Fastening strapping for steel roof
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| Posted in General Questions on
For my simple gable roof, I will be installing exposed-fastener steel roofing atop 2×4 strapping atop Grace Tri-Flex underlayment atop OSB sheathing atop trusses 24″ oc. Everything I’ve read indicates that screws would be best for fastening the strapping, as they offer far greater withdrawal resistance than nails, but I haven’t found any recommendations on the best fastening pattern. Would using two 3-1/2″ exterior deck screws at each intersection between the strapping (laid broad side down) and the truss top chords be best?
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Replies
The strapping too will be 24" oc, and the 3-1/2" screws are #10.
Jeff,
I'd do what almost all builders do and fasten the strapping with two 3 1/2" nails. While the screws do offer better withdrawal resistance, unless you are using structural screws, they have far less shear strength, and that where the majority of the load will be.
Thank you, Malcolm. Am I mistaken in thinking the OSB sheathing holds the trusses steady enough to prevent racking forces from exerting significant shear stresses on the strapping fasteners? If not, are you thinking that wind acting on the steel roofing itself exerts more shear stress than withdrawal stress on the fasteners holding the strapping?
Jeff,
No you are right: There is no appreciable racking load from the trusses. On a pitched roof the fasteners attaching the strapping are subject to live and dead loads which act on them primarily in shear, as they are applied vertically by gravity and horizontally by wind.
You can intuitively test this by imagining what would happen if you simply placed the strapping on the roof without fastening it. It would either slide down, or perhaps in very high wind up the roof, but there is no force acting through the sheathing trying to withdraw the screws.
I guess both my answers have been remiss in that what I should have said is that none of the loading is significant enough to worry about whether you use screws or nails. Most builders use nails because they work fine and are what people frame with.
Thank you for clarifying where the shear stress arises, Malcolm. I should have mentioned my pitch, 4:12, as a greater pitch would obviously magnify the shear stress due to gravity.
I read something a year or so ago about uplift forces on a roof, and although I seem to remember a more detailed discussion, the following article's discussion of purlin to truss connections vs. uplift force might have been it, roughly 60% down the page:
http://www.constructionmagnet.com/rural-builder/behind-the-hammer-with-mike-momb-common-post-frame-failures-and-how-to-prevent-tem
When I went looking for that article yesterday, I happened onto this discussion on an engineering forum regarding screws vs. nails in wood. They make several interesting points, among them how little wood actually makes contact with a nail to hold it in place:
http://www.eng-tips.com/viewthread.cfm?qid=317307
My understanding is that the uplift force on a roof is due to Bernoulli's principle, explained as it pertains to roofs about halfway down this page:
http://www.nsta.org/publications/news/story.aspx?id=49598
I don't know how much, if at all, that principle or the first article cited above apply to strapping atop a sheathed roof, as the air pressure inside the house is contained below the sheathing. I don't know how much pressure differential may exist between the airspace between the sheathing and the steel roofing vs. the air above the roofing.
I appreciate your noting that screws or nails would be adequate. Would spiral or ring-shank nails offer the most optimal combination of shear strength and uplift resistance?
Jeff,
Yes, as you say, there will be uplift. Stick-framed roofs often include collar ties to counter it. Trussed roofs have it built into their engineered structure, as does the nailing pattern of your sheathing.
I think it would be fair to say that there are a number of connections in a house where using structural screws would be superior to nails, but stepping back, the question is are there nailed connections in standard house construction that can cause future problems?
Leaving aside high seismic areas and those that experience extreme weather, I can only think of a couple: The attachment of deck ledgers, which has recently been addressed by most codes, and subfloor to joists, that may lead to squeaks. The rest, including roof strapping, simply don't necessitate anything more than adhering to the code specified nail length and spacing.
Please don't take this as a criticism, but one of the things that distinguishes owner-constructed houses from those of builders is that you almost invariably find some connections or details over-thought, with bolts or lag screws, additional structural members, or layers of membrane in areas that don't require them. That's not necessarily a bad thing. If something is got to be a worry to you, why not do a bit more so you don't have to think about it again.
I'm sure I overthink quite a few things, Malcolm, and I don't mind the suggestion at all; I'm much more afraid of underthinking something. I wish this fastening method were prescribed in the 2012 IRC; it would have saved me a few hours of trying to find an answer. Is there a code you follow in Canada that has a prescribed fastening spec for roof strapping that I could read online? If not, would you recommend spiral or ring-shank nails?
Jeff,
I only made the comment in the hope it would help reduce the worry some of these decisions can cause.
Here in BC we work from a modified National Building Code. It has tables prescribing the required nailing for all major building components at 9.23.3.4 and more specifically for roof framing at 9.23.14.8. I don't know if these are available online. While it deals with most situations, it isn't exhaustive, leaving many situations to a more general interdiction that 82mm length nails be used to connect 2x lumber and that they conform to a certain CSA rating. I suspect that is why you haven't found it in the IRC. It isn't important enough to merit a specific prescription.
If it were me I'd use two 3 1/2" nails at each truss. I don't know any framing nailer that could sink either spiral or ring shank nails that long, so if you wanted the added insurance they would afford against pull out you would have to hand nail. Seeing as there are so few fasteners involved, why not buy some structural screws from someone like GRK and sleep well?
BTW, good luck with your build!
I greatly appreciate your help, Malcolm, and if I've exhausted you on this topic, I totally understand. At the risk of continuing to overthink this, but perhaps with a benefit to someone else who has similar questions or would like to chime in, I've found the following options:
The structural screws by GRK would cost almost $300 for the 700 I need, and given that you don't think they're at all necessary, I wouldn't spend that much.
You're right that a deformed nail of 3-1/2" isn't available for a nail gun, at least not at HD. Interestingly, they only have one 3-1/2" nail of any kind for a nail gun, and it's only for a 28-degree gun, which I don't have. The closest they have in a collated nail is a 3-1/4" ring-shanked: a box of 2000 costs around $46. Would the ring shanks make up for the 1/4" less length?
HD also has a 3-1/2" spiral nail, galvanized, but it's skinnier than a common 16d; it's 9-gauge. Would it be too skinny? One advantage of hand-nailing is that I'll be able to feel whether I've hit the truss, although if I'm careful enough about marking lines, I shouldn't miss. 780 for around $50.
I also found some 3-1/2" ring-shanked polebarn nails at Amazon that look (no diameter specs) a lot fatter than the spiral nails; they're around $45 for 900. I just hope they're not TOO fat, as I don't want to split any wood.
Jeff,
If you've found 3 1/4" ring shanks to fit your gun I'd use those. You may find them hard to sink completely even with the pressure turned up. Might need to keep a hammer handy to get them flush. You will know if you've hit the trusses by the sound.
Many thanks, Malcolm, especially from my arm!