heres a question we had on the job one day. trusses are built with a arch/camber to them, we all know this. it is my understanding that this is to allow the trusses to flex under a load i.e. snow, what have you. also its been reported that you should not nail trusses to the top plates of walls, but rather you should use a nail on clip that holds the truss in place but also allows for the flexing of the truss. others of you have heard this and have used these clips. the question is that when a house is sheetrocked and after the lid is put up and then the walls are butted up tight to the cieling, how are the trusses able to do any flexing with their movement totally restricted by the fact that the sheetrock on the walls doesn’t allow for any downward flexing? you’ve all seen trusses that don’t sit flat on the interior walls of a house, mainly in the hall in the center of the house and this is due to the camber that is built into them. if this camber is negated by the sheetrock rendering it useless, then is it really necessary? please tell me what you think and don’t tell me to look it up in the archives.
thanks, cpaynter
Replies
I shim them and nail them tight. The whole structure should expand and contract as a unit.
Kip
I'm not a truss expert. Not even a carpenter but I looked into a related issue. Here is my understanding of the issue:
The trusses typically float over interior walls. They are actually weakened if they bear too much on interior walls they were not designed to bear on. Interior walls should not be nailed directly to trusses because of this camber and the need to accommodate changes in it.
Simpson and other metal reinforcement manufacturers make specialized brackets that are firmly nailed to the wall but the nails going into the truss ride in slots. This keeps the interior wall from tipping without restraining the interior movement of the truss.
On the exterior walls, where the trusses are designed to rest there is essentially no movement so few issues. Where the ceilings run up against interior walls the drywall should not be fastened within 18" of the wall. This unfastened length is supported at the edge primarily by the vertical edge of the drywall on the wall. this unfastened length is there to flex as the trusses camber changes with the load.
For a more definitive answer, from a professional who designs trusses and has a lot of experience in all aspects of truss design, drop a line to 'Boss Hog' a frequent contributor on this board.
Cpayner, I'm sure Boss Hog will set you straight but heres' what I know.
All trusses are NOT shipped with a built in camber. In my experience only the scissor type trusses are expected to "droop" after setting and loading.
Trussed DO uplift in cold climates during the winter. This uplift is very persistent cannot be prevented.
The truss institute suggests several methods to deal with this issue. Individual blocking (2x6's work great) placed on the flat between each truss, but not fastened to the truss maintain alignment and keep the corner drywall from floating upward with the truss.....IF..... the drywallers properly refrain from gluing and screwing too close to the interior partitions (keep all fasteners back 24" or so).
In short..there is no downward flexing.
blue
Warning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
"please tell me what you think and don't tell me to look it up in the archives."
If we all did that, we'd spend a heck of a lot of time typing the same thing over and over. The same questions come up over and over.
"trusses are built with a arch/camber to them, we all know this"
Some truss plants do, some don't.
Nailing the trusses solidly to an interior wall is a bad idea. Try reading the thread I did on Truss uplift - in the archives.
You can lead a horse to water, but, a pencil must be lead.
Boss,
None of the links in that post work for me.
"Page not found"
SamT
Link rot strikes again!
The trussnet.com site does not appear to be a live web site anymore.
The woodtruss.com site is still there, but it looks like they've moved things around. Try going to the home page. Delete everything after woodtruss.com on your browser's location line and try again. It looks like there's a search function at the home page if your browser is sufficiently up to date. Mine isn't, so I can't offer any more details. If you can get to the search function, search for uplift and see what you get.
Or you could Google for "truss uplift". There may be newer and better references in the last two years.
Thanks for leting me know. I'll research the stuff Monday and maybe re-do the thread with new links.I was in the grocery store. I saw a sign that said "pet supplies." So I did. Then I went outside and saw a sign that said "compact cars"...
My sheetrocker and I follow the practice of not screwing the edges of ceiling boards to trusses. We do the last screw 16" from the walls where the trusses bear.
The top edges of wall boards are butted tightly to the ceiling boards. Mud and tape in the corner joints per usual.
The trusses can ride up with the Simpson STC clip slots allowing the nails to slide. What we hope for is that the ceiling board flexes in that 16" span where it is not screwed, and we don't get cracking at corners.
What I have described is standard recommended practice for gypboard ceiling attachment to engineered wood trusses. It has worked for house after house.
i had one reply that said i shim them and nail them tight. in all reality simpson sliding truss clips and all i woudl be willing to be that nailing them to interior walls will work just as well as the other methods described by everyone. now i'm not saying that nailing trusses to the interior walls is the correct method, because we all know that it isn't. i'm just saying that i'd bet it would work just as well and does work just as well on thousands of homes every year. my past employer has been nailing them down for over 30 years and has never had a problem. we always had adequete attic ventilation however. our drywallers never held nails back a certian amount, also if the sheetrock on the wall was butted uptight against the cieling, not nailing it from 16 inches from the wall wouln't make any difference, it would still be held uptight against the bottom of the truss and if they moved, wich i doubt that they actually do, there would be cracks all along the tops of the walls. we never had cracks along the tops of walls. i just bought a house built in '70 and it has no cracks along the tops of the walls. yes it does have trusses. but i'll bet if i go up into the attic and expose along the top of the wall that they will be nailed down. you know that they will. so i'll say it again. nailing trusses down to interior walls, albeit wrong in the eyes of ultimate anal carpenterism, will work and has worked on houses for years and years. since the use of trusses for residential framing began
Uh-oh. Women, y'all grab the young-uns and take cover. This here's about to get a bit exciting...
I suspect that the relatively small length of many of these trusses and small movement of these trusses makes nailing them down less an issue. I also think the manner most of these trusses are nailed down has a mediating effect.
I have been in a lot of attics and have noted that the number of nails tends to be small, carpenters are lazy, and they tend to be toe-nailed, a far less secure method of nailing. That a lot of these joints have substantial gaps, allowing the nails flex, would further allow a certain amount of movement in these joints.
Another point is that the loads on trusses in a lot of the country just doesn't vary that much. I'm in Florida and there is startling little snow load. Those loads that do stress the trusses are of short duration. Like wind.
In other words nailing the trusses down, as is commonly and loosely done, works because people get away with it. Luck and probability, limited by sloppy nailing, fail to overcome the engineering margins. People get away with it more by way of dumb luck than design or planning.
4lorn...you're a little off base regarding the reasons that trusses are "poorly" nailed.
From the trusses perspective, interior partition nailing is meaningless. All the bottom chords are tensioned and do not need any partitions to support or stiffen them.
The only reason that partitions are nailed to trusses is to keep the partitions straight and braced until the drywall is applied...therefore significant nailing is not really needed. Keep in mind..that before you set the trusses, there were probably one or two braces holding the wall straight...and each brace had maybe two or three nails in it...and it held fine. Now, put one toenail in ten trusses and it will also hold fine. Once the drwallers are done hanging and taping, that partition isn't going to move a nanoinch.
Shimming and nailing trusses tightly is the wrong thing to do. Putting a few easy nails won't stop trusslift, nor tear anything apart if the truss decides to move a lot.
blueWarning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
My reasoning for shimming and nailing trusses on interior walls. Comes from having seen and repaired nail pops and cracks along the seem where walls meet ceilings.
I belive what causes it is. The homeowner goes up in the attic space and throws down some scrap plywood. Then carrys the Christmas decorations and several other items not being used by the family any longer up there.
If the trusses are floating even 1/4 inch, And you have a 250lb homeowner up there walking around. They are going to bounce. Thus creating nail pops and cracks.
Kip
Did you come here to tell us why we are wrong or are you seeking information?
Also, could you break up your posts into a few paragraphs? It's extremely hard on the eyes to read a solid block of text.
Jon Blakemore
i didn't say that anyone was wrong did i. i think that i stated that nailing down trusses was, repeat (was) the "improper" way to do it.
however, small loads such as wind, or random legth of nails used by lazy capenters doesn't lead me to believe that nailing down trusses to interior walls won't work as well as the methods described in earlier posts. just because simpson makes a hanger/bracket for something doesn't mean that its wrong if you don't use it.
its been done for years and years by reputable builders who aren't relying on luck and probability to overcome any engineering margins. i think we all know that nothing with happen if you nail your trusses down to your interior walls
Howdy.
I stacked track houses in southern california tracts for years. There, we top plated all interior (non bearing) walls with 1x4 so that they would be 3/4" below the trusses. We also 'floated' all the drywall backing on blocks attached to the lower cord of the trusses. We were required to use the simpson clips. This was because the roofs were all tile, and when they loaded the tile roofing, the trusses were supposed to deflect 5/16" +/-. When i moved up to northern CA they weren't as picky and would let us 'flat back' the walls with 2x6 nailed directly on top of the top plate. They also didn't use 1x4 top plates up here. Still use tile roofs I haven't seen any houses crack, but then again i was long gone when they were done taping..
I always assumed that they wouldn't have gone through all the trouble in southen CA if there wasn't a reason, so I tend to float my backing when i use trusses on my own jobs. That way it comes out with a flatter ceiling anyway if the concrete is not perfect ;) Have never had a problem with cracks at the ceiling. Header shrinkage is another thing though... I guess that would be another thread. RZ
random legth of nails used by lazy capenters
Now there's an authoritative, definitive, non-controversial statement. What makes you think carpenters (framers actually) would use random length nails? And, just so we know who we're dealing with, what do you do for a living? Does your house suffer from lazy carpenter syndrome?
Whenever you are asked if you can do a job, tell'em "Certainly, I can!" Then get busy and find out how to do it. T. Roosevelt
Don't forget, yer anal, too, Ed. I bet you even measure before you cut.
ROAR!!Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
The laziest carpenters seem to make the most intelligent choices. Show me a guy who's ambitous and willing to work hard, or show me the guy that is lazy and wants to get the job done easily and efficiently...and I'll take the latter ....every time!
blueWarning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
" i woudl be willing to be that nailing them to interior walls will work just as well as the other methods described by everyone. "
O.K., if truss uplift *IS* a problem, and it *DOES* cause drywall cracks in houses, why would you say nailing the trusses down should be O.K.?
Granted, it doesn't happen in every house. But it does happen. So how about explaining your reasoning a little better?
BTW - If you break your posts up into aragraphs it makes it a lot easier to read.Flies spread disease. Keep yours zipped.
Cpaynter, not all truss configurations uplift the same. You obviously haven't seen the ones that like to uplift. I can pretty much guarantee you that when they decide to, a few nails won't stop them. It is conceivable that they'd hold on to the top plate and raise that with them!
I lived in a house with substantial uplift properties. In the winter, I didn't have nice square drywall taped corners. They were substantially rounded or chamfered. If I ever decided to paint in the winter, I wouldn't be able to use different colors on the wall or ceiling because I couldn't differentiate which was which.
Ventilation may very well have something to do with it. A well ventilated attic in cold climates is the main culprit for creating uplift.
blueWarning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
"It is conceivable that they'd hold on to the top plate and raise that with them!"
I've been in a house where a wall was picked up by the trusses, causing a HUGE crack in the drywall. The HO wasn't impressed.
"A well ventilated attic in cold climates is the main culprit for creating uplift."
Actually, the opposite is true. More ventilation means less truss uplift.
Every house I've looked at that had truss uplift problems has had poor ventilation. And I've never looked at one with good ventilation that had truss uplift.Mom's Travel Agency - ask about our guilt trips.
I'll take your word for it. Maybe my old house needed more ventilation...but I don't think so. It was a low pitched roof...4/12... with a black roof. That might have something to do with it.
blueWarning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
Well, I certainly dont know everything about truss uplift. But what I said about ventilation is true as best as I know. My experience seems to agree with what I've read about the subject in industry magazines and such.
I do wish there was more research into it's causes. The truss industry and WTCA mostly seem to be concerned with making sure they don't get blamed and sued. No one is really trying to find out why it happens on some houses and not others.
Of course - That would take a substantial investment in time and money. And no one wants to come up with that. Maybe some college will take it up as a research project someday...I'm writing a book. I've got the page numbers done, so now I just have to fill in the rest.
http://www.askthebuilder.com/169_Truss_Uplift_and_Seasonal_Ceiling_Cracks.shtml
http://www.thisoldhouse.com/toh/knowhow/repair/article/0,16417,218584-5,00.html
http://www.cnr.umn.edu/BP/extension/faq/trussuplift.php
http://www.allaroundthehouse.com/lib.stu.na21.htm
They all seem to say the same thing. Does it sound right, wrong, indifferent, or anal to you?
Speaking of truss lift reminded me of a set of scissor trusses that we used to install for a custom builder...explain this "engineering detail" to me please.
Background: The scissor trusses in question were about 35' with an 18' front wall height and a 10' rear wall height. The roof was 8/12, the interior was flat in the front and pitched 5/12 in the rear.
The truss had a very weird rear wall "detail note". It said that the framers would have to provide a 1.75" HORIZONTAL movement at the heel. This was the calculated movement when loaded. They provided a special truss clip that allowed this slippage. The weird thing is that they didn't make any special changes in the heel height or tail length. I guess they thought the truss would magically move only at the heel, but not out on the overhang and wouldn't bulge up the plywood.
We never bothered to ask anyone, nor do anything special. In fact, we had already installed a half dozen of these roofs before the special truss clip ever showed up.
Comments?
blue
Warning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
Take a look at Simpson's special clip for scissor truss bearing. You'll see the provision for movement.
Looks to me like the inside finish needs some design considerations, also, if those trusses move as much as the nail slide holes in the clips.
Exactly Gene. If the movement is occuring.....why wouldn't they reduce the height of the truss and tell us not to fasten the roof sheathing and drywall to it?
I don't need to look in the Simpson site...I've already nailed those goofy clips on. I know for a fact that there wasn't any play in the truss though...our roof ply was layed tight and nailed to the truss! And furthermore, how are we supposed to deal with the gable end? That doesn't have the flex in it....
Goofy engineering for a residential application!
blueWarning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
I've posted about this before. This is a pet peeve of mine. I've seen the "sliding clips" that Gene Davis mentioned, but think the whole concept is ridiculous.
At a trade show once, I asked the Simpson guys how these things were supposed to be used. I said if you allowed the trusses to slide back and forth, what happened to the drywall joint? And the soffit material?
They had no idea. And I've never found anyone that can answer the question.
.
IMHO, the first thing that should happen is that the Freakin' truss designer should never allow a design like that to get built. Doesn't matter if it is a "legal" design or not, it's still stupid. Anyone with some common sense knows that a truss with 1.75" of horizontal deflection is going to cause problems.
BTW - How long ago was this? The latest stuff from the Truss Plate Institute only allow for a maximum 1.25" of horizontal deflection. So that shouldn't happen anymore. (Although 1.25" is still a lot)
It would help a lot if roof truss (vertical) deflection was limited to 360/240 instead of L/240/180. That's a heck of a lot of deflection.
With those standards, your 35' roof truss could have a vertical design deflection of 1.75" for live load and 2.333" total. That translates into a lot of horizontal deflection.Gun bans don't disarm criminals, gun bans attract them. [Walter Mondale]
Boss...there isn't any answer unless your not dealing in reality. We asked the lumber salesguy and the truss guy the same questions...of course we knew they weren't going to have any good answers...we just asked them for the laugh.
The last time I've built that house was at least 6 years ago...so it's entirely possible that the movement was 1.25" and my memory is fading.
blueWarning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
with substantial uplift properties
Maybe I'm not "getting" it, but it sure does seem like that if a person were rocking in an area where uplift occurs, that person could just detail for it. Like, butt J-bead up to the ceiling, then run the wallboard in the bead to allow for movement. Some inside corner or a 2" bed or crown moulding to cover the joint, maybe?
Now, if the ceiling joint is (as is all-too common around here) just going to get a second coat of popcorn, it's no big deal.
But I will admit to knowing a surprising (to me) number of people who assume all ceiling joints have cracks, curves, and/or waves and ripples in them.Occupational hazard of my occupation not being around (sorry Bubba)
I remember a job where I was called to look at a door problem.
Turns out thast the trusses were all nailed tighht to the interior wall.
But the bottom plate of that wall was only tacked strioght down in maybe three or four places with sixteens. So the whole wall was being lifted off the subfloor 3/8" of an inch in and around the seasonal shift.
That was tweaking somme of the doors adjacent to it.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
so according to boss hogg, i assume thats jefferson davis hogg, he has never been in a house that has proper or adequete attic ventilation and seen a problem with truss uplift. well then couldn't it be that all the houses we ever built, wich have never had this truss uplift problem had adequete attic ventilation.
and, if that is the case, that proper attic vetilation is the "key" to avoiding truss uplift. then doesn't it stand to reason that it is fine to nail the trusses down to the center/interior walls of a house, and then just go about venting the house the way we always did. then you won't have any problems, just like we've never had any.
wich brings me brings me back to my original question about camber built into trusses. if uplift is a problem, why is there any arch built into a truss. usually and arch is used to resist downward pressure, but apparently trusses don't have that problem. they move upward instead of downward. is this what i am led to believe?
I'm fairly certain that uplift is a problem that only cold climates experience.
I'll repeat...I've never had flat trusses with a cambere built into them.
I'll repeat....nailing the trusses down to the partitions is not necessary to protect the integrity of the truss...it will function perfectly even if there are no partitions to nail it to. The nailing of the partitions is done to hold the partition straight until the board is hung...at which time you could pull all those toenails and the partitions wont move!
Nail the hell out of them if you wish!
blueWarning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
well, thanks for the discussion everybody, it was fun