There are those goofy heating cables to prevent ice dams (on improperly built houses). A whole lot of them would melt snow and make the electric meter spin really fast.
I can’t imagine an inspector/building department approving an active solution (heating, shoveling, etc) when a passive one (properly sized rafters) is so easy. Code and inspections are predicated on the (not unreasonable) assumption that once a house is occupied, only automatic systems will be used.
If the design needs only a little more strength, doubling up (sistering) the rafters with another of the same size would increase roof load. Upsizing the roof sheathing (from 3/8″ to 1/2″ or to 5/8″) might also be needed. Neither of those changes would affect other measurements.
But why can’t the builder just increase the rafter size (from 2×8 to 2×12, for instance)? Or is it trusses?
Replies
I agree with David and it also seems a little strange that your contractor can't come up with any basic design changes. How did you get into this situation in the first place?
You could also sheet the face of the trusses with 3/8 plywood. That would greatly increase the strength of each one.
Gabe,
I like your idea of sheeting the face of the trusses to increase their load capacity; it should do that in spades. But won't that kind of mess up roof ventilation by blocking any lateral flow of air to the gable vents?
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
You would loose very little in the overall strenght of the assembly by leaving out a hole in the middle of a couple of open webs of the truss.
Most failures that I have investigated were partial and most were homes that had a generally weak assembly to start with.
If you know one component is weak you can normally figure out several ways to stiffen or improve on the structure.
Adding or laminating plywood is only one way. Adding blocking or strapping under or even over the bottom chord will greatly improve the performance. If you already have a finished house, adding continuous strapping to lock ALL of your trusses together is still the best way to SHARE the load so to speak and make it bearable.
Just a few thoughts
Gabe
Thanks David (and Gabe). The builder can indeed beef up the trusses and sheathing. All I have to do is pay. And ice dams will still be a problem as long as there are eaves, yes? Please note that I'm talking western North Carolina mountains - not the same level as the Rockies or west coast ranges. My concern started when I realized that a Florida beach house in the mountains might not be entirely suitable. Thank you both for your helpful comments on my dumb questions and perhaps even dumber situation.
CPW, in Florida, looking north.
The Truss manufacture can design the trusses, and should have for your local code requirements, weather it is 30,40, or 50 psf of loading. even a 3/12 pitched roof can br designed fo some pretty big loads.
Boss Hog can jump in here any time, but I am thinking maybe 2x6 top and bottom cords, etc.
Your builder should know this, and the local code requierments when qouting the job to you. A warning bell would be ringing in my head if he or the trus supplier did not catchsomething like this.
On the other hand if you are looking for a cheap fix, others have given you some good answers.
If it isn't built, fix it with a design change now, not after it is up.
Dave
Dave, Thanks for your input. I haven't yet gotten to the local codes, but inquiries have been sent to the builder (located in NC). And sheathing the trusses, while a good idea, may indeed introduce air flow concerns. Common sense, hey?
Thanks again, all of you, for opening my eyes a bit. I'll take it from here with the locals up yonder.
CPW in Florida, looking north.
Since the original message has been deleted, I can't remember exactly what it said. I remember something about heating cables to melt snow so there was no snow loading - I saw the message yesterday, but didn't have time to respond.
First off - Live loads aren't there only for snow. They're also for things like re-roofing, people working on the roof, etc. So removing snow load doesn't mean you don't need any live load capacity.
Also - There's no way you could justify an assumtion that the heating cables would be 100% effective. There's no way you could guarantee that the heating cables would be turned on every time it snowed. Or that they'd never burn out. And what would you do in case of a power failure?
The whole idea is just wrong in principle.
The idea of beefing up trusses by adding plywood to the sides isn't really a good one. (sorry Gabe) It might work if the plates were what was maxed out. But if the lumber is at it's stress limits, the ply wouldn't really do anything.
Dave, one note I wanted to add to something you said: "I am thinking maybe 2x6 top and bottom cords, etc."
Increasing the chord sizes doesn't increase the load carrying capacity of the trusses. (Assuming that's all you do) A truss with 2X4 chords that's designed for a 30# snow load will carry just as much weight as a truss with 2X10 chords designed for the same loading.
You can certainly increase the loading on a truss until it REQUIRES a larger chord size. But just adding the larger chords without increasing the loading does nothing but waste money.If you only knew the power of the Dark Side. [Darth Vader]
Glad you jumped in. I don't know squat about designing trusses, but do know that if I spec a truss system for x feet of free span, x lbs of live load, x lbs of point loading, etc., I get the proper design from my truss supplier. They give me shop drawing to submit with my prints when I apply for the permits. The guys get it right and I have never had a problem meeting code requirements.
Dave
Dave -
I wsn't picking on you- I was just addressing the perception that increasing chord sizes would automatically increase the load carrying capacity of a truss. I run across that a lot, and it isn't true.Men gossip more than women, but mean it. [Mignon McLaughlin]
Boss--
I am interested in this. I build a fair number of my roofs with what I call a 'rafter truss' which is nothing more complicated than a basic truss built out of the same size & spec of lumber the CBC dictates for rafters for the span etc in question. We build these RTs on site using 3/8 ply for gussett plates in addition to 3½'s nailing the joints together. They solve a lot of problems for us and make life a lot easier, and I've never had one even look like it was gonna fail: ceiling gyprock screwed directly to the bottom chords never cracks etc., etc. Our snow loads here are pretty serious, I oughta mention--around 3.0kPa.--and there's snow on the roof 6 months of the year easily.
Can you tell my why the point loads at each web/chord connection won't be absorbed and resisted better by a more massive chord, say a 2x8 or x10 for example?
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Boss Hogg's statement is wrong more often than it is right.
The internal members should be limited by their compression strength, the chords depending on the design are limited by either their tension strength or some bending criteria.
Larger wood for the chords allow larger design loads unless the internal members are at their compressive limits.
George--
Thanks for the input. What you say is the basis I've been going on up to now. If I'm in doubt about the bending moment on the bottom chord and the design allows it, I'll transfer the load down to a beam or supporting wall in the house right below where the webs meet on the bottom chord, thus cutting my span in half.
What Boss Hog is saying (I think) is that just bumping up the chord by one lumber size doesn't necessarily make the truss meet any particular snow-load criterion, if the truss design for the span in question wasn't predicated on the applicable snow load.
What I do is not that; I design the roof from the CBC rafter tables according to the pitch and dead loads, and then factor in the live loads: wind, seismic, and snow. But instead of hanging each rafter set off a ridge beam, I take each rafter set and turn it into a truss by adding webs and a bottom chord. Then we can set them with no ridge beam and no birds' mouth cuts, which speeds up the framing enormously.
It doesn't take long to knock out the trusses either; I'll make a jig to cut the acute angle with a circular saw, the other cuts are done on the chop saw, and we build a pattern frame out of 2x4 nailed to the subfloor so the lumber is just dropped into place and air-nailed together. Two guys can knock out thirty trusses before lunch that way and we place them after lunch, ready to sheath the next day.
This has passed our local inspectors' scrutiny more than once, so it's not a problem at that level. Performance over the 9 years since I first built a roof this way backs it up too--all my roofs survived the 1998 ice storm without even cracking the gyprock; three major roofs designed by engineers and built by large commercial GC's collapsed that winter within 5 kilometers of my home. Nobody hurt, but megabucks in lawsuits with everybody involved blaming everybody else. I don't think they're settled yet....
Boss &/or GeorgeR--am I on the ball, more or less, or do you see a problem with this technique that I don't?
Here's a couple of pics of a gambrel roof we built using rafter trusses for the upper section.Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
"What Boss Hog is saying (I think) is that just bumping up the chord by one lumber size doesn't necessarily make the truss meet any particular snow-load criterion, if the truss design for the span in question wasn't predicated on the applicable snow load."
I'm not sure if that explains it well enough. Let me try it from another perspective.
Take a fictitional roof truss that has 500# of compression in the top chord, and 500# of tension in the bottom chord. (At the heel, or part that sits over the wall) For starters, let's assume that both the top and bottom chords are 2X4.
If you figure out the plate size you need at that joint, it would come out to something like a 4" X 4" truss plate.
Now bump both the top and bottom chords up to a 2X6, but don't change the loading criteria. Since the top and bottom chords both still have the same 500# forces in them, you still need the 4" X 4" plate on the joint. So the truss isn't necessarilly any stronger.
That example is a bit oversimplified, but it illistrates the point I'm trying to make. If you want trusses to be stronger they need to be desgned for more loading before they're built. That way you'll have higher forces in the chord members, which will require larger plate sizes. (And MAY require larger member sizes)
BTW - I did a thread once about Building Your Own Trusses that may or may not interest you. Didn't know if you'd seen it or not.Be concerned about your future. You'll spend the rest of your life there.
Okay, let's all watch the ol' Dinosaur stick his neck out there again....
Boss, do I read you right? Are you telling me that the joint plates in a truss are more determinative of load-resistance than the lumber size? That seems to make sense to me; the weak-link proposition still holds true in my book.
If the joints are really the most determinative factor in truss strength, then we've got to pay the most attention there. What I do now is use 3/8" ply as gusset plates, glued and nailed on using 2" glued air nails staggered on 1" centers. I also toenail the joint using 3½" air nails. The size of the gussets is determined by the size of the lumber we're using; I generally make them as big as possible. Aside from buying metal gussett plates and pounding them in with a 16# sledge hammer, I don't see how I could do any better.
I did read your thread on making trusses from 2002; interesting stuff. I do have to report, however, that my experience with factory trusses recently hasn't found them to live up to the high quality you talked about then.
1. Lumber grade: All the trusses coming off the lines around here are built from SPF Stud KD. Nowadays, SPF=Spruce, and you're not even sure which species. (If you want fir, you special-order it and it comes in from BC with a big fat bill attached.) In general, it is not what I'd call the best lumber. You're gonna see wane, dead knots, skewed grain lifting out of the board, and hockey sticks. The short version is, the trusses are being built with lumber I wouldn't want to use to frame a partition wall. (I admit, I don't always have a choice; but then I'm not buying the stuff by the train-load.)
2. I routinely see butt splices in chords, and I'm not just talking about 40-footers. Perfectly ordinary 18 and 24-foot trusses for small chalets or bungalows are universally delivered with the bottom chord butt-spliced; sometimes it's sistered by a couple of feet of 2x4 to fill in the plate, sometimes not even. If I can order 20' or 24' lumber from my yard--and get it, KD no less!, in under a week--why can't the truss factory do the same?
3. The truss designs are done by computer. A lumberyard employee is given a quick training course on how to use the software. He then becomes an instant 'Truss Expert', even though his understanding of things may be no deeper than knowing which number to plug into what field on the screen. There is no engineering talent available at the point of sale to help the truss buyer make informed choices. In fact, the buyer may not even be aware there are choices to be made; the 'expert' will most likely make them for him without mentioning it, based upon hunches, desultory head-scratching, or what he had for lunch. No way to tell.
All this makes me wonder if the quality of the factory-built trusses I'm seeing is as high as it should be. I am somewhat comforted however by the knowledge that a truss is, just due to its form, incredibly strong. I have built railroad trestle trusses out of 2x6, 42"-deep section by 30 feet long, and used them to support a temporary bridge to drive my loaded pick-up truck across a small creek we couldn't get permission to stick a culvert in (mighta scared a goby fish, or whatever).
Add to this fact how much faster we can put up a roof using trusses, and I like working with them. I just don't like the quality I see coming off the line, and I've had good results using the method I described in my earlier post. That's why I wanted to know what you thought about it. Any more thoughts?
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Dang, you sure cover a lot of ground in one post.
"Are you telling me that the joint plates in a truss are more determinative of load-resistance than the lumber size?"
Not necessarilly. The only point I was trying to make in the example was ONLY that changes in chord sizes don't necessarilly mean increases in strength.
As you add loading to a truss, you may have to increase the heel plates, for instance. Add some more and you have to increase the top chord grade. Add a bit more and the bttom chord splice plate has to be increased, etc. Every truss is different.
"All the trusses coming off the lines around here are built from SPF Stud KD."
Are you sure? Stud grade lumber doesn't have much in the way of strength - I doubt it would be used in the truss chords. Webs maybe, but not chords. The chord lumber should be SPF #2 or better.
"I routinely see butt splices in chords"
What's wrong with that? The splices are plated together, and the plates are sized for the forces involved in the trusses.
The reason smaller trusses are spliced is that component saws aren't designed for lumber over 20'. If you're seeing 18' trusses that are spliced, it's probably because they are out of 18' lumber.
"A lumberyard employee is given a quick training course on how to use the software. He then becomes an instant 'Truss Expert'"
I don't buy that at all. I've trained about 25~ truss designers. It takes about a month before a designer is halfway productive, and about a year before they can work pretty much independently. Everything that's done is double-checked until I have confidence in what they're cranking out. They aren't given stuff to do that's beyond their abilities.
I realize that things may be a bit diferent where you're at. And the truss designers you deal with might not be terribly cooperative. But I think you're exaggerating a bit here.
"All this makes me wonder if the quality of the factory-built trusses I'm seeing is as high as it should be."
I agree. I also wish the quality is/was better at every truss plant I've been at. But most people who buy trusses aren't terribly concerned about quality. (I've talked about this before at length, but don't recall where so I can reference it)
You know how it's said that everybody wants their house built good, fast, and cheap - But you can only have 2 out of 3? The same thing applies to trusses.
What I hear on a daily basis is "how fast can I get my quote" - Why are they so expensive" - "How fast can you get them built". Those 3 lines are repeated over and over again, day after day. Faster and cheaper, faster and cheaper - That's what everybody wants. So it's no wonder that there are quality concerns at times. It's surprising that the trusses aren't worse quality than they are.
That's about the extent of my intelligent thoughts for the moment.......Who could be serious with men? What I really wanna do is to date a lot of pigs. Tell 'em to line up outside... and bring lots of emeralds. [Judy Tenuta]
Unless you are an engineer, you are not allowed to design outside of the perscriptive code.
In particular, your internal members are required to rest on load bearing walls not the ceiling joist.
The fact that your roof survived and others failed does not allow anyone to draw the conclusion that your roof was better built. To draw that conclusion would require that the "DESIGN" loads were similar and that the actual loads were similar.
Unless you are an engineer, you are not allowed to design outside of the perscriptive code.
Sure I am. I just have to have a licensed engineer sign off on it.
In particular, your internal members are required to rest on load bearing walls not the ceiling joist.
If the webs are transferring loads directly from the roof to load bearing walls, what do I need a truss for? Except convenience, of course. The structural reason for using a truss, as I always understood it, is to enable us to have longer clear spans than we could reasonably have using dimensional lumber.
The fact that your roof survived and others failed does not allow anyone to draw the conclusion that your roof was better built.
I'm sure the insurance companies for the architects, engineers, contractors, and subcontractors on those three collapsed roofs 5 miles from my uncollapsed roof will be comforted to hear that. And just so you know, my roof was specifically designed to accumulate snow (as an insulating factor) so if there were higher loads involved on either side, they were on my roof, not the ones that collapsed. See pic below after a nice little 3' dump of snow.
Sorry if I sound a little nettled; I get bent pretty easily when 'experts' (whether they know more than I do or not) start telling me what I can and cannot do instead of trying to work with me like an intelligent being from the same species.Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Don't take george too seriously.
I doubt he's even an engineer (As he claims) judging by the poor advice he gives.
I put him on ignore a long time ago due to his arrogant atitude.Q: How can you tell which bottle contains the PMS medicine? It's the one with bite marks on the cap.
Okie-doke. Funny, at first I thought he was going to give out with some useful input; then he pushed my button. Hope I didn't land on him too hard.
My old man was an engineer--a professional engineer, with a license; unlike like better than half of his graduating class from Cooper Union, he bothered to go on and take his Masters and sit for his license exams. Were he still alive, he'd have told me wood structures weren't his specialty (he designed heavy manufacturering equipment for the rubber and plastics industry for 45 years), but that we could try to figure out what I wanted to know together. He understood the value of specialization, but personally believed there was no reason an intelligent man couldn't learn anything he wanted to, whether in a formal, classroom-type environment, or as an apprentice, or just by solo studying.
I for one think our society has gone a little too 'documentation happy' in its (understandable) desire to assure that we don't get operated on by surgeons who haven't yet graduated from the University of Hard Knocks. When I was a younger man than I am now, it was possible to become a lawyer in the State of New York by working as a clerk for a lawyer for 5 years, I think it was, and then writing the bar exams. If you passed the exam, you got the right and title even if you were a high-school drop out. Of course, that won't pass these days; hasn't for damn near thirty years.
It's a shame, too; there's a lot of really talented people out there in all fields of endeavour with wealths of practical knowledge who could easily pass whatever certifying examinations a province or state set up to protect the populace from smiling frauds. But they're not allowed to write the exams to prove it, because they haven't got a sheepskin. Why not? Seems counter-productive to me.
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Now you're getting close to my heart!
Documentation excess stems from having more lawyers than doctors or engineeers, IMO. Same thing that leads to excessive insurance costs.
I would have lopved to know your father. But they came from a different generation, didn't they? Diferent world that spawned them back then.
Do the names, Rourke or Galt mean anything to you?.
Excellence is its own reward!
My dad would have approved of your signature line. What he used to say was, "Good enough is not good enough. Only your best is good enough."Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Boss,
This is one PE that has read many great and informative posts from you. Your understanding of trusses is beyond many a PE in my opinion. And I think you are dead-on in your Ignore assessment too. That was some terrible advice posted....that's not a mistake, it's rustic
Thanks for the kind words. Didn't know you were a PE.
Don't know if you've heard me say it before, but I tell people I'm a P.E. - It just stands for "Pretend Engineer".........................(-:I'd live life in the fast lane, but I am married to a speed bump.
I work in a very large firm, so PE can easily stand for PEon (or PE'ed on, come raise time).
Is a monkey riddle a baffoon?...that's not a mistake, it's rustic
I hope you can find a PE to sign off on your designs.
I hope you can always find an insurer.
You deserve both.
If you've got something constructive to say, go right ahead. You might find it more productive in the long run than publicly trying to pump up your own ego. And in the short run, it could help keep you off most people's ignore lists.
People post here to BT for information sharing, camaraderie, and amusement. The way I read you, you're only posting for your own amusement--and that doesn't seem to interest anybody else.
Try searching on the Web for an Andrew Carnegie course.
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
"Can you tell my why the point loads at each web/chord connection won't be absorbed and resisted better by a more massive chord, say a 2x8 or x10 for example?"
The connection is roughly the same if you use the same number of nails and the same size plywood gussets. That's why the member size doesn't really make a difference. Victory is won not in miles but in inches. Win a little now, hold your ground, and later win a little more.
I know you weren't sniping at me. Glad you jumped in this one.
I knew when I posted the guess at some kind of change in the truss design it would "red flag' someone, hopefully you.
Just goes to prove the old saying " it is better to keep ones mouth shut and be thought a fool, than open it and prove it." :-)
Dave