I think I’m about to learn more now, but I have to ask about this:
I was taught (in a State sponsored building trades program) that the purpose of collar ties was NOT to prevent ridge sag/outside wall spread! I was taught that you had to either use ceiling joists, trusses, or a supported ridge beam.
The idea was that the purpose of collar ties was to prevent, or minimize, rafter sag (between the ridge and the outside wall) and that collar ties were not to be relied on for anything else. Many houses that I have worked on (built between the teens and the thirties) were framed with 2×4 rafters, 2×4 or 2×6 ceiling joists, 1×6 collar ties (usually located about 1/3 of the way down from the top), and 1×6 or 1×8 skip sheathing. Nothing has yet been done to change the roof framing in lots of them. Yes, in a few of them there has been substantial rafter sag.
Have I been laboring under riduculous concepts for all these years? (OK, so I have been regardless of the answer to this question… Get it? LOL!)
Thanks for your comments!
It doesn’t matter how fast you get there, it just matters that you go in the right direction.
Edited 3/14/2003 8:09:58 PM ET by Hasbeen
Replies
Well, I had to come to the Breaktime University to learn the answer to this. Let me distill what I learned from a previous discussion.
Previously, I had always thought of collar ties as anit spread devices that sometimes doubled as ceiling joists.
It was here that I learned that these are actually rafter ties.
Colar ties are always required to be in the upper third of the rafter length. It is more strictly enforced (it is in the building code) in areas subject to tornadoes and hurricanes, because the main reason for collar ties is to prevent uplift in high winds, the sort of explosive action that can lift on the roof and tear the rafters loose form the ridge in an upward direction.
Excellence is its own reward!
piffin- How would they prevent that? I could see it if they ran perpendicular to the rafter, down to the ceiling joists. Then you might gain the shear strength of whatever connector you fastened them with. But it seems to me the collar ties we are all so familiar with would just pivot if subjected to the kind of uplift forces you're talking about.
Hasbeen - I think the way you described their usefulness is pretty much the accepted reasoning for them. I'd argue they are next to worthless in 99% of the applications I've seen over the years.
I agree that they are generally useless!
I also saw a number of homes built with 1x6 from ceiling joist to rafter, about half way down the rafter to maybe a 20 degree or so angle back toward the center of the house. The house that my trade school class built was done that way. That was in tornado country, maybe it was to prevent uplift!It doesn't matter how fast you get there, it just matters that you go in the right direction.
What Piffin said.
Keeps the roof from opening up like a clamshell in high winds.
Whatcha up to Norm?
My ideal weight if I were eleven feet tall.
Boss Man,
If you've got it, I would like to see something to supprot your "clamshell theory".
Currently I am not believing it, but am not discounting it either. I cant remember where I picked up the "wall spread theory" but that is what is stuck in my mind.
Can we have an engineer to rule on this! Anybody?
WAHDView Image
I don't know for fact what they are for, but if you just stand back and look at a cross-section of the roof framing, a few things become obvious. For a simple triangular roof structure, the rafters are in both bending and compression, caused by dead and live loads of/on the roof. If the ridge is not structural (ie the ends are not supported any differently than at any other set of rafters), then all the roof load is on the outside walls, under the rafter/joist connection. That makes a big triangle, with the ceiling joists in tension.
With collar ties a third of the way down, and no ceiling joists to react against the spreading forces, the collar ties have to do it all. That puts them in tension, and being only a third of the way down, this tension force is much greater than if the joists were taking it (three times as much). The rafters are already in bending, and the collar ties would add a significant bending load at the point where they attach. The shear in the tie/rafter connection would be huge (relatively speaking).
If the collar ties are there to prevent rafter sag, that means they are in compression. This would add to the spreading force at the walls, rather than reduce it. The typical 1x material I've seen won't support much compression without buckling, and would be much more effective half-way down the span than near the top anyway.
The keeping the clam shell closed principle seems the most reasonable to me, given the material, connection methods, and (required?) location near the top. The ridge can't open unless something gets longer, or a connection fails (like rafters/ridge beam shear, or collar tie/rafter shear). They may not do much in the event of a direct tornado hit, but most houses are more likely to see the associated high winds, or a hurricane, if coastal. Rafter to ridge connections are basically compression connections, and don't look (to me) like they can withstand much in the way of separation, or tension. Hence, the collar ties.
At least, that's how I see it.Be seeing you...
Tom,
I hear what your saying about clamshell as it relates to loading.
The question is, whats up with the clamshell and uplift as it relates to collar ties?
I understand the uplift, what I don't understand is how collar ties are to prevent it. Its kind of like tieing your boat off to your boat and not the dock.
WAHDView Image
Asking for an engineer's opinion won't help - If you took all the engineers in the world and laid them end to end, they wouldn't reach a conclusion.
(No offence intended to engineers - Just poking fun)
Let me throw my 2 cent's worth in and see what you think.
For starters, imagine a stick framed gable roof with ceiling joists parallel to the rafters, a ridge board, (NOT a beam) and no collar ties. The rafters are (typicaly)just toenailed into the ridge board. Not much there to keep the roof opening up like a clamshell with stong wind uplift. (Watch one of those storm related shows on the Discovery channel, and watch how the roofs blow off)
Now add collar ties to that same roof system. They don't change the dynamics of the structure much, unless there's wind uplift. Then they would serve to tie the 2 sides of the roof together. It would pretty much have to stay in place or blow off the whole thing as one unit. (The wind uplift would find the weakest link)
Now take out the ceiling joists. This changes the dynamics of the roof dramatically. First off, you now have ceiling load on the collar ties and underside of the rafters, which increases the horizontal thrust at the outside walls. Since the collar ties have to resist that thrust, they pull inwards and downwards on the rafters, which adds a lot of bending stress to them. Rafters in this situation may not work which would work fine with the ceiling joists in place.
.
I wouldn't say it was completely WRONG to frame a roof with collar ties and no ceiling joists. But it takes a different kind of analysis for the rafters. That's why I question it when people talk about removing the ceiling joists on old houses and relying on collar ties.
That's my opinion, and it's worth every penny ya paid for it...............(-:The first 90 percent of a project takes 90 percent of the time. The last 10 percent takes the other 90 percent of the time.
How does the wind get under the roof in the first place, Boss? Does it create a vacumn type uplift on the lee side as it hammers the windward side?
The uplift is created by the wind passing over the top of the roof - Much like the air passing over the top of an airplane wing.Time wounds all heels.
I frame houses here in southern Alabama, in a "high-wind" area, and we have been told by our inspectors that, like Piffin said, collar ties are to prevent uplift from pulling the rafters up and away from the ridge. We normally use 2x6 rafters at 24" o.c., and add 6'-7' 2x4 collar ties to every pair of common rafters. We have also begun to clip the attic bracing (bridging) to the rafters.
-The master of wrap and strap
Any tie between the rafters add to the stength and stiffness of the roof framing.
They not only reduce bending (the span is reduced), but they also prevent spreading at the top and bottom of the rafter. Clearly, the location of the tie affects how well it does any of this.
The ties and fasteners should be engineered to use ties to their best advantage.
Tornadoes will pull any roof off of wood frame construction. Hope for a near miss.
GeorgeR srote: They not only reduce bending (the span is reduced), but they also prevent spreading at the top and bottom of the rafter.
George, how do collar ties reduce spreading at the top and what forces would cause spreading at the top?It doesn't matter how fast you get there, it just matters that you go in the right direction.
I am one of those engineers.
You asked how they prevent the top from spreading. If you have a tie near the top (and the bottom is constrained), wind uplift can not pull the two planes of the roof apart. (A strap acroos the top of the rafters might do a better job.)
It is so hard to answer engineering questions. The engineer looks at the loads he feels are appropiate and then designs load paths and nailing patterns. Then we guess after the fact.
I got the room built. How do I get outside?
It's interesting to note the diversity of opinions regarding the use of ceiling joists and collar ties seeing it has variously been a standard feature in housing for how long now and as yet there is not a group or organization that has nailed down a definition of specific purposes and exact specifications of correct placement.
Forget primal scream, just roar.
Makes sense to me, George! Thanks for the explanation.
Oh, to get out, open the window and jump!It doesn't matter how fast you get there, it just matters that you go in the right direction.
I have been taught and it makes sense, Collar ties prevent the walls from spreading. Think of a simple gable on a one storey rectangular house whom the owners wish to have vaulted ceilings, therefore no ceiling joists, no half storey ect.
With pitched roof joists bearing on the walls there would be a tendency for the ridge to want to fall downward and therefore transfer the force to the walls thereby pushing them outwards. Collar ties, wall ties, there are many names, act against these forces and complete the load path so that it is in equilibrium, triangular.
Where I am located (Southern Ontario) it is generally agreed that collar ties may be placed at a maximum hieght of 2/3 of the roof hieght (with exceptions for low slopes)and are sized as cieling joists for the resulting span, the Code here states that one must be used every 4' min.
Thats my understanding, anyone else?
Edited 3/14/2003 2:36:02 PM ET by CDN_Builder
Edited 3/14/2003 3:45:12 PM ET by CDN_Builder
My thinking is that the example you gave of common frame roof with no ceiling joists is simply something that shouldn't be done at all. Hence, my original statement: "I was taught that you had to either use ceiling joists, trusses, or a supported ridge beam."
If a customer wants the look you are describing, the best solution is probably scissor trusses.
And yes, I'm glad I asked. This kind of back and forth talk on a subject like this is the best part of the forum! IMHO, of course.It doesn't matter how fast you get there, it just matters that you go in the right direction.
I would disagree, the scenario I have stated would be the absolute minimum in terms of support, but it works. it works because of its efficiency, it is basically the simplest truss there is a triangle, all loads are in equilibrium and transferred to the foundation.Roof joists and collar ties would be sized from the code depending on your building areas snow load. To prevent uplift I would install metal ties to the wall studs.
Also, the "collar ties" act as a pivot point preventing the top ends of the rafter from separating from the ridge board, as this happen if the walls were subject to collapse inward which at the same time are being subject to the gravity and outward force of the roof. Designed properly this roof is in perfect equilbrium and capable of maximum snowloads, as long as proper rafters, ties, and fastener choices are heeded.
Well thats my opinion. I've seen it, I've done it, it works.
Cheers
Some have said that the purpose of collar ties is to revent separation of the ridge and the rafter tops in the event of drastic uplift from wind.
If that is the case, why wouldn't you put them immediately under the ridge instead of 1/3 of the way down?It doesn't matter how fast you get there, it just matters that you go in the right direction.
Some do. The code limits it to no more than a third of the way down..
Excellence is its own reward!
Has,
Our crew always cuts them 6-7 feet long, regardless. Nail 'em where they fall. I doubt any carpenters (in my area at least) are gonna measure to see exactly where the top third of the roof begins.
Well, I concede that they might help prevent this clamshell opening in a strong wind thing, but I agree with whoever said if that's what they're for, you be better served using steel straps nailed flat on top of one rafter, running up and over the rifge, and down onto the top of the opposite rafter.
Until I read this thread, I really figured collar ties were just for hanging the attic lights and banging my head on;-)...actually, sag was the only part of the equation that I was seeing. Now, I understand the ridge separation deal, but I don't see how this works with the airplane wing analogy...wouldn't that be lifting the roof off at the other end, the top plates? I've got a metal roof with plenty of ventilation, so, I could see wind getting under it, and blowing it apart at the ridge...but on a shingled roof with soffit and ridge vents, does the peak gets sucked upwards?
As a true windbag, I need to know these things<G> EliphIno!
Well, I can explain the airplane wing part, 'snort, but somebody else will have to tie that to the roof opening like yer Aunt Emma's arms when you show up for Christmas Eve with a fresh keg.
The top surface of an airplane wing is arched, so it has more surface than the bottom. So when the front edge of the wing "splits" the body of air, the part that goes across the top of the wing has farther to travel, so it gets thinner than the air that travels under the wing, thereby creating "lift". At least, I THINK that's right.
I still fail to see how collar ties help prevent sag, though.
Dang, I still don't get it, my Aunt Emma's always got her own keg...so, I'm guessing we're talking suck lift here, and not uplift, am I getting close Boss?
The sag is compression, stuff pushing down, four layers of shingles, ice, snow, Santa and his raingear, a pissed off higher being...
I'm thinking up as something from undeneath, Ted, you're giving me the rolling over thing, which I get, but why not spray Top Coat° on everything? I mean what's not more top to coat than a roof? Whassa matter? can't get a grip? OK, hips are not supposed to blow off, whyzat? EliphIno!
BS Billy, it's ok son.
Georgie, I don't read codes, every time a new one comes out the last one is incorrect.
Heres what I was talking about:
http://www.taunton.com/finehomebuilding/pages/h00011.asp
Take it up with them.
I'm pretty sure a curve ball bends from centrifical force, Boss, not air pressure. It's all in the laces.
Well you are completely wrong about that. Thats why curveball pitchers have a tough time in Denver ... too little air pressure. The laces create more drag creating uneven pressure on the ball which affects it's flight. Centrifugal force may have some effect but it isn't likely to be large enough to be detectable.
The lift force of a subsonic airfoil is produced because the air moving over the top goes further that the air below it. Pressure is invesely related to velocity and so a low pressue region is created over the airfoil and lift is produced. The amount of lift produce is depentant on the shape of the wing, the size of the wing and the square of the velocity of the flow over the wing. A roof can produce lift in the same manner and because it has a large suface area and the wind speeds over it in a tornado are quite high it would probably generate a fair bit of lift. The greatest lift forces would occur just to the lee of the ridge line in a gable roof where flow seperation would occur , probably within the top third of the roof. Not a bad spot for a collar tie. This is the area where you would expect the most damage to shinges in a high wind and that is why the roof would tend to open like an oyster along the ridge. As for the baseball check out a book titled The Science of Sport, I can't lay my hands on my copy right now so I can't give you the author. As i recall it has a chapter on the dynamics of baseball. Very intersting and a great conversation stopper at the bar after work.
Happy housebuilding
John Quinton
Quinton Construction
Thanks... I don't question the collar tie purposes ... I'll check out the book. It sounds interesting.
Here's an explanation I found about curve balls:
http://entertainment.howstuffworks.com/question444.htm
Here's another about how lift is created by an airplane wing:
http://travel.howstuffworks.com/airplane3.htm
Lotsa cool stuff at this website, BTW - Great place for a kid to poke around.Look, if you don't like my parties, you can leave in a huff. If that's too soon, leave in a minute and a huff. If you can't find that, you can leave in a taxi.
Thanks for all your posts on this thread! I didn't get the uplift thing about collar ties at all.
What sy we all head over to the Woodshed for a round on me?It doesn't matter how fast you get there, it just matters that you go in the right direction.
Back in the dark ages, in architecting school, I was taught that a collar tie was used when there were no conventional ceiling joists (coffered, miter, cathedral, etc., ceilings). The collar ties "took the place" of the joists in preventing the spread of the walls below (from any roof loading). This is why they had to be in the bottom third of the rafter (closest to the ceiling). Further, I was taught that what are commonly called collar ties, are, in fact, rafter ties, and that their purpose was to add rigidity to the "apex" of the roof/rafter system. For rafter "sag" (or to resist uplift), you add in rafter jacks, which tie the rafters to the joists (typically, midspan to midspan). I remember this, because the jacks become a "hidden" mechanism for creating a bearing wall (not a good thing to discover while remodeling).
When I was learning about lateral & wind loads, they taught that what stripped the roof off entire in high winds, was a failure of gable walls, not the roof membrane itself. Also, that the weakest area was in the 'hanging' gables, framed between a lower roof and an upper. Which seems odd, until the mechanism is explained. Around 150-160 mph (an outflow from a downburst or in-line wind event), the lower roof membrane is "pushed in." That flexes the joint to the gable, creating a space. Once a space is opened, the wind then "inflates" the roof from the attic out. Florida changed their reinforcing and strapping requirements after Andrew, due to gable failure destroying so many roofs (some talk about reducing the number of ties between the commons & the studs, so that the roofs would not come off in one piece, too).
To borrow the line from Dennis Miller, that's just my opinion, I could be wrong.
This is how I understand it:
The faster air moves, the less pressure it has. When the wind goes over the top of the roof, it's moving faster than the air inside the attic. So you get uplift.
That's also roughly how a curve ball works in baseball - The ball's spin makes the air move faster on one side, and the ball moves towards the lower pressure.
That's the best I know how to explain it.Life is like a dog-sled team. If you aren't the lead dog, the scenery never changes.
Piffin is right.
Collar ties restrict wind up lift.
Rafter ties stop the spread.
That's why Rafter ties many not be place higher than 1/3 up from the BOTTOM, collar ties 1/3 down from the TOP of the vertical distance.
Ceiling joists are typically your rafter ties.
In the event of a tornado, the wind will rip up one side of the roof, curl over and suck the other side of the roof off opening up the roof. That's why you need collar ties.
Everyone is so involved with the terminology and the code requirements.
The ICC 1&2 family dwelling code requires (802.3)
Connection of rafters to the joists or to "rafter ties" as close to the plate as possible.
There is no tie required at the top (unless there is no ridge board).
Note the absence of a "upper 1/3" or "lower 1/3" requirement.
Codes differ. When in doubt, consult your AHJ.
I got the room built. How do I get outside?
A collar tie is not going to keep your roof on if a tornado is close, period. straight line winds maybe depending on the wind speed a collar tie might help. I dont know where your at but we get quite a few here in Kansas.When they say on the news after a tornado that damage was light it means it it tore a tree out of a pasture or ripped a a pole barn apart.It doesnt mean it pulled a roof loose. look at some pictures of the Andover Kansas tornado of a few years ago.
Ever watch one of those storm shows on the Discovery channel? Ever watch the roofs blow off of houses in sections? Or in one huge piece?
When a tornado goes through an area, there's probably no way to save the houses that are in the direct path of it. But what about the ones that are a half mile or so away? They could still see winds high enough to do serious damage.
So there's good reason to design for wind. Why do people point to their wrist when asking for the time, but don't point to their crotch when they ask where the bathroom is?
here's how I understand the uplift part, right or wrong.
If a tornado or a high wind brings along an extremely high pressure differential ie. lower pressure outside and regular pressure inside, the difference is the force applied in an upward direction on the roof. Because of the airfoil nature of the shape of the roof likened above to an airplane wing, that uplift is greatest at the ridge. I have witnessed this in that the greatest number of wind damaged shingles I have needed to replace in my life have been within a couple of feet of the ridge.
Now there is more fastening between top plates and the joists when hurricane ties are used than there is between ridge board and the joists so with greater uplift there and less fastening, it is not hard to imagine the roof opening up like the cargo bay doors of a space shuttle in a tornado with the fastening at the top plate and soffit acting as a hinge.
In the previous discussion we had on this (where I was educated on the subject and where the southern building code was quoted) there were comments from those who had witnessed the aftermath of tornadoes and testimony that convinced me that upper third collars were effective to prevent greater roof damage.
Personally, I think the anti-sag benefit is incidental and minimal..
Excellence is its own reward!
Dang, I've been putting those things in for over 30 years and thought it was all about the sag, glad I nailed the hell out of them now...this is why I love this place... EliphIno!
Ain't sag what you get when there are too many beers pulling your belly down over your belt buckle? I guess enough roof framing will prevent some of that...
.
Excellence is its own reward!
I'm pretty sure that's Dunlap's Disease...like in, my belly done lapped over my belt...I can't find my belt right now, ummm, I'll get back with you.... EliphIno!
Okay, now hold on there piffin. You're onto something there with the shingles closest to the ridge being the first to flap up in high winds, but is it possible that's because as the wind travels up the surface of the roof it gets compressed, because of the resistance of the taller obstacle?
Dunlap disease is a sure sign you're living right.
I suppose so. But I know that I have been lifted up off a roof near the ridge once in high wind. It's an un-nerving experience that doesn't leave much time for on the spot analysis..
Excellence is its own reward!
Do you mean that you have a metal roof over your home with no felt or similar under the metal? In other words, metal put straight onto skip sheathing or purlins.It doesn't matter how fast you get there, it just matters that you go in the right direction.
Yep, right on the skip sheathing. The theory is, let that metal breath and evaporate any condensation quickly. It works around here. EliphIno!
Glad u asked?
This has been a very fortuitous post. Collar ties are to prevent rafter sag, except in high winds when they serve to prevent spread. The class is all right (correct). Today everyone gets an A+ and goes home early.