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tji rafters

sunsen | Posted in Construction Techniques on February 10, 2015 09:56am

I’d like to substitute tji i-beams, (or whatever they’re referred to), for 2×8 rafters on a plan. Has anyone here ever used them in framing a roof before? I think the smallest ones are 9+” deep so I’d need to do a seat cut for sure in order to have everything work out height-wise. Is it possible to make a seat cut on these things? Do you block the web? If a seat cut is not allowed, how are they secured? Hurricane ties or something?

Anyone with some experience in this regard who has some input would be appreciated.

Tom

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Replies

  1. DanH | Feb 10, 2015 10:13pm | #1

    I saw a couple of houses up in Fargo done with them, but I didn't pay that much attention to the details, and it's been several years.

  2. User avater
    deadnuts | Feb 10, 2015 10:43pm | #2

    how about checking w/ the manufacturer

    Weyerhaeuser has information on how you can, as well as how you cannot, frame rafters with their product on their website. You can download the pdf here:

    http://www.techsupport.weyerhaeuser.com/hc/en-us/articles/201758760-Blocking-Requirements-for-TJI-joists

    1. sunsen | Feb 11, 2015 09:07pm | #5

      Thanks, I'll head on over there and have a look.

  3. Geoffrey | Feb 10, 2015 11:19pm | #3

    Tom,   TJI's can be used for roof framing but require much more involved blocking and bracing than standard 2X lumber.

     You  would probably need someone to spec. out the correct size rafter for your situation, usually you can get your local TJI

    supplier to do that for you. I would strongly urge you to use the fire rated version (which also requires a ceiling of 1/2"

    sheet- rock to achieve that rating).

    If you plan on using them elsewhere on the job you should be using the fire-rated version w/ 1/2" sheet-rock, in other words-

    no open ceilings. There has been recent concern with the lack of fire protection on these types of framing members.

    The standard version is definitely  a fire concern, which is why I have never liked them from the get-go and would stick with

    solid-sawn lumber. just my humble opine.

    check out this link.  Weyerhaeuser :: TJI® Joists

    Good Luck!

    Geoff

    1. sunsen | Feb 11, 2015 09:06pm | #4

      I think I've got the fire angle figured out. I'll be foaming everything on the underside of the roof. I'm not exactly sure why they'd be more of a fire risk than solid stock anyway. Does the stranded part burn easier? Or is it the increased surface area?

      I've been using only TJI's for floor joists for years. I really like the uniformity of the floors. That's why I want to give them a try on the roof of this house I'm building for myself. I really want a flat, stable roof. 

      1. cussnu2 | Feb 12, 2015 01:49pm | #7

        I believe the fire issue is precisely that the thin web absolutely burns too quickly which immediately compromises the ability of the member to hold up what it is supposed to.  Fireman see them as a direct threat to their lives because they lose strength so quickly in a fire versus a 2x that can burn for a very long time before it will structurally fail.  I know if I were a fireman called on to enter a burning building, I would absolutely rather do so in a solid framed floor and roof than to do so in one constructed with TGIs.

        1. calvin | Feb 12, 2015 06:39pm | #8

          There has and continues to be discussion on this matter in the multi family housing arena which by itself does spill over into single family dwellings.

          http://www.jlconline.com/fire-safety/fire-storm-is-light-framing-at-risk_o.aspx?utm_source=newsletter&utm_content=jump&utm_medium=email&utm_campaign=JLCNL_021215&day=2015-02-12&he=126eb9abfd6326e2bfc9486408b084c36f1455d8

          .

  4. User avater
    BossHog | Feb 12, 2015 05:46am | #6

    The biggest drawback to using I-joists as rafters is that you have to have a ridge beam.  You can't simply use a ridge board, as you might with rafters.

    The literature for manufacturers I've seen does allow for a seat cut at the wall.  But part of the bottom flange must rest on the wall.

    Where they attach to the ridge beam it typically requires special hangers and blocing. 

    Overhang details vary from one company to another.

    If you've used them for floors before you know what brand you're dealing with.  A look at their website should give you the option to download their engineering details.

  5. sunsen | Feb 13, 2015 10:05am | #9

    I spoke to my engineer yesterday. He said, "Why not use the rim joist stuff? It's way stronger than standard lumber and doesn't have all the fastening issues of the tji joists." 

    Anybody frame a roof with this before? I think it's called LVL or something like that. I just order the stuff along with the TJI's I use for floor framing and I don't remember the exact term.

    1. mark122 | Feb 14, 2015 12:54am | #10

      LVL's will be an expensive roof frame. we mostly used them for large hips, valleys etc around here and dimensional lumber for commons and jacks. if the price tag doesnt bother you then yes they do not have the resitrictions TJI's have and you can span them considerablly more than dimensional lumber.

      oh and your framers will curse you on delivery...those big boys get heavy!

      1. DanH | Feb 14, 2015 07:08am | #11

        Yeah, I think you'd need a crane for LVLs.

      2. sunsen | Feb 21, 2015 05:59pm | #22

        Framers? What framers? I'm the framer.

        Turns out the LVL's can be purchased at 1.75" x 7.25" dimension-wise. I don't recall them being super heavy and at this dimension they're essentially like plywood versions of 2x8's. They're four times as expensive as solid stock though, which means I'm going to have to think about it. 

        They're the only engineered material I've been able to locate that will do the job. TJI joists are limited to 2' eaves and mine are 4'. Also, the LSL rim joist material isn't strong enough. Cheap though. 

        1. mark122 | Feb 23, 2015 06:50am | #23

          is your main concern the 4' overhang? or the possibility of sagging you are trying to avoid? how long are your rafters spanning?

          1. sunsen | Feb 23, 2015 08:48pm | #24

            I neglected to mention that along with a 4' eave I want to do a knife edge, no gutters or fascia to provide for looser tolerances. So yes, my main concern is any sort of deviation that is inherent in solid stock. If I do go with solid stock I'm sure I'll be installing full 1/2" ply gussets at each rafter in effort to line things up. I'm just weighing whether the extra cost of the LVL's will equal that of the alignment process. Also, this is not a cut up roof so there are some long runs to deal with. I think it will look pretty clean if I can pull it off though so what the heck, might as well give it a shot.  

        2. qualityjob | Feb 25, 2015 02:10am | #25

          lvl

          My area doesn't allow TJIs without sprinklers as they are considered a fire hazard ...  I use LVLs for floor joists and other things.

          The material costs are high but they are pretty easy to carry (1 or 2 guys depending upon the size), completely straight, they don't warp/twist, they don't deflect easily, and my framer loves them so I probably save a little on labor costs, though that is hard to quantify. 

          If you have 4' eaves I think you'll end up appreciating their qualities, they are (I believe) rated 4x the strength of conventional lumber.

    2. john7g | Feb 16, 2015 11:39am | #12

      Framers?

      Have you thought about what the framers are going to say and how much extra they're going to charge you when you use anything but dimensional lumber?  Some may not even want the extra hassle and won't bid thereby narrowing your field of choices.  

      Waht value for your project do you gain using engineered materials over standard diemensional lumber. Just because you can spec it doesn't mean you should.  

    3. User avater
      BossHog | Feb 17, 2015 07:54pm | #13

      Rim joist material typically comes in 12' lenghts, and is not intended to be used for rafters.

      LVLs would be exhorbinately expensive for rafters. Probably 4 to 6 times the cost of I-joists.

      1. strawmyers | Feb 18, 2015 12:19pm | #14

        This is an honest question, not sarcasm: what roof covering are you using that requires TJI/LVL level straightness?  It's amazing how much a dimensional shingle will hide... especially for the price difference versus going with the above options.

  6. jimblodgett | Feb 19, 2015 10:44am | #15

    We framed the house we're currently building with 16" I joist rafters. 

    They did cost a little more than the materials would have been to frame and furr 2x12s to the same depth but this is the second time I've framed a roof with them and I really like them for a variety of reasons.

    We used a third top plate, ripped at 22+1/2 degrees (5/12), to land the downhill end of the rafters on.  Worked great. The blocking and fastening details were clearly specified in the pamphlet we got from the manufacturer.

    The single biggest difference framing with them is building overhangs and rafter tails, but I enjoy carpentry, so even those details just seem like intesting challenges to me more than added expense or frustration.  Piece of cake, really, just a little different than framing with solid sawn stock.

    The one unexpected issue we ran into was insulation widths.  This house we're building is super insulated (why we chose 16" depth in the first place) and as you know batt insulation is typically cut to fit snuggly between framing members.  I joists webs are only 1/2" thick, not 1+1/2" like most 2 x materials.  So batts manufactured to fit beautifully between sawn lumber don't really fill the cavity between I joists.  Just something to think about that I wished I had forseen.  Don't know where you're building, or how much you care about this level of detailing, but in my mind, for this house at least, it matters. 

    1. calvin | Feb 19, 2015 02:05pm | #16

      Jim

      Pitchers and catchers reported!

      unfaced batts are available wider from steel stud suppliers (or most commercial drywall and ceiling suppliers).   Works well when dealing with steel studs or drop ceilings.  If a person needed wider for friction fit between I-joists, maybe they would fill the bill.

      high if 3 today, low minus 15

      yippie

      1. DanH | Feb 19, 2015 03:20pm | #17

        Gee, it only got down to 11 below this morning, here in tropical southern Minnesota.  And it's 7 above right now.

      2. jimblodgett | Feb 19, 2015 07:03pm | #18

        "Pitchers and catchers

        "Pitchers and catchers reported!" Ahhh.  Life begins anew.

        You're right about insulation widths, too.

        Temps in the high 50s by day, 30s at night out here on the glacial till the past couple weeks.  Stayed warmer last night and we got a little rain today.  Might build a small fire tonight after dinner, but didn't have one in the shop today.  Very mild for Feb.

    2. mark122 | Feb 21, 2015 09:32am | #20

      how do you get super insulated with fiberglass?

      how do you guys achieve "super" insulated with fiberglass? i cant see how you can achieve it with fiberglass, to much air movement.

      1. jimblodgett | Feb 21, 2015 02:42pm | #21

        dilligent detailing

        We used several techniques - thoughtful framing layout, minimum electrical, plumbing and mechanical penetrations in the shell, lots of spray foam to restrict air movement from one bay to another, meticulously installed insulation, airtight drywall...other things I'd rather not post on a public forum that may or may not be "legal" but are recommended by a building scientist who's kool-aid I drink. 

        Bottom line, I believe in the principle - "What happens in the wall, stays in the wall (or ceiling). Do everything possible to stop air (and of course water) from entering the framed cavity and the rest is easy".  So that's what we do.  Been using fiberglass - blown when possible, eco batts when I have to - for many years.  You can keep your cellulose and spray foams, I'm dancing with who brung me.

  7. cussnu2 | Feb 19, 2015 10:09pm | #19

    Base e ball Ben berry berry good to me

  8. oldhand | Feb 25, 2015 08:54am | #26

    back to the fire thing..

    I just read what happens in case of fire they fail quickly because the glue melts and they fall apart  before they burn up. Solid lumber deforms before failing,  slowly losing it's strength.

    Seems to me like a very real draw back.

    1. sunsen | Feb 25, 2015 09:30pm | #27

      Well, to be honest, I'm not planning on having a fire. I think the odds are low of that happening. 

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