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Attic truss has a diagonal brace (red) linking all but outermost king posts (blue). I would like to flip the orientation so that instead of starting at bottom at one end and ending at the top, it would start at the top and end at the bottom. Everything else would remain as is.

Does the orientation of the brace matter?

enter image description here

enter image description here

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4 Answers 4

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If there's only the one, no, it doesn't matter.

If (your drawing, being partial, leaves the complete framing picture unclear) there are a pair of them in the structure such that the pair form an X brace, you'd have to switch both.

Or, if your description is a bit off, the orientation can matter (I have similar braces at each end of my roof, which are indeed specified to run from the peak at the end truss, but they don't go the full length of my roof, which is what your text says this does. Those would not work quite the same inverted. If this does, as you say, go the full length, inverting it would make no difference.)

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  • Why doesn't it matter if there's only one? For two, why is symmetry important?
    – popham
    Nov 11, 2023 at 3:36
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    If there's only one, it was probably just a construction brace, not needed after the rest of the structure is in place
    – keshlam
    Nov 11, 2023 at 14:00
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Ecernwal's right, but chances are that brace is doing nothing. That sort of thing is often installed during construction to stabilize framing, but once the roof is sheathed there's no way the assembly can lean in the manner that such a brace would prevent.

I've installed many engineered truss systems, and never is there a brace specified that's askew like that. They're always aligned with the ridge and tight to the center struts, with specific nailing schedules to each truss. No truss designer will specify a brace fastened akimbo and only at the ends.

Every engineered truss package comes with clearly detailed bracing schedules. There are no standard requirements. They don't all have these. There's really only one way to know for sure, and that's by consulting either the original paperwork or an engineer.

So yes, you can relocate that brace, but when you do, nail it flat to the vertical truss webs on each truss with two 12d nails or larger so it's actually doing something.

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  • See e.g. Fig 1 on Page 34 (4th page, link starts on page 31) regarding permanent diagonal bracing. structuremag.org/wp-content/uploads/2014/09/bracing-issues1.pdf Lack of or improper permanent bracing is mentioned as a frequent factor in truss failures here: warrenforensics.com/2021/02/10/…
    – Ecnerwal
    Nov 10, 2023 at 16:08
  • Yes, but "proper" is clearly described in the paperwork provided with a truss package. There are no generic standards.
    – isherwood
    Nov 10, 2023 at 16:47
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    I thought in high wind prone areas, like Florida, such a brace is required/recommended to help prevent the gabled side of the roof from folding down under wind load. Even though the sheathing helps, without the brace you basically have a box that you're trying to stiffen with fasteners. That will be weaker than a triangular brace.
    – SteveSh
    Nov 10, 2023 at 17:10
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    The load path to the foundation (who says it has to go to the foundation anyway) would be to the floor-of-the-attic joists, to the walls, down to the foundation. And the gable ends of roofs, even a trussed roof design, may not be a truss itself, but rather just a standard stud-framed wall,
    – SteveSh
    Nov 10, 2023 at 19:08
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    @SteveSh, load path to foundation is a necessary condition for equilibrium. Without it, the structure collapses. Your load path through the ceiling stresses a gypsum board diaphragm instead of the roof sheathing diaphragm to resist the horizontal load. There's another quirk of redundant systems like you're proposing: If one system is extremely stiff compared to the other, then the stiffer system can fail before the second system picks up any load.
    – popham
    Nov 11, 2023 at 1:04
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Leave it alone. According to the SBCA's Guide to Good Practice for Handling and Installing Bracing of Metal Plate Connected Wood Trusses,

Web [permanent individual truss member restraint] and diagonal braces used for installation stability purposes and installed at the locations specified for [permanent building stability bracing] can become part of the [permanent building stability bracing] system.

Your diagonal braces are therefore part of the "permanent building stability bracing system." An inspector should have demanded removal of the diagonal braces if that was not the case. The truss design drawings would have specified admissible locations and configurations (admissible yet probably not necessary), but I doubt that you'll find any trace of those documents.

In reality I suspect that designing these braces for "permanent building stability" actually involves verifying that they're not too stiff. If they were too stiff, then they could damage the structure before roof sheathing deflects sufficiently to do the actual resisting. (Put more succinctly, "I suspect that @isherwood is right.") Unfortunately that's just speculation on my part.

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  • I'm not asking whether a brace can be removed, but rather whether its orientation can be flipped.
    – ipavlic
    Nov 13, 2023 at 19:16
  • I don't get how the wind only exists on one side. Imagine the house were rotated 180 degrees, which is the same as rotating the brace 180 degrees.
    – ipavlic
    Nov 13, 2023 at 19:58
  • @ipavlic, why do you want to toggle the brace?
    – popham
    Nov 13, 2023 at 21:28
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Here's the way I look at the use of a brace.

This shows a typical gabled end roof, like you find on many ranchers:

enter image description here

Here's the same roof structure looking at it from the side.

enter image description here

Now when the wind blows onto gabled end of the roof, the flat gable wants to collapse, or pivot about its base. The diagonal brace helps to prevent that from happening.

enter image description here

Without the brace, the roof wants to collapse, or parallelogram, like this:

enter image description here

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    This reasoning only applies to removing the brace, not to changing its direction. If you flip the brace direction, it's now in tension rather than compression, but still does a good job of preventing collapse. (Normally, the roof covering also acts to prevent collapse, but this doesn't apply if you've got something like a thatched roof rather than the common shingles-on-plywood construction.)
    – Mark
    Nov 10, 2023 at 23:43
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    Now superimpose the roof sheathing onto your parallelogram with the left edge aligned with the gable end. The top has moved down from the ridge, but this movement is restrained by the fasteners connecting the sheet to other rafters running under the sheet. The same mechanism works in exterior walls to keep them from deforming into your parallelogram. That wall bracing is developed in Chapter 6 of the IRC.
    – popham
    Nov 11, 2023 at 1:10
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    What I don't get is why would the wind always blow from one direction. The roof is gabled on both sides, so if changing the direction would lead to higher risk, the risk should already be present for the wind blowing into the other gable.
    – ipavlic
    Nov 11, 2023 at 1:34
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    I mentioned in my answer that the two roof surfaces act as robust shear planes. What you describe in your diagrams cannot possibly happen without massive catastrophic damage. If that much force is at play, a couple nails at the end of that board one one ceiling joist do virtually nothing.
    – isherwood
    Nov 11, 2023 at 2:44
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    Think of the walls plus roof edge as a stiff, strong perimeter. There's a hinge, however, at the ceiling elevation. You can brace that hinge with a gypsum board ceiling diaphragm under IRC R702.3.6, but that requires annoying detailing around the ceiling's perimeter. Alternatively you can use bracing to stabilize the hinge.
    – popham
    Nov 14, 2023 at 0:22

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