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I am designing a home which includes 38′ truss spans. The truss company says their design software indicates that in cold weather the bottom cord can rise nearly 1/2″. Whether this is due to the cold, or to anticipated snow load (design spec is 65 pounds) is not clear. To deal with this, they give me a brochure detailing how to “float” the last few feet of ceiling drywall abutting central partitions. I find this highly unsatisfactory, for a number of reasons.
I have proposed a system using 3/4″ threaded steel rod which would tie the bottom cords, near mid-span, to the steel girders in the basement, by running down through partitions (house is single story). The truss company will not condone this, but just refers me to the drywall procedure.
Anybody know if my approach will work without causing the trusses to separate at the tie-plates? Any other ideas? Can’t accept a moving ceiling!
Thanks,
Geo.
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NO choice George (1st law of Mother Nature: It's not nice to fool with Mother Nature)
The Canadian Building Code indicates that interior partitions should not be attached to the engineered trusses used in housing today.
It even goes so far as to recommend a 1/2 gap. I have never left the gap in elevation but I have never attached them either. I leave them float.
Pre-engineered truss are designed to carry the full load at the exterior wall where the top an dbottom chords intersect. Forcing your truss to somehow be ridget in the middle and flex where it is not designed to may do damage.
If your truss supplier says they don't approve your modification, this means that you have no warrantee and this may affect your occupancy permit as well as any outside financing.
Tread carefully when you try to re-invent the wheel.
*There is a way around this, but it is hot cheap. Use two sets of trusses. One holds the roof and the other the ceiling and interior partitions. It was done here locally by an industrialist who wanted a perfect house. His gutters and downspouts were custom manufactured, welded stainless steel. Don't think he had many leaks.Dennis
*That one made me laugh! And this guy actually got someone onto his site to build?!!Blue
*Gabe's right george! If you don't want the trusses to experience uplift, uplift your house and move it to a warmer climate. And pray the iceage doesn't come early!By the way, the method that is recommended allows your drywall to stay stationary. Only the trusses move.If you don't leave the trusses unattached, they will pull the double top plate away from the studs. Don't worry though, they'll get pushed back down each spring. Isn't spring wonderful?In my last house, I couldn't paint in the winter because it was too hard to tell where the ceiling started. The tape was pulled away to a 45 degree angle each year in the upper bedrooms.Blue
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There was a discussion of this issue a year or so ago in FHB. Try a search in the index section of the homepage. Could help, but I routinely get call-backs here in MN for the same problem.
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Oh my,this is such an easy answer.Save all your scraps of 2X4's,2X6's,and 3/4 subfloor scraps.Cut the scraps 22" long(assuming trusses are 24" O.C.).Using the framing nailer and rolling scaffold,go around and nail the 22" scraps to the top of the interior partition wall plates.Overhanging the top plate by 1-2",you now have something secure to screw the sheetrock to.The lid should be screwed to these nailers at the center(between trusses),then the next screw must be 16" away from the wall partition.This allows the ceiling sheetrock to stay attached to the wall at the intersection of the wall/ceiling joint.The only part that moves,is the field of the ceiling.This will never be noticed to the naked eye.This technique won't allow the tape to be pulled up to 45 degree angle as Blue stated.You can also purchase special clips from Simpson that do relatively the same thing,but why spend more money when you have all this scrap lumber sitting around?George,never attempt to fasten a truss in any manner other than it's intended bearing points.In your case,the outside walls is the only place the bottom chord should be fastened.
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maj-
I tried this on my house (constructed fall 98, Alaska). I did this because I had already noticed the gap develop prior to rocking. I used 4x12 1/2" rock, so the transition is even less noticeable, as the long board helps in any curvature that may result.
As for the trusses, I have a combo of both scissors and straight for different areas, and I attached them as required, only at the exterior walls, with hurricane clips. This is my first go at trusses, as we normally due a 12:12 pitch. We did this as a spec ranch with 6:12 and architectual shingles and it looks good, but I know I could have site built much better trusses than I received from the local plant. at one point, there is a belly between two chords that is off as far as 1/2". This took some fancy finish work, but the results look good.
Think we'll be going back to rafters, though. One thing I have considered (actually, I've seen a 12:14 roof done this way) is using a system of TJI's and 2x4 webbing to give a 16 - 18" depth for insulation and air space. This would be engineered, of course. I am interested in hearing thoughts on this subject...
Also, why do the trusses rise 1/2"? Just curious...
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Fred I could never figure out if some of the comments are for real or if there's some leg pulling going on.
I have difficulty thinking that adding 30 to 50% more of empty cubic footage, to the house mass will result in any savings. It will make the house harder to keep cozy and warm with the convection currents etc. (not to mention, one butt ugly ceiling structure.
*Rick, every once in a while, I get a truss with that 1/2" belly, or some other defect. The truss compant is happy to come out and look at it, and will pay us to fix it. If they don't want pay us to fix it, they send out their own service crew (rare). I think your inexperience with trusses led you to believe that you had to work around the inferior product. You don't. And the trusscompany is usually glad that you alerted them for quality control purposes.Of course, if I just have to make a 1/2" rip a few feetlong, I'll probably not bother them. Exception: If the 1/2" is critical to the truss design, I'll want the poroper fix (usually a 2x4x8' scab. Additionally, they haveto supply a fix document for inspection. Blue
*George,Only the builders who 'cut corners' here in RI have ever experienced truss uplift. I never knew what it was until I read about it in magazines.Only thing that differentiates most of us in RI and southeastern MA from those who cut corners (and the rest of the USA) is that most of us fur our ceilings with 1x3 stock. It's so common here you only notice it when you don't see furring on a job.Though not scientifically proven yet and all I have is anecdotal evidence --- It Works.We strap ceilings 16" oc and use 1x6 ledger board (rough cut spruce) where a wall will run parallel to the furring.I understand UBC won't allow 1x stock so if you're under their rules you'll need 2x stock.The only other difference between most of the USA and here that may have an effect (but I doubt it) is that we almost exclusively use veneer plaster. But I've seen drywall jobs with furred ceilings preform fine.It's worth a try if you don't like floating your ceiling / wall corners.
*George, never short change the truss company. If there is any problem you will be left out on your own. There are three ways to combat the uplift problem that I am aware of.1 Use the float system. If this was installed and you did not know it your house would not be any differant then the one down the street.2 Use a ceiling joist to span the load walls, that is seperate from the truss.3 Use a three part truss, two sides and a top truss. This must have a load wall in the house.
*Dear Fred;I may live in Canada, but when it comes to construction I'm from Missouri. I quit attending conferences a long time ago because I found that the speakers always had a hidden adgenda that motivated them to come up with outrageous claims so that they would be invited to do the circuit.(Those that can, do, and those that can't, teach)I earned my Gold Seal Certification by working on more than 500 million dollars worth of commercial construction during the past 35 years.The one thing that I attribute to my success in life is that I always built to the best of my abilities, with my eyes and ears wide open and I listened to what my trades had to say first and the engineers second.I also know that you can build a house in a set fashion for 10 years and never have a problem, but one day, when the conditions are right, all hell will happen with that house, and all the experts won't have a clue as to why and won't agree on the possible cause.There are things that look good on paper, work well in a lab, may even work well for builders in one area, but nothing replaces practical experience and common sense.When I visited England a few years back and checked out some of the castles and Cathedrals, I was reminded how little we have learned in our craft since the time that these monuments were designed and built.We have been so busy re-inventing the wheel that we never took a good look at the original.
*I had a house a few years back that had truss uplift. Fought the problem for awhile until a friend gave me some sage advice. He looked at it as an opportunity for some cove molding fastened to nailers in the ceilings but not the walls. The trusses were free to move over the designed temperature and load range and the occupants weren't dismayed by looking at the seasonal crack.Have since found that especially in cold climates truss lift is common and is taken care of either by some type of sub-structure or clips as posted above or by use of trim.As for FredL's idea of just adding the attic to the interior volume. That wouldn't help because the primary cause of truss lift the engineers tell me is roof loading. Plus, it just adds to the volume of the structure that must be heated.So, clip, use a substructure or hide by trim; they all work. Just don't fasten anything to the trusses that interferes with their design function:):)
*George - As a former truss guy, I've dealt with this quite a bit.First of all, what causes it ? No one knows for certain, but the best available theory seems to be this: The bottom chord of the truss is in the ceiling insulation, so it stays warm and dry. The top chord, as it cools off and warms up, gains moisture from condensation. The top chord expands a bit, the bottom chord contracts a bit, which causes the truss to "lift" a bit. Truss uplift can also be caused by the shrinking of framing materials. A 2X12 drying from 19% to 8% moisture will shrink about 1/8". So if you have a 2X12 beam in your basement, and 2X12 floor joists, this will account for some shrinkage. George, I honestly have no idea what running those rods up through your walls will do to your house. But won't it be incredibly expensive ? You have a reasonable solution to the problem. (floating drywall) Why not take advantage of it, and spend that money on something more practical ?Another point I'd like to bring up is ventilation. Every house I got called out on for truss uplift problems had poor attic ventilation. I went out once on a $450,000 house with the problem. It had an indoor swimming pool, and a large indoor hot tub. There were no proper vents at the eaves. All the bathroom vents and kitchen exhaust fans terminated in the attic. The guy tried to nail the trusses down to the walls to keep them from lifting. The trusses ended up picking up the walls, and causing huge cracks in the drywall. The guy was acting as his own GC, by the way. Another case I remember was a do-it yourselfer. He had never heard of proper vents. He had used vented soffit and ridge vent, so he thought he had plenty of ventilation. But when he blew in his attic insulation, he crawled out and filled his soffit up with it, thinking that would keep his house warmer. Another contractor thought he had a great solution to a truss uplift problem .He just went up in the attic and cut all the truss webs in half. The homeowner found out, and sued the contractor. Turns out the contractor had no insurance, and didn't own anything but a pickup truck. (but I bet his bid was the cheapest !) Naturally the only solution the homeowner could come up with was to sue the truss manufacturer. The insurance company settled out of court, and raised our insurance rates, even though we had done nothing wrong. Sorrry - I'm kind of getting off the subject here. My point in this is to illistrate that good ventilation seems to reduce truss uplift. If you want more info about it, check out http://www.trussnet.com. Click on the link called "partition separation". (The truss industry doesn't call it "truss uplift" anymore)George, I hope your experience with your new house is "uplifting" in other ways............
*Fred only two points were made in my post,The first is that I'm not impressed by someone having designed 1500 homes in Nevada, I'm impressed by someone who built 1500 homes in Nevada.The second is I truly believe that your methodology is based on theory and not experience.
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Thanks, Blue.
The particular truss was an extra I ordered for nailing purposes, next to the transition from scissor to regular truss. Since there are 2 trusses next to each other here, the inspector was not concerned with it's bearing capacity. I won't use this particular truss plant again, because I had trouble with them all the way. I think they just used inferior lumber. I also think plywood gussets would be much stronger, nailed on each side.
I appreciate the pointers. I read this site regularly, and also try to help out with areas more in my expertise (if it can be called that!)
Always learning in Alaska,
Rick
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Dear Fred.
Yes it's true that I don't know your friend Joe, however if he's forgotten more than I'll ever know, I think I'll pass on meeting him.
Seems to me he would have trouble remembering the meeting anyways.
With regards to learning about electricity from a book, remember, before the 50's all the books claimed that electricity flowed the opposite of what we now know to be true.
How how will it be before your information is proved or disproved and how much will it cost, if it's wrong?
Take a little time off and visit our National Research Council Campus and Central Housing and Mortgage Corporations, technical libraries.
And when you do, take a good look at the buildings your standing in, I was the project manager for the construction of them.
*MIke, I don't know if you heard yourself, but you implied that the addition of 1x3 furring strips prevent uplift!If that were the case, all truss manufacturers would specify it!Your trusses are uplifting too, but the connection at the points that would be affected, are probably minimizing the effect, thus tricking you into thnking it doesn't occur. The blocking that the truus institute reccomends does the same thing. And Geaorge would have never known about it, if he wasn't the GC, and talked with the truss man about it.Sometimes the truss guys just say too much!And you also stated that those who suffer uplift are utting corners! That's absurd!Do yourself a favor, and follow the truss link that somone posted, and learn what uplift is all about. Then you will have a better chance of recognizing those "corner cutters"!Blue
*Fred, I'm only going to challenge you on one tiny little part of your theory. You said: So we always lower the total fuel use of a house whenever we build on an addition. I have an experiment that will prove you wrong. Let's assume that my dog fido had to stay outside. So I built him a nice dog house, and insulated it tightly on the roof, walls, and floor with foam. i did such a nice job, that a 100watt light bulb was all I needed to heat the house up.Well fido decides he wants a bigger pad, one to entertain in. So I take him and his doghouse, out to my barn (its 28 x 40)and I nail one wall of the doghouse to the barn wall (throw the roof and other walls out), and insulate the barn with foam. Will I be able to use a 50 watt bulb now? Some how I'm skeptical!Clue me in!BluePs Tell your guys the masks work better when placed over their mouth and nose!
*Come to think about it, the house that I lived in had poor attic ventilation, and the trusses uplifted. The attic wasn't damp though. I thought the uplift was caused by the cold. Somehow. You'd thin that the truss institute would have did some studies and had concrete answers by now. We're not going to stop uplift here in this chat room. Instead, we are just giving the proper techniques for treating the symptoms.Blue
*Fred, I don't have a theory, only observations.In my original post on this subject, you will note that my comment was that it isn't smart to try and fool Mother Nature.Just as a high rise must have flexibility, so should a house.The original request was for comments on his installing a steel beam in the basement and tying the steel beam to the bottom chords of the trusses with tie rods, to prevent them from moving.One way or another the house, whether in whole or in part, will move and flex as it breaths.I am of the opinion that given the truss company has indicated that they would not approve of such a proceedure and therefore I indicated he should rethink his plan of attack.It was a conservative opinion, safe and to the point.PS I'm STILL a great builder.
*I'm sorry Fred, but your last paragraph has a bit of circular reasoning in it."So stabilizing the trusses..." leads to:"Then the trusses would be more stable..."I'm confused.Rich Beckman
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I live in Winnipeg, Manitoba, Canada - a cold climate for certain. I have the "truss lift" problem in my home. The trusses span 30 feet. In the winter, the ceiling drywall lifts away from the partition walls near the center of the house in two of the bedrooms on the second floor. This must be caused by cold and not by snow load as it happens regardless of how much snow we have on the roof. The only way that I have to hide the problem is to attach mouldings to the ceiling and let them float on the wall so that the moulding will move up and down and the crack will not be visible. I do not think you could tie the trusses so that they would not move. If you do, you will create other problems. The truss will move at a different point. The other answer is to stick frame the roof as it does not appear to be a problem in that type of roof framing.
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FredL - I'm not sure if I understand your question: "So how can we possibly get truss uplift if the other parts of the truss are getting bigger?". The idea, as I understand it, is that the top chord has a higher moisture content due to condensation, and expands a bit. The bottom chord is dry, and is stable. The expanding top chord is apparently what causes truss uplift.
I didn't claim that proper attic ventilation prevents uplift - I doubt that's the case. But, on every case I've been called out on, they *ALL* had poor attic ventilation. I do think poor ventilation aggravates the problem. I assume that ventilation would keep more airflow around the top chord, and keep it drier.
I'm not trying to pass myself off as an expert on the subject - I was just trying to pass along the experiences I've had with the problem.
*Adding volume to the heated space has virtually no effect on the thermal performance of the building. Heat loss is a function of the surface area of the building and not the volume. I, too, fail to see how loading a truss can cause up lift. The geometry just doesn't work.
*Let me try to get this straight, Gabe. We can't trust the engineers or scientists, because they don't know anything about building. Their agenda has nothing to do with making better buildings. Therefore, the best approach is simply to continue building without innovation. But, when we go on building without change all hell can happen and no one can explain why or agree about why things failed. But the best thing to do is still go on building the same way, because experience is better than theory. So when we introduce a new problem, say better thermal performance, the best way to achieve the goal is to build things the same as we did before. So the best builders are ones who keep on making the same mistakes but still keep their eyes and ears open. Open for what? I'm sorry, but I'm confused.
*Mike,You're right...It's a bunch of yak, yak.Jack : )
*Mike,b I'm right behind you buddy, It's a bunch of yak, yak again!Jack : )
*Never thought I'd get involved with one of y'all's famous neverending discussions but I guess I have.First of all, I'm not sure I can make the answer I got from the engineer and that seems to track with my experience lucid enough for this forum. But I'll try.It seems that truss lift is a function of truss length and loading. With trusses the loading(thrust) vectors are not what we would intuit; that is vertical. Instead they are modified by the eave overhang length, chord length, web design, and pitch. The result is that there are in fact upward thrust vectors in the chord center section of all trusses, but they are large enough to be noticable on certain long trusses. Some can be visually affected by temperature as that may also cause large enough distortion thrust vectors to move the truss chord. The end result is that there is enough upward force to move the center section of the truss upward. Some building manuals call out construction so the movement doesn't pose a visual problem. Those were mentioned in my previous post and by others.If anyone really wants to understand this at a greater depth than this forum I suggest you sit down with a truss engineer and graphically analyze the force vectors together. It is really simple to understand that way.Summary: There is no reason why a properly engineered truss can't be used with nearly any span. But with longer spans compensation will need to be made for distortion that doesn't affect the truss' function but may affect the building owner's visual comfort. In all cases follow exactly the truss manufacturer's instructions unless you are big into doing expensive and dangerous experiments.
*The attic, though cold, is not a wind tunnel. The blown in insulation will continue to do it's work, event though the trusses have lifted. Batt insulation might be affected, but batt insulation is a bad way to go anyway, unless further covered with blown.I'm still not convinced that more is less.Blue
* Randy, good point about the snow loading theory. My uplifting also was consistent all winter regardless of snow.I solved the problem with a much easier solution. I quit looking up!I don't think I would be happy with a molding up there, but another soulution that would appeal to me would be a "pan ceiling" style drop at the edges. Of course that might not be possible on all existing situations.Blue
* The problem that I see with the shrinkage theory is that if the bottom chord shrunk, it would become tighter, and less likely to raise up. Blue
* I don't want to understand this anymore, and you have given us plenty to think about. I wish my buddy Jethro was here to help me cipher!My experince with uplift was on very small, simple trusses. 24 footers. 4/12. I'm not losing sleep over it.One thing that I do lose sleep over is this:If cold expands things, why do we add heat to a nut to expand it?It seems to me that heat expands, and cold contracts, then cold expands, then heat contracrts!It's no wonder I flunked physics! Blue
*I'll state my prejudices right up front so there's no misunderstandingi I don't like trussesand I don't think they should be used in residential applications. I have managed to avoid ever using them thus far in my carreer although I did bid on a house addition (which I didn't get) last fall, and had to do a little research into trusses for my own sake, as they were on the drawings. I found out several things which lead me to believe that the termi " properly engineered truss" is a misnomer for residential applications. They may be i engineeredto function well as a light weight, long unsupported span, i roof framing systembut they don't seem to be engineered to provide an adequate i residential ceiling.To wit: truss uplift is treated with cosmetic solutions, instead of beingi engineeredout of existence.They are, after all, designed to provide large unemcumbered floor areas for industrial/commercial applications, where i "seasonal uplift"(as it's called around here) is not an issue, and have been adopted by the housing industry as a cost saving device, and as with most cost saving devices there is some penalty to pay.I was surprised to discover during my research that there was an up charge for i raised heel trusses, and that because of this, they weren't commonly used for residential work, yet the benefits of the raised heel area for increased insulation and mitigating ice dams is undeniable. The i raised heelwas also an i add onand was subject to narrow limitations, instead of being a governing factor in the design of the truss!!! This fact alone made me suspicious of the degree of i design engineeringthat goes into these things forb residential requirements.Flame away-Patrick
*Patrick,I like the room in the attic trusses, otherwise I'm with you all the way...Trusses especially today waste so much space...remember Grandma's huge walk in attic?J
*Patrick - No flames, just a couple things I wanted to mention. You seem surprised that "raised heel trusses" were more expensive. They have more lumber, more labor, and more plates than a conventional truss. That's what makes them more expensive. In central Illinois, I would guess less than 1% of the houses I've worked on had raised heel trusses. No one wants to pay for them. I don't understand what you mean about raised heels "subject to narrow limitations,". Could you explain yourself some more ?I really can't figure out whay you're "suspicious of the degree of design engineering " that goes into trusses. In my experience, trusses are questioned and second guessed to death. Every truss job I sent out had to be designed to many specifications by many people and/or organizations. (I won't try to list them all) But you can stick build anything around here, and no one questions anything. Why is that ?
*Blue - I don't think your "shrinkage" theory hold water. If the top chord has a higher moisture content, it will expand, or get longer. The bottom chord will stay the same, or dry up a bit and get shorter. If the top chord gets longer, it pushes the peak up. (It can't push the walls out, since the bottom chord connects both ends.) The webs attached to the peak will obviously raise up with the peak. And, of course, the bottom chord will raise up with the webs.
*RonI'm noti surprisedthat "raised heel trusses" are more expensive, justi perplexed (?)at the vicious circle that keeps them out of standard use because they're more expensive when they should arguably be the i standard style!!The "narrow limitations" comment was referring to the amount of i raised heelI could order. It wasn't variable, or customizable. I could either have 3 1/2" of bottom chord/insulation space or I could have, I think, 6" of raised heel/insulation space. The i raised heelwas simply an add on wedge, not a re-engineered design, and that is what I mean by being suspicious of the i design engineering for b residential applications.I find b absolutely no faultwith the engineering design of trusses for span and loads as a i roof framing system.I think it is arguable, however, that the attention to engineering design doesn't extend to addressing the "uplift" problem in residential applications.It shouldn't surprise you that stick built roofs are not as carefully scrutinized as trusses. Much larger framing lumber is used both for rafters and joists and though they are connected at the plates, they are independant when compared to interdependant truss members, whichi by comparisonare cobbled together from bits of 2x4 and need to be properly i engineered!!!
*Never said that Mike.Some of my best buddies are engineers and scientist down at the campus. We have some rather animated discussions every once in a while about all kinds of building products and methods.Over the years we all benefit from each other's thoughts and opinions.The point you missed is that I don't rely on self appointed experts, doing the talk show circuit of seminars. If you think about the products that have been introduced over the years only to disapear within a few years because they were the 8 track of the industry, you'll know what I mean.In one of our local towns there is a mill that produced transite piping for sewers, because it was a local company, the council saw to it that it was the only pipe used in town for nearly a decade.The company closed it's doors. Now every home that has this pipe is finding out that they rot and have to be replaced.Council doesn't remember it's part in this venture.Homeowners, now have to pay to have the pipes replaced outside their homes and also pay for the city to do the connections. avg $5,000.00 each.We must never close our eyes to new products and methods of construction, but that doesn't mean rushing in with them closed like sheep because some sales oriented expert wants to sell you some magic beans.A lot of consumers are fighting back when they have been wronged, the contractors pockets are not as deep as the suppliers and manufacturers. Who do you think has the better odds of being screwed.Lets not argue about theory, leave that to the scientist, I'm nobody's guinea pig.
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After reading all these posts, here's my 1 cent worth. If an engineer like me explains how loading causes uplift, then the only loading I can think of that would cause "seasonal" uplift would be snow. This doesn't seem to be the cause since it also occurs without a snow load.
Moisture seems like the likely cause, and Ron's explanation is logical.
The first principle of woodworking is "wood moves".
The second principle is "if you don't allow for the first principle, you'll have problems."
If you try to fix a raised panel inside its frame, and don't allow it to expand and contract freely, it'll split.
If you don't allow a hardwood floor room to expand, it'll buckle.
If you try to hold down the bottom cord of a moisture expanding truss, you'll probably split the bottom cord or tear it from the rest of the truss.
You can obviously treat the symptoms with the different techniques described above.
The only proposed technique dealing with the problem (moisture) was Fred's. Bringing the entire truss into the living/insulated space sounds logical. Doing that, however, sure sounds like a project. It seems like the best time to accomplish this is in new construction. Treat the roof like a cathedral ceiling and go from there.
The only clear consensus is, "DON'T MESS WITH THE STRUCTURE OF THE TRUSS".
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Thats what I always wanted to know! We stick build some roofs that are so flimsy, and they pass without question!
*Around here, it is not uncommon to have heels that are raised. Ive set many trusses that range from 8" to 72". The height of the heel is detremined by the exterior elevation. It is determined by the window height and the sizes of the cornice members.It is a well known fact, that erecting an energy wise house is always more costly than the standared 1970's model. If you want it, you gotta pay for it!Your other concern is the ceiling system. The problem of uplift is so minor, except in a few people's minds. The proposed fix is so simple that we have collectively used more energy talking about it, than the fix itself requires! Just block and floost, and I guarantee that you'll never notice the slight variation any more than you notice the slisght variation that is inherrant in all framing members! It seems ironic that you'll install joist with their crowns, but reject a truss that is straight, but might crown!If perfectly straight ceilings are the only thing you will accept, you have to stop building with wood. You probably will have to enlist the scientists at NASA to help you achieve the perfect building. Good luck!I have not seen a conventional ceiling that was a stiff as trusses, unless it was far overspecced. To me that is a huge waste of our resources! Save the owls, and beef the buildings up where it is meaningful! Holding a static piece of drywall is not my idea of an important place that needs the extra wood. Do you also use 1" drywall on your ceilings? Just switching to 5/8" on your ceilings would make it better!How much better is better?You've also neglected the most important benefit of trusses: the ability to clear span and provide flexible design opportunities. How many 22' wide rooms have you framed recently? The last house I framed had a geat room ceiling that measured 22x26 without interupton by bearing headers, or posts. It would have been financially irreponsible to erect that house, and maintain the design criteria below, without trusses. When you look at the whole picture, a few blocks, and 16" of floating drywall seems very minor, very minor indeed!Get into the 21st century, and save the owls! Blue
*George, if the floating drywall bothers you, and you can't figure out how to move that house into a warmer climate, then you have two choices.Change the laws of physics, or if that is too complicated, frame the house conventionally! Blue
*What part of my shrinkage theory doesn't holdwater?If the bottom chord shrinks, does it get looser?Or do the walls move in closer together to accomodate the shrinkage? Blue
*Patrick - If your truss supplier only gave you 2 options for raised heels, you need to find another one. Trusses are no longer designed by taking standard pictures from a book and figuring out the member lengths. Every truss on every job is run through a design cycle on a computer to determine lumber grades, plate sizes, etc. There's no reason you can't have any heel height you want. I'm personally not surprised that raised heel trusses are used so little. With the current mentality in America, cheaper/lower quality stuff is what most seem to want. (Why do you think wal-mart is doing so well ?) I also doubt that the extra cost is justified. You only gain more insulation in a very small area on the house. You have to pay more for the trusses, add more siding, and of course more labor. You could easily add $1,000 or $2,000 to the cost of a house. How long would the payback cycle be ?As for the stick built vs. truss framing issue - I think stick framing should have to meet the same design criteria that trusses do. Stick framing isn't automatically stronger just because you use bigger mambers. It just LOOKS stronger. Many times no thought is put into the connections between the rafters/ceiling joists/ridge board. I doubt most stick framing I've seen could actually hold up to full design loads for this area. Trusses, however, are supposed to have a 3 times safety factor. You mentioned that"attention to engineering design doesn't extend to addressing the "uplift" problem". What would you suggest be done ? We can't change the properties of lumber, can we ? Maybe we should sue somebody. I know ! - God grows the trees, let's just sue him and see what we get out of it. As I said before, trusses are NOT just "cobbled together from bits of 2x4". Not by a long shot. Every truss that goes out that door has to last for 50+ years. Since there's no statute of limitations for engineering, they have to be able to be defended in court for that long. If you want to stay in business, you have to do a decant job.
*You go,Blue! Finally I found something to agree with you on(just kidding).It does not take alot of time to block the partition walls,and you get to use up all the scrap that the homeowner just can't stand to see thrown away!I also tie all the intersecting wall corners together with a 2X6 fastened to the First 2X4 stud at the outside wall intersection,letting 1" overhang each side.This lets you kill two birds with one stone.There will never be cracking corners in the houses I build.BTW,I also hang all the rock in my houses,so I know that the screws will not be in a truss within 18" of the interior wall partitions.Let the wood move as it will,no one will ever stop wood from moving,that I think we all agree on.So, we just have to deal with it to the best of our ability.Furniture builders contend with this problem all the time.They have found ways to fasten wood together,but yet still let it move as it will.
*Well George you have seen all the biases and discussion. One thing seems clear: Long trusses will move and for some reason(I happen to believe it is load as that is what all the engineers have told me and not moisture as presented here)and must be compensated for . If you don't like that then it seems you need to move to another kind of structure design. Perhaps Post and Beam, maybe using Engineered Beams, or maybe use Steel. Or, you could redesign for shorter spans. Personally, I've done all these because my bias is to do whatever works within the budget. If the budget isn't big enough cancel or get a new budget.After all the two primary reasons for trusses are the ability to have long clear spans and cheaper construction costs.I am always curious what decision is made after all the smoke of a discussion like this. What are you going to do George?
*Hear! Hear!Blue
*You're right on one point. I reread my post and I should have stated the situation locally as follows: We don't experience the problems that truss uplift presents. I realize the real problem that bottom cord uplift presents but anecdotally speaking, straping overcomes the problem. On the other point regarding 'cutting corners' I didn't mean to imply that those who don't strap are cutting corners. That was the intent of using '*'. Not using furring is considered corner cutting locally. Just like ommiting joist blocking and using OSB... Sometimes brievity gets me in trouble.Thanks for the objective observation.
*
I am designing a home which includes 38' truss spans. The truss company says their design software indicates that in cold weather the bottom cord can rise nearly 1/2". Whether this is due to the cold, or to anticipated snow load (design spec is 65 pounds) is not clear. To deal with this, they give me a brochure detailing how to "float" the last few feet of ceiling drywall abutting central partitions. I find this highly unsatisfactory, for a number of reasons.
I have proposed a system using 3/4" threaded steel rod which would tie the bottom cords, near mid-span, to the steel girders in the basement, by running down through partitions (house is single story). The truss company will not condone this, but just refers me to the drywall procedure.
Anybody know if my approach will work without causing the trusses to separate at the tie-plates? Any other ideas? Can't accept a moving ceiling!
Thanks,
Geo.