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I bought an old house with tongue-and-groove flooring and no subfloor. It looks like it was retrofit with a modern HVAC system and floor vents some time in 2008. Whoever installed the return vent made some questionable decisions - they used the space between joists to form the return duct, and also cut away the floor such that the edge is not properly supported on one side:

floor cut away to create HVAC return vent

You can see that the floorboards are already cracking from people walking across them.

What can I do to support their edges? You can see that the return vent actually spans more than one joist cavity. I don't think I could add a block parallel to the joist visible in the picture without completely taking apart the vent or lifting up the flooring (which I'd rather not do). If I add any blocking perpendicular to the joists inside the vent, I would be partially obstructing the return airflow.

I also can't make the vent longer by cutting back the flooring to the next joist over, because at that point it would extend in front of the new doorway we just built.

Are there any other options? Some kind of bracket or brace that would support the edges but still permit airflow?

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  • how deep are the supporting floor joists?
    – Ack
    Commented Mar 24, 2020 at 0:35
  • @Ack they're 2x8 nominal.
    – alexw
    Commented Mar 24, 2020 at 0:52

3 Answers 3

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I'm not an HVAC guy so I don't know the CFM requirements but I believe that you could use 2x4 blocking that would leave over 3-1/2" depth should be plenty for air flow.

Assuming that you are comfortable with that air flow, use 2x4 blocking with Simpson A35 clips and screws for attachment. You might be an extension on your power driver to reach the far side. Use (2) #8 x 1" screws for each side of the A35.

Workflow. Attach the A35s to the cross members, attach the members to the floor joists. Attach the A35s to the blocking member, attach to the cross members

Here is a very rough sketch of it.

enter image description here enter image description here

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    Ah, this is an interesting idea! So basically the way that first occurred to me with perpendicular blocking, except using 2x4s instead of 2x8s to permit airflow.
    – alexw
    Commented Mar 24, 2020 at 2:42
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One option to consider would be a set of pillars under each individual floor board, wedged or glued firmly in place, perhaps only secured on the bottom to allow for thermal movement.

This concept extended within my grey matter to suggest a plank running across all the floor boards, but with holes across the span to provide for the airflow.

The pillars could be hardwood dowels of an inch diameter or larger, or strips of lumber with the grain oriented for maximum support.

A plank with holes would be more restrictive, but having more than one option isn't a bad idea.

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  • Maybe a 1 x 4 flat up against the 5 floor boards and two 2x2's supporting it+
    – JACK
    Commented Mar 24, 2020 at 0:55
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I'd probably just get some angle iron.

One piece parallel with the joist with the flange surface inline with the floor. Two lateral pieces parallel with the floor boards on the edge with the flange under the first piece. A final piece flange parallel with bottom surface of floor boards. This final pieces takes the loading, causes the previous two pieces to cause uplift on the first piece. I'd go with something like 1/4" thick steel but you could probably go quite a bit thinner really you just want it beefy enough that it doesn't bend with an expected weight load.

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