*
I’ve got a 40-acre patch of woods and the inclination to build a large cabin myself (I built our current home with success, but am not a builder by profession).
The plan I’d like to use calls for using room-in-attic type trusses 24′ long with a 10/12 pitch and a 12′ wide room within. My problem is that this has to be a do-it-yourself type project and for a number of reasons I don’t see a crane being a part of the building process for me.
Can these things be stick built? I was thinking of layering 2 24′ 2×8’s (for the bottom) over single 2×8’s on the roof slope portion, then using 2×6’s for the interior room shape within the “triangle”. I also thought it might be stylish to beef these up a little, put them 4′ on center and expose the bottom chord to the room below in some fasion.
Can this be reasonably done without consulting a structural engineer?
-RandyR
Replies
*
I've got a 40-acre patch of woods and the inclination to build a large cabin myself (I built our current home with success, but am not a builder by profession).
The plan I'd like to use calls for using room-in-attic type trusses 24' long with a 10/12 pitch and a 12' wide room within. My problem is that this has to be a do-it-yourself type project and for a number of reasons I don't see a crane being a part of the building process for me.
Can these things be stick built? I was thinking of layering 2 24' 2x8's (for the bottom) over single 2x8's on the roof slope portion, then using 2x6's for the interior room shape within the "triangle". I also thought it might be stylish to beef these up a little, put them 4' on center and expose the bottom chord to the room below in some fasion.
Can this be reasonably done without consulting a structural engineer?
-RandyR
*
Randy,
It seems to me that if you are willing to go through all the trouble of building trusses on-site, you might as well just stick build the rafter/joist system. No need for trusses. Of course, at a 24' span, you will need a center beam to support the span of your floor/ceiling joists unless you can tie those joists to the rafters above. The latter would probably need some input from an engineer as to it's assembly.
Wish I could join you on those 40 acres.
Best luck,
Pete Draganic
*
"Can this be reasonably done without consulting a structural engineer? "
Short answer; No
Longer answer; Maybe, but even assuming you are lucky and nobody gets hurt, a collapse is going to cost you a lot more, in lost materials, than consulting an engineer in the first place. This is no place to cut corners.
jay
*
Randy,
Prepare yourself for the insults...just look at my thread "Gussets for roof trusses".
B
*I think Randy made a good suggestion. Using a center beam, your floor system could be framed with conventional lumber, I-joists, or floor trusses. But, they would have to be designed for the added load that comes fom the roof bearing on the kneewall. (See the posting titled " Gussets for roof trusses " for more talk about building your own trusses)You might want to consider raising the pitch, or building some height into the roof system at the outside wall. Assuming about 8" over the wall with rafters, and a 10/12 pitch, you would only have about a 58" kneewall. Your ceiling would slope a long ways into the room.
*Ron - Why bother with a bearing kneewall? 12' span is obviously no problem. Much easier to get a continous air seal if you can sheath all the way down the rafters to the exterior wall. If you want to screen that portion off visually for storage, put up some face frames or paneling. Otherwise you have a nightmare to get sealed and insulated.Also, with engineered joists, I thought 24' spans were OK. Too bouncy no matter what?
*
Pete & Ron,
Thanks for the input. Of course, a center beam in the room below defeats the purpose of having RIA trusses - large open spaces below (the room I had in mind would be about 36' long and I would strongly prefer no posts). As for the kneewall and the required heaviness of the bottom & slope cords, don't these work together like the webbings of a traditional truss to make each piece act stronger than it would be on its own? I know that if I bought these from a manufacturer, designed for 2' o.c. they would use a single 2x8 for the bottom and a single 2x6 for the sloped portion and kneewall. Add a 2x4 connection for the "ceiling" of this little room and a bunch of mending plates and there you have the "engineered" truss. My plan simply beefs up each portion and substitutes lap joints (and lots of nails and adhesive) for the mending plates.
I realize there are good reasons to want to be able to shift the blame to an engineer for any potential lawsuit, but what I'm asking is that - from the experienced point of view of you folks out there who do this for a living - shouldn't this work? If not, where are the weaknesses?
Thanks, RandyR
PS There are essentially no framing codes in the area this would be built, so getting approval is not a consideration.
*
As long as you're still in the design phase, if you'd be willing to change your roof design from a gable to a gambrel try this site, http://www.barnplans.com. They sell plans for an engineered gambrel truss that would give more room upstairs than a stick framed gable and instructions for laying out the jig. Yeah, I know, then your building would look more like a barn but hey, I like gambrel roofed barns. In fact I cant hardly drive by an old gambrel roofed building without pulling in and asking if I can go up into the loft/attic and see how it was framed. Good luck, JonC
*Bill,Good point about sheathing/insulating all the way to the exterior wall. Seems like a lot of extra work, but then, nothing comes free. I'd be interested in reading more about enclosing (making livable) attic space. Anyone wanna keep this thread going?B
*Randy,Perhaps I'm confused, but I'm "looking" at a section along the 36'axis of your attic. I see a "truss" that consists of triangles towards each sidewall with an irregular hexagon between those triangles, and possibly another triangle above the hexagon shape. I don't understand this to be a rigid shape.B
*When I got home today I found the Feb JLC and it had a feature related to this - basically how much easier it was to seal the top of the house at the roof rafters than at the attic floor. (Non- venting was integral to this in the feature but leave that for another thread.)I've been planning and am about to begin - like this weekend - converting attic into living space - sheath under rafters - no kneewalls - pack above sheathing with cellulose. You comment about extra work - seems easier to me to keep it all in one plane than jog down knee wall and then across ceiling joists. Sealing around all that would be iffy. (Also this is a hip roof with four hip roofed dormers and the way all the planes intersect is kind of visually appealing.)
*
B,
I think you are picturing it correctly, and unless I'm wrong (you all will correct me if I am) this is exactly how manufactured RIA trusses are made. I've seen them, although I'm not 100% sure of the dimentions used. The key is using heavier than usual dimentions for the outer edges and some larger than usual mending plates at the joints. The bottom span must be the dimention necessary to span the floor of the room created above, in this case 12'. Sizing it for a 24' span is unnecessary because the kneewall "webbing" connection with the slope acts the same way as a support would from below.
-RandyR
*JonC,Thanks for the suggestion, although I would shy away from the barn look in this case. It brings up a question, however: If a gambrel style RIA truss can be stick built with a roof slope that has a "kink" in it and therefore fundamentally less stable than the triangle, why can't my conventional RIA truss be stick built? Curious....RandyR
*"why can't my conventional RIA truss be stick built?"I see no reason why it can't, but I still think you should have a profesional engineer design it. If you goto barnsplans.com, you'll see that even though the trusses are site built, they specificly mention "engineered" design. It seems to me that you must have at least a little bit of doubt about your design, or you wouldn't have asked about it.One rarely sees "lots of nails" specified on engineered plans, it is more like, "n number of x size at c spacing" (I think I'd consider split rings myself).I think in this case consulting a profesional is cheap insurance.You might find this book educational, it won't really tell you how to design a truss, but will give you some idea of what is involved AITC Timber construction manual (amazon.com)Jay
*Jay,Thanks for the input, and of course, you are absolutely right. I never expected anyone here to say "Oh yea, you can do that, no problem" but what I was hoping was that someone would have tried or seen similar work in a situation that proved successful and might pass on some tips. After all, it doesn't seem to me to be a far-out question. RIA trusses are a very handy item, but they are impossible to handle pre-made without a crane and a work crew. As long as the dimentions of lumber used (and quality) are not substandard and as long as the connections are strong enough, there is no logical reason why they can't be pieced together in place.As for engineering specs, of course they are wise. But so is never driving faster than 55mph. We've all seen those obnoxious "I'm writing this to avoid being sued by a moron" stickers stuck on every product you buy now. Sometimes I wonder how mankind ever made it to the 20th century without scientific proof that something was 100% foolproof.-RandyR
*
If you're going to build these yourself - how will you lift them from the deck without a crane? Or do you plan to build staging and a portable (?) jig for setting all the pieces in place in the air, and holding them until the glue cures? It seems that the chief advantage of trusses is speed of erection and less time in the field.
Also, I believe most trusses you can order today use msr - machine stress rated - lumber. Virtually every piece is checked for allowable loads and used in the truss where it will work. And the allowable stress values of this lumber is quite a bit higher than most you can buy locally.
Your driving 55mph may be a suitable analogy - just realize that the stress on a 2X4 in a truss may require an allowable stress several times what is permitted in the typical 2x4 from the lumber yard. So instead of driving 55 you're driving 120 mph non-stop for the whole day and night.
You'll have to forgive me but - since I could of sticked frame the whole roof and second floor in the time since you first posted the question for less money in materials then what I would spend for site building trusses - it just doesn't make sense. And consider that someday - if stick built - I could add a dormer or make any number of alterations that I couldn't with trusses.
But I got to love that pioneering spirit. I wish you all the best with your home.
*Bill - I just wanted to reply to 2 things you said:" most trusses you can order today use msr" That may be true in some areas, but not always. I think truss manufacturers tend to use MSR in larger sizes, say 2X8 and up. Also, you mention dormers and attic trusses. Dormers CAN be used with attic trusses, IF you have the luxury of knowing where the dormers are going ahead of time. The trusses are generally doubled or tripled around the opening, and the opening is filled in with stick framing. I'm not trying to pick on you, just trying to clarify a couple of points.
*OK - I appreciate that. What lumber rating does a truss manufacturer use? Does a truss guy just pick up any bundle of 2X4 with a #1 stamped on it and go? I thought there were too many allowable defects in a typical load of lumber for use in a truss, especially for tension.Of course you're right about the dormer trusses. I meant to address after commiting to a trussed roof without dormers it would seem very hard to add one in the future. I guess as a designer I assume that needs and circumstances will change and that the ability to modify a building is a given. Stick framing would make it pretty easy to add an extra bedroom up stairs or do a tall, open space; trusses seem to complicate it a lot.
*Bill, Ron, et al,If one was making their own trusses, selecting members from the tables, wouldn't the tables indicate the stress rating required? And, TENSION? Wouldn't the most likely failure mode of any truss member be buckling, due to excess compression?
*In the simplest case - a big triangle with a square inscribed - the bottom chord and the horizontal representing the ceiling of the attic room would be in tension - as well as stresed for bending. The top chords (rafters) and the knee walls would be in compression. I think.But I've understood from texts, magazines, talks with engineers, lumber manuals, etc. that in a "lot" of graded lumber, that some allowable number of pieces will not meet the minimum values. This is not a problem with stick framing - like who cares if one stud is weaker than the sticke - nothing is going to fail. But consider one big knot in a tension member - maybe near the connection. I don't know how truss fabricators cull these out but think the do.Hopefully Ron will respond or some truss manufacturer will jump in.
*Bill, I agree with your comments. No question that the members representing the top and bottom of the "square" as you say are subjected to serious bending. In fact, I'm hard pressed to call a RIA truss a true truss, but then again, I don't claim to be current with treminology. But when it comes to that large knot, I rather see it close to a gusset in a tension member, than mid-span in a compression piece.
*To Bill and "B" -For starters, Bill asked "What lumber rating does a truss manufacturer use ?" Generally, it's whatever is easiest to get a reliable supply of, and will do the job. Typically, around here anyway, that's southern yellow pine. The grades vary, depending on what mills they're buying from. You also said that "....too many allowable defects in a typical load of lumber for use in a truss" That's not necessarily true. Visually graded lumber isn't always bad, and MSR isn't always good. The truss builders and saw guys are supposed to cull bad lumber, but don't always. They can be as contientious or as lazy as any other worker in America.Maybe it would help to review how lumber is graded at the mills. Visually graded lumber is assigned a grade by a guy who basically looks over the board, and marks it according to whatever grades his mill is selling. For instance, they may sell select structural and #2, but sell #1. MSR lumber is run through a machine that actually bends the board a bit, and measures the amount of force required to bend it. A grade is assigned to it based on that test. There is a third type of lumber called MEL. (Mechanically evaluated lumber) It's actually X-rayed every 1/2", and a grade assigned to it based on the composite of the X-rays. Both MEL and MSR are more expensive due to the cost of the testing, and the overhead of these expensive machines. On to truss design. The guy called "B" mentioned "selecting members from the tables". There are no such tables. To design a truss, you basically "build" it on the computer screen. Loads are added as needed. A typical attic truss is loaded 4 different ways in most cases. Then you "run" the truss, and the computer does most of the work from there. The amount of bending stress and tension or compression are calculated for each load case. The computer does some of the deflection checks, and some are done by hand. Each member in the truss is assigned a CSI, or "combined stress index". The CSIs must add up to less than one. Hopefully, the trusses also go through a reality check in the designers' mind. But this doesn't always happen. Then you run the plating module to come up with a plate for each joint. This also requires a reality check, as the computer can't "think" about handling, etc. Bill, you also said "the horizontal representing the ceiling of the attic room would be in tension". It's actually in compression. I could draw you a picture and show you why, but it's hard to explain. And "B" (Could you maybe make up a nickname ?) said "I'm hard pressed to call a RIA truss a true truss". A simple truss can be broken down into a bunch of triangles, and used to be designed as a "pin joint model", or "empirical analysis". But that's pretty much gone out the window with the advent of computers in the design process. They now are designed with a "matrix" analysis, which checks each member in several locations. I must confess that even after 15 years in the business, I don't completely understand this process. But back to your point about attic trusses. The analysis of an attic truss requires the assumption that each joint and each member has some inherent stiffness. Again, this something that's a bit over my head. But the analysis method is backed with a great deal of testing on actual trusses. Hope this wasn't too much, but I'm trying to cover a lot of ground here. I'd be happy to go over any of this in more detail if there's some interest. But I don't want to bore others.
*Thanks. I guess your right about that ceiling member being in compression. Seems like it might need a vertical web in the center.I thought I had read the msr grading machines actually printed a value on the piece and the guys in the truss plant would actually look to match thoswe values with the minimum called for bu the engineers (computer program) drawing.So - is the tolerance for lumber defects higher for stick building than trusses?
*Bill - You bring up some good points, as usual. Lumber grades are actually printed on ALL lumber, even visually graded lumber. The guys in the truss plant *should* make sure they are the right grades for the truss they are building. But, they aren't perfect. This is particularly a problem when building different truss types of the same span. For instance, you order some 30' common trusses, and some 30' scissor trusses at the same time. The 30' commons use a #2 top chord, but the scissors take #1 top chords. Since they're all the same length, they can easily get mixed up. About 1/3 of the people who applied for jobs at the last plant I worked at couldn't read. So asking them to read grade stamps is a bit of a stretch. As for your comment: "is the tolerance for lumber defects higher for stick building than trusses? " I think there are good and bad truss plants, same as carpenters. Truss plants might be a little more paraniod about liability, but I see little difference otherwise.
*Sorry Ron - let me rephrase that question about tolerance. It seems to me that a single defective stud or joist is much less likely to cause a building failure - or at least a catstrophic building failure - in stick building than if that piece is used in a truss.(I re: the msr - I thought each piece was labled with certain values specific to that piece - like Fb and E - unlike visually graded lumber that has certain minimum value - even though some pieces may be much stronger than others.Thanks immensly for taking your time to answer these questions.Bill
*Lumber strength and house strength....Remodeling involves demolition much of the time.I have also had to tear down a structure or to and can tell you they don't come down easily. Giant sections of support can be removed and nothing budges. Snow loads are a different matter altogether and a missing link is all it takes.I have used 28' span room-in-attic trusses, set them with three man crew no crane, and like them. Rafters are nice though especially if you want be able to make changes later on as others have stated.I really think if it was me I would try to hall in manufactured trusses somehow. Homemade rack-trailer a couple at a time or whatever it takes.Jack : )
*
I've got a 40-acre patch of woods and the inclination to build a large cabin myself (I built our current home with success, but am not a builder by profession).
The plan I'd like to use calls for using room-in-attic type trusses 24' long with a 10/12 pitch and a 12' wide room within. My problem is that this has to be a do-it-yourself type project and for a number of reasons I don't see a crane being a part of the building process for me.
Can these things be stick built? I was thinking of layering 2 24' 2x8's (for the bottom) over single 2x8's on the roof slope portion, then using 2x6's for the interior room shape within the "triangle". I also thought it might be stylish to beef these up a little, put them 4' on center and expose the bottom chord to the room below in some fasion.
Can this be reasonably done without consulting a structural engineer?
-RandyR