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Ceiling truss detail–any opinions?

| Posted in General Discussion on November 8, 2001 01:37am

*
Hi all. I’m a long time lurker, infrequent poster.

I’m building my garage and plan to use open web floor trusses for the ceiling/attic floor. Instead of just running a kneewall over the door opening, I’m thinking of using an extra truss there and scabbing a 2″ wide kneewall to the truss. This will give me something to nail sheathing to and allow the truss to sit completely on the 2X6 top plate.

Ok, the truss atop the wall probably is overkill; the header is 3 #2SYP 2X12s for a 10′ span. But I’m putting a single pane window into the gable above this that I might open up some time in the future to allow hoisting of stuff into the attic. I’ll probably need extra bracing if I do open it, and adding a truss is an easy thing to do right now.

That said, can anyone see any problems with my building it this way, or does anyone have any opinions about this?

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  1. piffin_ | Oct 31, 2001 06:04am | #1

    *
    I've done this a few times. The only problem is that the trusses are built with a slight crown. On the open floor it will settle out a little with weight from materials and live load to nearly level. At the end wall, if you nail sheathing to it first, it won't settle. If you platform frame the wall won't set right level on it without a rocking chair effect.

    so you need some barclamps to pull it down in the middle, fasten it with screws, and go ahead with the job.

    1. blue_eyed_devil_ | Oct 31, 2001 01:22pm | #2

      *Sorry to disagree Piffin but here goes..You should never, ever attempt to pull down a crowned truss. blue

      1. Boss_Hog | Oct 31, 2001 02:21pm | #3

        *Not all trusses are crowned. Depends on who built them.But Blue is right - Pulling the crown out of one probably isn't a good idea. I really don't see how adding a floor truss over a bearing wall will provide any "extra bracing" for you.

        1. piffin_ | Oct 31, 2001 08:30pm | #4

          *OK Blue - Why not? I'm ready to learn something new.Do you just span across it with sheathing and not nail into it?

          1. John-Texas | Nov 01, 2001 04:52am | #5

            *Piffin, Blue, Ron, thanks for the replies.>I really don't see how adding a floor truss over a bearing wall will provide any "extra bracing" for you.Ron, it's amazing how much a few words can help. I think I've got a better handle on it now. The vertical studs of a kneewall will do a better job of transferring downward forces than the diagonals of a truss could, and any racking force is easily transferred through the sheathing and attic decking.Eventually, I'd like to have a 500lb hoist on a gantry, so I'll have to somehow distribute 1000lbs of weight inside the building (I understand this is traditionally done with an engineer's stamp.) For now, I'll probably just add an extra truss in the first four feet to give myself something to work with later.

          2. blue_eyed_devil_ | Nov 01, 2001 01:10pm | #6

            *Piffin, pulling down a truss stresses the truss. If it is built with a camber, it will come down on it's own after it is loaded. If you've pulled it down, then fastened it tight at both ends, somethings got to give (buckled compression members?).If you are in a cold climate, you might be setting yourself up for some serious truss uplift damage. If possible, it's better to let the bottom chord float up and down without restriction. I lived in a house that had substantial uplift. Every winter, the ceiling would raise up about 3/4". The drywall tape would form a 45º angle.Any engineers want to chirp in with better info?blue

          3. Boss_Hog | Nov 01, 2001 02:26pm | #7

            *John -If you're going to use a hoist down the road somewhere, plan for it now. Trying to beef up trusses later to do the job can me problematic. Talk to your truss guys about what you want to do, and plan for it now. Floor trusses probably aren't good candidates for supporting a hoist, but adding some beams now might be a good option. They go in a lot easier as you're building it than later on. I once did roof trusses for a 60 X 100 (I think) shop which had a hoist system under it. They wanted two separate 1,000# hoists that could travel anywhere in the shop within 3' of the outside walls. We had to design the trusses for 10,000# concentrated loads where the beams were suspended from the trusses. But since it was planned as the building was constructed, it was no big deal. (Although it probably cost a fortune)

          4. piffin_ | Nov 02, 2001 02:03am | #8

            *OK blue, I understand about regular cieling/roof trusses and wouldn't think of pulling them down. That is where you get the uplift problem you describe. Mr Texas here is talking about a floor trusss that forms cieling of the first floor and joist for the second. The rest of the floor system will load down with sheet rock, framing, live load etc. but if you've placed one at each end wall and nailed the sheathing to it while still unloaded and crowned up, it won't be able to come down with the rest of the floor. This will give you a serious floor leveling problem. BTW, I'm not being beligerant - just looking to search out a good answer with this argument.Suppose you place a set of bookcases on the floor in this location. It will lean into the room and possibly spill the books.So I get an open web floor truss with about a 3/4" crown. I use the clamps and pull it down to only about 1/4" and use screws plus nails in the exterior sheathing to hold it there. The clamps put no more load on it (just preloading is all) than the eventual load on the rest of the floor system so it isn't hurt. I recognize the problem of a drum effect by fastening the ply subfloor to it immediately but that seems less important than having the wall above do a rocking chair and set above the other perimeter walls perpendicular to it by 3/8"I honestly have never considered this a great problem before or heard complaints from customers. But then most of these I have done were for apartments or studios above garages where free open space below was a major reason for using the well engineered floor truss system. They are really great for large open spans and the mechanical subs love them too.Hope to hear more on this one - hopefully from engineers.

          5. Mike_Willms | Nov 02, 2001 03:41am | #9

            *I'm confused. Why not just frame the wall continuous to the top of the floor trusses? If you're looking for added strength by setting a floor truss on top, why not use an LVL instead? I would honestly never-in-a-hundred-years set a floor truss on top of an exterior wall anyways. Maybe I'm not seeing something, what would it gain?

          6. piffin_ | Nov 02, 2001 04:26am | #10

            *Why not just frame the wall continuous to the top of the floor trusses? >>>>'cause most times you frame all four walls the same height and tie top plates together.LVL sounds good to me, though the cost might get steep on a sixteen inch one.I guess the idea just grows out of the process with plain solid 2 by lumber where you use same product throughout the system. I get confused ordering too many different things;>)Keep teaching me.........

          7. blue_eyed_devil_ | Nov 02, 2001 12:01pm | #11

            *I'm also confused Mike. Trusses are relatively expensive and I don't understand why anyone would use them as a rim joist, especially knowing that they are going to sheat them later.Piffin, I wouldn't reccomend pulling down a truss. If it'ts simply being used as a rim joist, I'd cut it instead.blue

          8. Boss_Hog | Nov 02, 2001 02:42pm | #12

            *Piffin -I aren't an injuneer, but I'll give this a shot since I work in the business. The reason I suggest i notpulling a truss down is that it's going to try to return to it's original position for a heck of a long time. If it eventually pulls it's way back up to it's original position, it might crack drywall or something like that. A truss fastened to a wall can sometimes pick the whole wall up with it. Pulling 1/4" out of a truss as you suggest probably wouldn't do much to it. But if a guy got carried away, the truss might get damaged. Not likely, but possible. I know of a guy who stood a bundle of roof trusses up and set a bunk of lumber on the peak with a forklift to try to take the crown out of them. (I'm not sure how he held it there - didn't actually see it) That's dumb, and might damage the trusses.

          9. piffin_ | Nov 03, 2001 01:25am | #13

            *Thanks all, I'm starting to get my head wrapped around this idea.

          10. blue_eyed_devil_ | Nov 04, 2001 03:20am | #14

            *Ron, I agree. I've seen guys go to great lengths to pull the trusses down. The truss does indeed spend it's lifetime trying to get back up!Piffins concern is with the theoretical hump left at the wall line. In theory, all the trusses will settle straight, and if we don't get the crown out of the rim, the floor will flare up at the walls. Piffin has just stated my reason #132 why I don't like floor truss systems.The more obvious problem is getting the rocking chair effect out of the floor in it's relationship to the wall. Since we do a good job of framing our walls straight and sheathing them, when we stand them up, the ends will be floating (if sheathed with osb). The ¼" gap at the floor/deck connection is unsettling to those of us who care. Like I said, I just cut the truss if it is used as a rim, or we stick frame the end kneewall ourselves. Stick framing poses other problems however (uneven shrinkage factors- reason #159)blue

          11. Mike_Willms | Nov 04, 2001 03:45am | #15

            *Reason #132 is applicable to all floor systems. TJI's don't have a crown, but will sag when loaded...as will any type of floor. Piffin,I can see the benefit of tying the top plates together, but what about the hinge effect caused by setting a knee wall or floor truss on top of the parallel wall? My way, the floor sheeting keeps it solid. The end walls can't seperate from the lower walls since the sheeting ties them together. The floor sheeting would literally have to tear down the middle of the house for these walls to seperate. In the end, it has the same effect of keeping the walls tied together, maybe better.

          12. piffin_ | Nov 04, 2001 04:26am | #16

            *That hinge effect is what I don't like so I've never done with kneewalls. Supposing that I now build with truss next to the wall (almost a ballon style set) and run subfloor up to the wall. Now when it moves with weight My baseboard shows an openning like roof trusses can leave at the cieling. I like it all tied together.Blue stated the case well but I don't see too much concern about the truss raising back up because I don't load it down 100% - maybe 60-80% to approximate what the design load will place on it for the rest of the floor. I like his idea of just cutting the sucker but then we're back to a kneewall so It is maybe best to build a kneewall and set the subfloor ply right over it. That kneewall can slow production down for a crew who's ready to rock'n'roll on with that decking.

          13. John-Texas | Nov 04, 2001 11:09am | #17

            *Piffin, for what it's worth I had the truss mill make the kneewalls as well as the trusses. The unit price was six bucks less on a 20' span.

          14. blue_eyed_devil_ | Nov 04, 2001 02:25pm | #18

            *Piffin, how does a truss change your concern about hinge effect?Like John from Texas, we've had the kneewalls sent out. Gimme dimensional lumber any day....blue

          15. Rich_Beckman | Nov 04, 2001 06:25pm | #19

            *> Piffin, how does a truss change your concern about hinge effect?Thank you, Blue. I'm not sure I'm following this discussion very well, but that question occurred to me. Since you are asking it, I must not be too far off in my visualization.Rich Beckman

          16. piffin_ | Nov 05, 2001 04:21am | #20

            *bnecause the truss is tied to the floor system with the ply subfloor - You're all right, this got too theoreticall and abstract with suppose this and if that. I never thought about ordering a kneewal from the truss builder. That's why I'm here - to learn.You gotta realize that when you're on an island, new ideas are a little slower to percolate. When I moved here about fifteen years ago - they were just finding out about things like PT, construction adhesive, joist hangers, trusses, and premixed concrete. To be fair - the ferry was a little smaller back then so big trucks couldn't make the trip out. But most of my better ideas I got from years back when I was woreking elsewhere.But I'm just full of ideas I haven't even thought of yet!

          17. blue_eyed_devil_ | Nov 05, 2001 01:32pm | #21

            *Piffin, what Island are you speaking of?The hinge effect remains whether it's a standard truss or a kneewall truss. A few lacers will alleviate any of that worry. blue

          18. piffin_ | Nov 07, 2001 04:59am | #22

            *blew,your e-mail doesn't work - came back at me.Send to me for reply

          19. blue_eyed_devil_ | Nov 08, 2001 01:37pm | #23

            *Piffin, interestingly, my email links to an old outdated address, but my preferences indicate that my email is [email protected], which is correct. I wasn't able to find the dialogue box to delete the old one.blue

  2. John-Texas | Nov 08, 2001 01:37pm | #24

    *
    Hi all. I'm a long time lurker, infrequent poster.

    I'm building my garage and plan to use open web floor trusses for the ceiling/attic floor. Instead of just running a kneewall over the door opening, I'm thinking of using an extra truss there and scabbing a 2" wide kneewall to the truss. This will give me something to nail sheathing to and allow the truss to sit completely on the 2X6 top plate.

    Ok, the truss atop the wall probably is overkill; the header is 3 #2SYP 2X12s for a 10' span. But I'm putting a single pane window into the gable above this that I might open up some time in the future to allow hoisting of stuff into the attic. I'll probably need extra bracing if I do open it, and adding a truss is an easy thing to do right now.

    That said, can anyone see any problems with my building it this way, or does anyone have any opinions about this?

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