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This board is nailed to the underside of the joists, and does not extend across the full width of the deck.

I'm planning to add a commercial under-deck ceiling, and would need to remove that board.

Would there be an issue removing that board? What does it do?

Diagonal board

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  • I can see light through the decking (as expected) - this is to allow water to run off the deck and slow the rot of the deck boards. This water will run through the deck and pool on top of your new ceiling with nowhere to go. It will rot out the ceiling (fairly quickly) and will probably take the bottom of the joists with it. To avoid this, you will need to install a solid floor on top of the deck and seal it just like you would a flat roof. This isn't just a cosmetic/maintenance issue - it's a safety issue [con't]
    – FreeMan
    Nov 4, 2020 at 14:33
  • ... you don't want deck joists collapsing due to unnoticed, hidden rot and have all your party guests, or just you and/or your family, to fall through to the ground below. I would suggest you not tackle this project without consulting a roofer to figure out how to seal the TOP of the deck.
    – FreeMan
    Nov 4, 2020 at 14:34
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    @FreeMan We'd be installing a commercial under deck system like RainTight, Trex Rain Escape, UnderDeck, etc. They are usually designed with a PVC ceiling that slopes to a PVC or vinyl gutter and drain system.
    – Chris Vesper
    Nov 4, 2020 at 15:24
  • Good to hear it!
    – FreeMan
    Nov 4, 2020 at 16:16

3 Answers 3

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It’s a temporary brace...probably used temporarily during construction and they forgot to remove it. (Look close and you’ll see the end of the board is not fastened.)

The height of the deck above the ground has nothing to do with the use (or need) for a brace.

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  • Yes, it's not even bolted, it's just nailed to the underside of the joists. It's ability to prevent warping is probably fairly limited.
    – Chris Vesper
    Nov 4, 2020 at 15:35
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With the deck that high the diagonal board stops the deck from wobbling. I might add a steel cable x to do the same thing in a much smaller profile. It could be drilled in but I would want both directions with cable.

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  • I've checked out a number of deck building plans and guides and I haven't seen plans for a diagonal support like that. The deck is anchored to the house with a ledger board which I would think would prevent racking. Do you have a source for a requirement for diagonal stiffening braces on a raised deck?
    – Chris Vesper
    Nov 4, 2020 at 15:37
  • Well pull it out and then have 2 people walk the same direction close to the rail and stop if you feel the deck wobble as it may the brace is to stop that. Wobble also loosens fasteners screws or nails over time. A deck 1 foot off the ground will not need this like a deck 8-9’ off the ground. Sheer wall bracing is similar a rigid steel strap or cable in a x pattern.
    – Ed Beal
    Nov 4, 2020 at 15:39
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    If the deck "wobbles" it's not properly fastened to the support posts (or the posts are loose in the ground, a bigger problem). A properly framed deck isn't going anywhere.
    – Carl Witthoft
    Nov 4, 2020 at 18:11
  • Funny just about every talk deck or house support has them in Sonoma and Marin counties CA and why would shear walls be needed are they not well attached to the foundation? where I learned the trade and they are well fastened to both the structure and the foundation support.
    – Ed Beal
    Nov 4, 2020 at 18:29
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Diagonals are needed in square /rectangle construction to prevent distortion ( called "racking") and movement/wobble ; with no diagonals it will distort as a trapezoid . I would put in 2 X 6 diagonals between many of the joists. It could also be done with cables or steel straps but these are tension members only. A ceiling of something like 1/2 plywood wood also stiffen it. Generally the exterior wall corners of homes will have plywood sheathing to prevent this possible distortion. The diagonal board was to provide stability during construction ; houses under construction will many many of these temporary diagonals. Apparently they forgot to remove it and put in permanent diagonals.

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  • I've checked out a number of deck building plans and guides and I don't see specifications for a diagonal support like that. You do have diagonal supports on the vertical posts, based on the height of the deck.
    – Chris Vesper
    Nov 4, 2020 at 15:25
  • Same principle in a different plane; it will not help the horizontal plane of the deck floor. And I don't see enough diagonals on the uprights to do a good job. However, the stairway is providing some lateral stability to the deck..
    – blacksmith37
    Nov 4, 2020 at 15:39
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    Most of the cross brace supports I have installed on tall decks have been on the legs. This adds stability to the deck in a similar manor. Started my construction life in Northern California lots of houses built on hills and all the tall decks had bracing either on the deck or legs.
    – Ed Beal
    Nov 4, 2020 at 15:59

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