0

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.

gomennathan
  • 554
  • 3
  • 12
  • They must surely be designed to carry snow loads. – Harper - Reinstate Monica Oct 27 '23 at 05:52
  • 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 https://codes.iccsafe.org/content/IRC2018/chapter-3-building-planning#IRC2018_Pt03_Ch03_SecR301.5? – popham Oct 27 '23 at 07:43
  • What's the length of the truss's central bottom chord? – popham Oct 27 '23 at 07:53
  • Loading it full of books and your rock collection, or living in it, is entirely different from accessing it (only.) Indeed, storage of strictly lightweight things is also fine, so long as they are, indeed, strictly lightweight and well distributed. Problems arise when folks get sloppy about that. – Ecnerwal Oct 27 '23 at 12:06
  • @Harper-ReinstateMonica I'm sure not all trusses are designed for the same snow loads, and probably lower load trusses are sold in less snowy areas as a cheaper alternative. Plus snow load is on the top of the truss, me walking is all on the bottom, it's not obvious to me that the capacity is the same. Also, does your comment imply that it may become dangerous when there is snow on the roof? – gomennathan Oct 27 '23 at 15:49
  • @popham The trusses look to be 34 ft wide and 6 ft tall. Typical Fink W shape. I think the bottom chords are multiple pieces, not one 34ft long timber. Is there a specific critical number? I could try to go up there and measure. – gomennathan Oct 27 '23 at 15:53
  • @popham And the house is about 50 years old, dunno about trusses but I'd guess they're the same age? – gomennathan Oct 27 '23 at 15:53
  • @Ecnerwal This is the part that confuses me. I guess it depends on how much of a bookworm you are, but a bunch of books on a few shelves seems like it would be similar weight to a human. So why is it okay for human but not books? Books are even a static load, unlike a moving human. Is it that books + human is bad? How exactly do I judge when I've put too much load on? Are people talking about like, 1000 lbs of books all on a 1-2 sqft footprint? – gomennathan Oct 27 '23 at 15:57
  • Also, I guess another part of this is distribution. If I put both feet on the same truss that's 200 lbs on one element. If I put left foot on one and right on the other, that's only 100 lbs per element. If I stand on a 8 ft plywood spanning 4 trusses, then it's 50 lbs per element (plus the weight of the plywood)? So even though the rudimentary flooring adds some weight, it actually relieves the load? – gomennathan Oct 27 '23 at 15:59
  • And while we're at it, I imagine the presence of load bearing walls under the truss is probably also a factor, no? – gomennathan Oct 27 '23 at 16:02
  • "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?" You're "at risk" of a heart attack just waking up and getting out of bed in the morning, especially Monday morning (go look up the stats), so yes, you're "at risk". The risk of your trusses collapsing during this activity is, IMHO, minimal. But really that part makes this opinion based - what's acceptable risk for me may not be acceptable for you. – FreeMan Oct 27 '23 at 16:08
  • Converting your Fink W trusses to double vees instead of double yews, I assume that you're walking on the chunk of wood from one vee's corner point to the other vee's corner point. How long is this chunk of wood? (34 ft)/3? If the trusses are 25 years or younger (I don't care otherwise), then are they spaced on 24 inch centers? – popham Oct 27 '23 at 16:54
  • "Also, does your comment imply that it may become dangerous when there is snow on the roof?" No, I'm implying (hopefully correctly; one can hardly tell the way the climate keeps surprising us) that the government required the builder to consider maximum foreseeable snow loads when designing the truss. – Harper - Reinstate Monica Oct 27 '23 at 19:07
  • 1
    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 '23 at 19:43
  • 1
    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 https://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 '23 at 20:06

1 Answers1

4

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.)

RMDman
  • 28,521
  • 2
  • 24
  • 64
  • 1
    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 '23 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 '23 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 '23 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? – gomennathan Oct 27 '23 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"... – gomennathan Oct 27 '23 at 16:02
  • I would say that "designed for what I'm doing" as in "you're over thinking this". If you need to do some maintenance, just get up there and take care of it. While anything can happen, the likelihood of a truss failure from a person moving around up there for a short period for some maintenance is highly unlikely. As noted in my anecdote, mine have held up to movement & storage for 3 decades. No guarantees about yours – FreeMan Oct 27 '23 at 16:05
  • @FreeMan, your garage location could be a relevant distinction between your and OP's situations. The truss manufacturers tend to use a piece of software called MiTek, where here's what their documentation has to say about it: https://www.mitek-us.com/wp-content/uploads/uploadedFiles/_RedesignSite/Content/documents/engineering/tech-articles/design-tips/Limited%20Access%20Storage%20Load.pdf. – popham Oct 27 '23 at 20:36
  • @Freeman I don't get the point about "overthinking". It's a basic question about how much a truss can carry, relevant to many people. Sure, maybe some people think it's not interesting or unimportant, but obviously the question is for people who do want to know. – gomennathan Oct 28 '23 at 22:25
  • This answer states that what you want to do is just fine. You've then come back and asked for more details like "what if the person is 200+ lbs?", "what if there are sheets of OSB @ 80lb each", "What are the caveats?", "What is the quality control?" THAT part is the over thinking. There's enough of that overdesign built in that 1 person can say (and 4 more agree) that what you're looking to do is well within all the things you're worrying about. If you're not willing to accept that, then either do the math yourself or find a truss co and ask them. – FreeMan Oct 29 '23 at 00:00