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This is a feasability type question. I know that pre-fabbing truss roofs isn’t anything new. I’ve never seen it done, but think I may have the right application to give it a try. The nickel tour: Scissor trusses forming two gables. Truss over-frames form a cross-gable roof, with the second gables a few feet lower than the first two.
Since their down there, may as well paper and roof, too. We’ll frame the top floor walls with a single plate, and land the roof with vent-blocks, fascia, barge, skylights, unit separation, bracing, hurricane clips, everything. The top plate to the wall will go up with the roof, and get screwed to the first one from below.
How to rig this thing, what kind of additional bracing could I expect to have to put into the webbing, and any views whatsoever from anybody are welcome.
Nathan
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Nathan,
A while back a guy was posting on the board about his framing prowess and posted some pictures of a gable roof that he framed, sheathed and felted on the first floor deck, it was really quite interesting.
I am always looking to make things more efficient, and do some truss prefab on the ground wherever I can. JLC had a recent article on prefabbing hip truss packages by Mike Guertin, I think that was a pretty good synopsis of the system I've been using for many years for these types of situations. One problem I had with the article was that he maintained it was the safest way, bar none was my impression, to set a hip package. I'm pretty sure it can be done safely any number of ways.
Prefab on the ground requires more thought, and probably a more experience crew to do it efficiently. It also requires a good bit of flat ground. Be prepared for the possibility of less efficiency the first time out.
Probably the biggest issue in prefabbing trusses is double handling of components, generally with a crane, which is expensive.
Your situation, unless I am not reading it right, could probably be set, sheathed and felted in a day by a good crew of four, I would think. unless its really big. I'm not sure what the advantage would be. There are probably some regional considerations too.
I have had good luck on hip truss prefab when I have 4 or more hip/mono packages involved, I can usually work those together on the ground while three guys are setting commons, gable reverses, etc.
Let us know how it comes out.
*Here's the JLC article:http://www.jlconline.com/jlc/archive/framing/truss_roofs/index.html
*Nathanleave the clips off until the roof is set. We tried it that way once and the part of the clip that ties it to the top (middle) plate kept hanging up on the walls as we were setting the roof. Made for a real PIA to get it aligned with the thingys hanging down every where. Hats off to you! Good luck and be careful. Stay out from under it as much as possible. The guy mentioned above said they would watch from the street as the roof was lifted. When the crane had it over the building and only a few inches above the plates then they would go up and give it final guidance into place.jim
*jim,between the SE, the truss engineer and the architect, I don't think I'm getting any eager takers to help with the rigging question. What I've come to thus far is that the scissor trusses need to be pulled up in such a way so that the load on the scissor trusses is dispersed and transfered to the seat cuts. This way, the overframe trusses just go for the ride. The overframe ridge is at the same elevation as the main roof ridge. There are two pitches to the roof, as the building is not a square. The long axis of the building are overframe gables. If the scissors were bobbed 1 1/2" on each end, that gable could sit on the top plate. The gable sheathing would break on this plate, so then a traditional two-point wall sling on each gable wall would pick the top plate with continuous nailing, carrying the scissors along for the ride. Rig the gable truss with two leave-in-place fall protection anchors, located at 1/3 intervals. Total roof weight with ply, paper, cedar and architectural panels is about 8,300 lbs.?
*NathanI am no engineer truss designer or fartitect, I have set a few roofs I built on the ground though. Due to cultural differences I've never had to worry about felt or shingles. The way we rig our truss racks differs with the size and shape of course. There really isn't room here to give you the whole can of worms, that would take you coming out here and working with me for a few years.We've had good luck running a larger piece of lumber(2x6 to 11 7/8 ml depending on size of rack) at the king post of the trusses. Thiss spreads the lifting force somewhat evenly across the trusses. If you get the papers that come with the trusses that show proper bracing and lifting techniques you won't find the answer there. I've learned it mostly by trial and error, luckily more trial and little error and no one's ever got hurt on crane day. If you're going to set the whole house as one unit you should try to run a false ridge, as mentioned above. You'll need to use a spreader bar and lift the roof from four points on the ridge. You'll need to leave some openings in the roof to do this. Call your crane company, they may have some ideas as well, after all that is what they do.I AM NOT QUALIFIED TO OFFER EXPERT ADVICE ON THIS SUBJECT AND WILL NOT BE HELD LIABLE FOR ANYTHING THAT GOES WRONG.;^)jim good luck with your roof! Let me know what you decide and if you can post a photo of it up in the air.
*thanks jim, I'll do it just like you said to...seriously, it'll work. I know if they can build Hoover dam and put the Colorado through a hillside, I can get this thing up in one piece.laternathan
*> the scissor trusses need to be pulled up in such a way so that the load on the scissor trusses is dispersed and transfered to the seat cuts.Bull. The trusses can be designed so you can lift them at other points. If the truss company you're dealing with won't cooperate, there should be plenty more around. > If the scissors were bobbed 1 1/2" on each end, that gable could sit on the top plateThis is a great convenience, if you have enough bearing to work with. I don't like doing it on 2X4 walls, though. That only leaves 2" for bearing. Since nothing in the real world is perfect, that doesn't leave much margin for error.
*nathanI know that you know this,but, I'll remind you anyway. Make sure that whatever you build your roof on that it is level. If you build it on a kooky set up it won't set flat on the walls. Believe it or not the sheathing will hold almost any error you make.Are you going to build it on the floor, then move it aside, or build it on false walls on the ground? It isn't completely necessary to have it level in relation to the front back line, just parallel to the bearing walls. For instance you could set one false wall up LEVEL and have the other that supports the other end of your trusses at a different elevation and LEVEL. Having them at the same elevation does ease the assembly tough.jim
*Thanks. Can you tell me why I'd want to pick the roof from somewhere other than the two overframe gables if I'm looking for a pick of both the scissors and the overframes? Also, these are 2x6 exteriors. Thanks again
*Nathan -I assumed you i wantedto pick them up at another point and they had said you i hadto pick them up at the ends.
*Lets just say the engineer strongly suggested that the pick act on the scissors in such a way that would mimic the forces put on them while their at rest on the roof. The truss co. suggested they would get the gable going off of the plate instead of the top chords. I put them together and came up with the concept of picking the sheathed overframe gables sheared to top plate(s) which bear the scissors. So I ask you this, respecting your view and experience, does this seem like a reasonable course to pursue with an engineer? I'm also curious what modifications to the scissors would be necessary to allow me to set the picks on them, and have them push the overframes up, rather than be pulled up.ThanksNathan
*thanks for you sharing Jim. I will most definitely have the crane co. give me their advice. I'll try to get the digi cam working to post a photo or two. Probably three weeks or so.laternathan
*JimI have six foundations in two columns about thirty feet apart each side. We're making each building exactly the same, fit the best we can on each fndn. The g.c. sills are the forms, which we'll set single or double plates on, squared up and tacked down. There is about two feet from existing grade to top of fndn. A real manufacturing plant, under the big skylight.
*nathan lucky you to have the perfect setup for just such the endeavor you're embarking on. I usually build my roof in sections spread all over the country side. The roof sheathing figured to lock together as the pieces are set and any overframes done up in place. I do think you could successfully ridge pick your set up as I mentioned before. I don't see how you could lift it at the bearing pints as your engineer suggests(if I understood your previous post)again good luck and if you do them all this way the last one is sure to go way smoother than the first.jim
*I don't think picking them up at the ends is wrong - I just thought it could be difficult to rig if the scissors were very long. If you wanted to pick the scissors up elsewhere, it would just be a matter of deciding where you were going to pick them up at, and re-analyzing the trusses for the different force paterns. (Like an upwards point load where you would be picking them up at) No big deal with the software we have nowadays. Hell, I can put so many load cases on a truss it'd make yer head spin. I know it makes mine spin sometimes...(-:
*Nathan Have you tried it yet. Hopefully you have and survived with some greeat pictures.JIm