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I am in the process of adding some plywood decking to the floor of part of my attic, which is built from webbing-style trusses. In the picture below, the cross-brace marked with yellow arrows is a real thorn in my side, impeding travel from one area to another. How can I know if it is structurally required, or just a remnant of temporary bracing before the attic/roof was complete? Those exact same trusses are cross-braced at least 4 other ways, not counting the roof sheathing, or the 1x4 furring strips I installed below my flooring.

I'm worried that the green splotches of spray-paint are in someway meaning 'permanent cross-brace is needed here'. :(

(Please ignore the shoddily draped phone & cat5 wires, and the not-yet-nailed-down plywood flooring-- this is a work in progress.)

[EDITS: Added second picture for Bryce, and 3rd & 4th pictures for shirlock homes.]

general picture of cross-brace in question

For user Bryce: shot of the roof above the brace

For user shirlock homes: the high end of the brace

For user shirlock homes: the low end of the brace

Outside view of garage, showing roof-line, truss orientations, and brace orientation. Note that all of the trusses in the house run in this same orientation, regardless of what the roof-line is doing. For user shirlock homes: outside view of roof

View of garage ceiling, showing brace position relative to trusses. For user shirlock homes: inside view of garage ceiling

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A good start is to see if it's under tension, or just kind of hanging out. Check if it's securely nailed. What's the shape of your roof: those angles look crazy. –  Bryce May 7 at 7:35
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Looks like a game of KerPlunk. –  Tester101 May 7 at 10:26
    
Bryce: I added a picture of the roof, directly above the brace I am asking about. (I laid on my back to take the shot.) In that picture, the roof slopes downward to the right. You can see perpendicular cross-braces above the trusses, between the trusses and the roof sheathing. These give the sheathing something flat to lay against. I'll also be adding additional pictures of the ends of the brace, for another commenter's request. –  Ryan V. Bissell May 7 at 17:32
    
Possibly important detail: this brace never contacts any truss in a flush manner. Always at weird angles (out of necessity, because the trusses, while pre-fabbed, are not identical.) But it is nailed to every truss it contacts. –  Ryan V. Bissell May 7 at 17:40
    
Bryce: how might I determine if it is under tension? (Aside from removing it and "seeing what happens"... which I am seriously thinking about doing, because I can't find any obvious clues that suggest it is under tension.) –  Ryan V. Bissell May 12 at 22:37
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3 Answers 3

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Only the truss designer can say for sure whether they are required or not. A structural engineer might not be able to fully calculate the load transfers in the trusses, as they will only be able to estimate the joint strengths.

I'd hazard a guess that they are required or they'd not be there. From what I can tell from your photo (though it isn't quite clear), it looks like it could be one of the bracings that are required (in the UK at least) by the accepted standards for trusses (for an example, see here: NHBC pitched roof guide).

I'd be reluctant to remove it without first ensuring that you have sufficient equivalent bracing in place.

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Your answer is informative, but I can think of one instance where such a cross-brace as this one would be present, but not required: a temporary brace designed to hold adjacent trusses vertical and parallel, before the roof above is added. But who's going to take the time to remove a temporary brace? That would just increase the contractor's labor cost, reducing profit. (IMO.) –  Ryan V. Bissell May 7 at 16:59
    
It seemed to be at an odd angle for temporary bracing. Having seen the additional photos though, I'm now not sure it looks sufficient secured as a structural member. I've seen temporary bracing both removed and left in place... –  John May 8 at 8:37
    
I think this is likely to be my accepted answer-- due to your first paragraph-- even though it may not be the answer I myself wanted to hear. :) Will accept something in the next 24 hours. –  Ryan V. Bissell May 12 at 22:39
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Although it is very difficult to see from the limited view in your pic, here are some observations/questions. Is the 2X4 in question actually crossing the trusses at a downward angle and not flush fitting on the truss? Are there similar crossing 2X4's elsewhere in the roof structure? In all the truss roofs I have built or done inspections in, I have never seen cross bracing similar to what I see in your pic. It is common to see some cross bracing left behind from the construction phase however. Cross bracing and strong arms are always used as trusses are assembled. Normally the cross bracing is on top of the trusses and removed as the sheathing is added, but I have seen some under the trusses, but usually perpendicular to the truss near the top. I guess that I would be looking at where this strange brace starts and ends to give me an idea if in fact, it is structurally tied to the gable end and an internal support point. So, the bottom line is that I would not rule out the possibility it was used as part of the structure, but I have my doubts.

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Yes, the 2x4 in question is crossing the trusses at a slanted angle, and not flush fitting on any of the trusses that it is nailed to. (The trusses are pre-fab, but not identical, so a flush fit would be impossible here.) There are other similar-crossing 2x4s elsewhere, including some that are flush fitting on perpendicular uprights. (Those do seem structural, as they tend to be 12 feet long, horizontally installed, fastened to 6 trusses. But this skewed one is the only one I have a desire/need to remove. –  Ryan V. Bissell May 7 at 16:54
    
shirlock: I have added pictures of the ends of the brace. –  Ryan V. Bissell May 7 at 17:44
    
And, what's with the splotches of green spray paint? They frequently accompany these awkwardly-installed braces (and never the obviously structural ones.) Does the green mean "yes, OK to remove"? I don't think I would have chosen 'green' to mean "no"... –  Ryan V. Bissell May 7 at 18:02
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It is actually somewhat common for truss designs to require strong back bracing mid span of longer internal compression members to reduce the effective slenderness ratio of the member to something reasonable. The fact there are other braces on the truss does not mean anything, the need for bracing is determined for each individual member.

That said, all strong backs I've ever seen were always exactly perpendicular to the truss span in order to brace identical members of identical trusses. The way this thing runs makes me skeptical that it is a permanent strong back, but there remains a possibility it is required as the members it is bracing are rather long and appear to be compression members. I never could intuit compression versus tension members of internal webs, so I could be wrong on that aspect.

I don't think there would be any issues with removing it temporarily for access during your work, but I would recommend you replace it after the work is completed just to be safe.

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Do you have any insight to share on what is being communicated by the green splotches of spraypaint on the truss members? These splotches only seem to be present at locations where an oddly-placed brace is, never for the perpendicular ones. –  Ryan V. Bissell May 7 at 20:02
    
to be perfectly honest, without seeing the entire system and how they relate to each other, I don't want to mislead you. In over 20 years of building and inspecting, I haven't seen this type of cross bracing. Maybe if you can get a few more pics showing the relationship of the members, I can evaluate it better. –  shirlock homes May 7 at 21:49
    
Sorry, i just saw your additional pics. If that is a structural brace, it is pretty flimsy. It doesn't seem to span an area that I would expect. Can you show me a pic of the roof from outside that is supported by these trusses? there must be some architectural detail there. –  shirlock homes May 7 at 21:55
    
shirlock homes: it's almost dead center in the attic space above my attached garage. It doesn't get within 10 feet of an exterior wall. At one end, it is within 2 feet of an interior wall (the wall between garage and hallway,) but it runs mostly parallel to that wall, on the garage side. If you still think an exterior shot would help, I'll provide some. I could also provide a photo of my garage ceiling, marking the approximate location of the brace above. –  Ryan V. Bissell May 8 at 2:05
    
Update: I misread your request. I will provide pictures, but it likely won't be before tomorrow, since it is now dark outside. –  Ryan V. Bissell May 8 at 2:14
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