 1-1/2" wood screws.

Can I add a piece of 2x4 and attach that firmly using four 2-1/4" wood screws and then attach the ceiling flanges to the 2x4?

Here's what it would look like without the 2x4: Ceiling Flange attached to Subfloor only

Here's what I am proposing (is this okay?): Ceiling Flange attached to Subfloor with a piece of 2/4

  • 2
    What size and type of fastener you should use is dependent on how much weight or lateral force the flange is going to be subject to. What are the rods going to be doing?
    – RMDman
    Nov 14, 2022 at 19:58
  • 4 rods will be holding approx. 50lbs total. They are supports for concealed ducted air conditioner/heat pumps which require supporting at each corner. Manufacturer specifies they should not use struts, but 3/8 threaded rods connected with washers and nuts.
    – Paul Evans
    Nov 14, 2022 at 20:18
  • 3
    A sub floor should have 2x joists about 16 inches from each other. Do not know the weight of your duct work, but instead of screwing 2x4s to OSB, would screw them to the joists near by. For hanging stuff, do not have to worry about the screws pulling out of the OSB.
    – crip659
    Nov 14, 2022 at 20:41
  • 3
    Imagine using only the 2-1/4" screws by themselves. Adding the 2x4 does not increase the amount of engagement that the screws have in the OSB.
    – spuck
    Nov 14, 2022 at 21:19
  • 1
    Why are you not looking to attach your flanges directly to the floor joists? Your 3/4" subfloor isn't just floating in the air, it's got something holding it up - take advantage of that.
    – FreeMan
    Nov 15, 2022 at 15:49

1 Answer 1



Your 2-1/4" screws will each have about 1/2" of useful penetration into the OSB. That's just enough to delay catastrophe until your unease about the plan subsides. Then you may see the whole thing come down, along with a nice big wafer off the bottom of your subfloor.

Anchor the two-by to adjacent joists, or run the screws through from above. If you're going to rely completely on tension, rather than shear, it had better be substantial.

If you're unable to do either of those (or along with either of those), slather the two-by with heavy duty construction adhesive to spread the load and add holding strength.

  • 1
    Personally I'd bolt rather than screw from above. A set of nice big countersunk bolts, with nuts and washers on the bottom. Ideally self-locking nuts (you'll need an assistant).
    – Chris H
    Nov 15, 2022 at 12:23
  • 1
    Countersunk in 3/4" material? That might leave you with half an inch of support in distinctly layered material. Doesn't sound great to me.
    – isherwood
    Nov 15, 2022 at 13:45
  • I work in metric, but M6 in 3/4" would be plenty and give you 15.75mm or about 5/8". 1/4-20 would be a touch more given the different CSK angle. My "nice big" was perhaps misplaced given the loads involved, but I probably wouldn't go below M5. And with machine screws into wood you don't need to (fully) cut away the countersink - use the fact that wood compresses. I assumed protruding heads wouldn't work, but unless you imagined the screw heads proud of the surface your "screw from above" has exactly the same problem.
    – Chris H
    Nov 15, 2022 at 13:59
  • 1
    countersunk not counterbored. Dimension t here. Conical so it really does compress. Washer only on underside to protect surface as you tighten the nut
    – Chris H
    Nov 15, 2022 at 14:03
  • 1
    My perspective comes from carpentry, where things are probably not as standardized as in steel fabrication, for example. IMO, a counterbore the depth of a bolt head is functionally identical to a conical countersink for the same purpose. In other words, a counterbore can be a subset or type of countersink. It's all somewhat pedantic, though.
    – isherwood
    Nov 15, 2022 at 16:59

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