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Is a cat walk in the attic above insulation safe for trusses?

ERIC WHETZEL | Posted in General Questions on

Will I undermine my roof trusses at all by adding a catwalk in the attic with a series of 2×4’s? I’m trying to make it easier to do visual inspections of the roof and any repairs in the attic by setting up a walkway above the insulation (about 2′ deep).

I wish I had done this during construction, but I figure better late than never.

The Flatrock Passive House blog has a good picture of what I’d like to do:

http://flatrockpassivehouse.blogspot.com/2017/08/the-attic-hatchway-landing-and-catwalk.html

I think my roof trusses are a little smaller, probably around 2-2.5″ wide, more like what’s shown in this YouTube video:

My set-up would be only for getting around in the attic, not any storage.

Is this safe to do?

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Eric,

    As long as you installed the prescribed permanent bracing that was specified, it's quite safe to add a catwalk.

  2. ERIC WHETZEL | | #2

    Is it ok to use 2.5" decking screws instead of nails?

    Thanks Malcolm!

    1. Expert Member
      MALCOLM TAYLOR | | #4

      Eric,

      The shear strength of a #8 deck screw into dry framing lumber is approximately 150 lbs. So two each end should do it as long as you don't intend to take friends up to wander around, use the catwalk as a platform to perform work in the attic, or as Walter said pile it full of heavy junk.

    2. Expert Member
      BILL WICHERS | | #6

      Malcolm is correct that the screws should work, but...

      You can’t count on even load sharing between the two sides of the platform since the crawler (that might be you :-) may lean far to one side while doing work from the platform. Wood is a naturally variable material too, it’s no where near as consistant as steel is so there is always the possibility of a screw going into a split or soft spot you didn’t see.

      Play it safe, use three screws in each end of each support. Screws are cheap. Fixing the ceiling you just fell through because you cheaped out on screws is a major pain.

      Bill

  3. walta100 | | #3

    Most likely it will be fine but like almost all internet advice it is worth what you pay for it.

    Please understand only the engineer that designed your trusses understand all the forces involved in your case. Also just because you are smart enough to not to load this catwalk with thousands of pounds of junk does not mean the next owner will be your engineer may not approve for this reason.

    Walta

    1. Expert Member
      MALCOLM TAYLOR | | #5

      "Please understand only the engineer that designed your trusses understand all the forces involved in your case."

      No. Every builder who receives the truss drawings accompanying their order knows the value and direction of the forces working on every member and connecting hardware of the trusses. That's the miracle of engineering software. There is no mystery.

  4. Expert Member
    Peter Yost | | #7

    Hi Eric -

    Technically, I would definitely check with the truss company. Each web/chord of any truss is designed to move with different loads according to the attachment to each other member. So, those horizontal connections locking in one web to another does not allow the truss to move with different loads. Not worried about the small and momentary load of a person walking up there, but the live load the roof bears. I got my a_ _ chewed out once by a structural engineer for locking in truss members like this.

    So, I said technically because I just checked on this with a licensed architect and while he admitted that I was right, he said: won't be a problem.

    You are welcome for how immensely satisfying my "comme ci, comme ca" answer is...

    1. Expert Member
      MALCOLM TAYLOR | | #8

      Peter,

      I asked my truss engineer an associated question about temporary braces used during erection (as opposed to the permeant ones called out on the drawings): Is there any danger to leaving bracing between any of the members? His answer was no. The more the better. I take that as an endorsement that the horizontal supports for a catwalk do not affect the performance of the trusses. I'd be interested to hear others chime in.

    2. Expert Member
      BILL WICHERS | | #9

      Peter, I’m curious if you remember what the structural engineer that chewed you out was concerned about. I’m an electrical engineer, not structural, but as I remember from my limited work with statics (every engineer has to take a little bit of the others classes), trusses act as interlocked triangles and aren’t all that complicated. As long as you don’t put forces that bend/bow the bracing or put wierd forces (like upward tension) on them I’d think you’d be ok. That’s especially for temporary and relatively small forces like a catwalk.

      I’ve had issues at work where an inspector doesn’t want utilities (electrical, piping, sprinkler lines) hanging from the lower chord of a bar joist in a commercial building, they want it all hung from the top which interferes with insulation and sealing between walls. My structural engineer has said it doesn’t matter where you hang loads on a “true bar joist” (top and bottom chords are the same size), just try to put point loads at panel points. I would think for standard trusses it would work the same way.

      Bill

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