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My roof is made up of Fink trusses (chords in W shape). The trusses are made of 2x4s. The bottom edge of the truss acts as a ceiling joist and drywall sheets are attached below it, forming the ceiling.

Occasionally, I need to crawl into the attic for repairs. To make life easier, I laid some OSB boards across so I don't have to tiptoe on the joists (bottom edges of trusses).

However, looking at some questions on this site makes me think that this is a bad idea. Apparently, the trusses are designed to have just enough strength to carry themselves, the roof and ceiling and adding flooring would overload the truss. For example, this answer seems to suggest at least 2x8s are needed as joists.

Does this mean that when I crawl around the attic to fix a light fixture, I'm at risk of destroying my roof and ceiling?

Note, I am talking about occasional, brief maintenance access only. Not regular living or storage.

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  • They must surely be designed to carry snow loads. Oct 27, 2023 at 5:52
  • How old are the trusses? Are you in an IRC jurisdiction? Are there attic areas that satisfy conditions 1, 2, and 3 from footnote g of the table at codes.iccsafe.org/content/IRC2018/…?
    – popham
    Oct 27, 2023 at 7:43
  • What's the length of the truss's central bottom chord?
    – popham
    Oct 27, 2023 at 7:53
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    The difference between you and your books is that the books sit up in that space waiting for the 50 year return interval snow and wind loads to show up. When those loads show up, you will have robbed from the capacity to resist them. There's a relatively small probability that you're going to bumble around up there during these 50 year return interval events.
    – popham
    Oct 27, 2023 at 19:43
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    And the other difference between you and your books is the time duration of the load. See the Table 2.3.2 from page 11 (pdf page 3) of awc.org/pdf-viewer/?idp=1414&idf=17 for the load duration multiplication factors. Wood carries more load for short durations than it can for long durations.
    – popham
    Oct 27, 2023 at 20:06

1 Answer 1

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Trusses are engineered to allow just what you are doing.

Stepping on the bottom chord or spanning a few with planks for occasional brief maintenance is fine.

The answer you linked was addressing someone turning the attic into living space and adding flooring. That cannot be done. Trusses would have to be designed for the purpose of adding flooring for storage or such if that were your need.

You need not worry about your ceiling unless you are an unusually big person of heavy weight. ( reserve any mention of specific weight to not offend, but you all know who you are.)

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    As anecdotal evidence only (not suggesting that this should be done), I put OSB flooring down on my garage trusses 28 years ago and started piling junk, errr... stuff, on them. They've held up just fine. Again, not saying this should be done, but I'd imagine that there's more than enough extra capacity designed into these because the manufacturers know that people will do this even though they shouldn't...
    – FreeMan
    Oct 27, 2023 at 12:49
  • @FreeMan, My experience with truss manufacturers is just that. Sometimes the trusses over garages are designed with an "open" center to allow the owner to place planks for storage. They are designed to allow extra weight , but not advertised as such, suspecting that if people knew, they would go overboard as is often done.
    – RMDman
    Oct 27, 2023 at 13:04
  • Indeed. To clarify, mine are not designed for storage (I wish they were). They are a "W" design, and it's really inconvenient to move up there and get boxes moved around. I'm just posting this as an anecdote to say that storing "a few things" or even "a lot of junk" won't, necessarily, lead to immediate collapse. Therefore, your answer, that walking around up there on occasion for maintenance is perfectly acceptable, is 100% correct.
    – FreeMan
    Oct 27, 2023 at 13:06
  • When you say "designed for what I'm doing", do you mean just say 200 lbs person (including tools) walking around? Or would putting down a bunch of OSB boards (~80 lbs each for 4x8ft) also be part of the "design parameters"? Is there a caveat like OSB boards are okay for the day, but not permanently? Oct 27, 2023 at 15:47
  • @FreeMan great point and I agree. The problem is how do you know how much "extra" a particular manufacturer built in - is it a few tens or hundreds of lbs? Plus, the other issue is that the "extra" is not subject to quality control, so for all you know, one of the trusses you step on could turn out to happen to have a lot less "extra"... Oct 27, 2023 at 16:02

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