I’ve been reading up a great deal on collar ties and have come to this understanding. Collare ties placed in the upper 1/3 of a roof( by themselves) will do little to stop wal thrust / rafter sag/ or decrease rafter span. However, if collar ties are placed in the upper 1/3 and used in conjunction with ceiling joists fastened at the top wall plates, and the ceiling joists are at minimun 4 ft on ctr, the collar ties will help decrease rafter span / sag. Seem logical??
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Replies
If your rafter and collar tie are strong enough high or low placement of the collar tie is irrelevant. They have to be sized and placed in correlation with each other. Think about this before arguing! If the rafter will not sag and the collar tie will not allow the rafters to spread you have no movement. PERIOD! It is the same principle as a cathedral ceiling will no collar ties. If the ridge and rafters are strong enough not to sag you have no movement. This was just explained to me by a structural engineer. I had to think about it for a bit before I understood what he was saying.
This is an age old argument. COLLAR TIES DO ABSOLUTELY NOTHING TO STOP RAFTER SAG OR OUTWARD THRUST ON WALLS. The whole purpose of a collar tie is to stop the roof from blowing apart in a wind storm. Wind blowing through roof vents create enormous pressure in the roof space. The ridge holds the rafters from falling into themselves. The ceiling joists hold the walls from being pushed out. Collar ties hold the rafters at the top from separating from the ridge when pressure is caused from wind.
Why do vaulted ceiling have no rafter ties? No roof space to create pressure.
Ever wonder why a roof blows off during a tornado, and the walls are left standing? Pressure differential in the inside space of the roof.
Edited 2/12/2008 5:32 pm ET by Bridge_Dog
Edited 2/12/2008 5:33 pm ET by Bridge_Dog
I'm not trying to start an argument, But, I have come to realize that my rafters are undersized. Accordingly, I have collar ties on each rafter pair and ceiling joists on 4 ft centers in a vaulted ceiling. The engineer stated that although the rafters are a bit undersized, this arrangement was adequate.
Sorry for yelling, I just get so pizzed off at this dissucion. I will be more civil in my post to you. How can you have a vaulted ceiling with wall ties?? I need more info to help you.
The vaulted ceiling only spans half the width of the building. Picture a low sloped triangle ( roof is 5/12) cut down the middle from top point to middle of bottom span. There is no ridge BEAM underneath rafters at top,(or at all for that matter) but beam underneath(ceiling height) running down middle of triangle. Rafters were connected with gusset plates at ridge. I made the lower beam to support ceiling joists becaue I couldn't beam it at ridge. Anyway, the existing rafters are short 1' 5" according to code. So I put in collar ties on each pair in the peak, and ran (2)2x10 ceiling joists every 4 ft to hold walls from spreading. The 2x10's run from wall plate to center beam. Also, coming off that beam I put purlins suporting the rafters at ridge. Clear as mud???? If I could post a pic I'd do it.
The beam, the supports up, and the ceiling jopists / ties are doingf the load bearing and preventing wall spread.
The collart ties up above are preventing uplift at ridge so higfh winds don't open things up. They may be unnecessary with the gusset plates. Your framing is a mix of all sorts of marginal things that are working in concert together according to your engineer there and it is with an engineer that any one should think about such a mish mash of things together simply because it is a-typical.
The normal rules apply to normally framed structures, not unique designs
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COLLAR TIES DO ABSOLUTELY NOTHING TO STOP RAFTER SAG OR OUTWARD THRUST ON WALLS.
When collar ties are placed at the right height, they will stop the walls from outward thrust without using a structural ridge.Joe Carola
Looks like a rafter tie to me. A collar tie is put in the upper 1/3 of a roof at 4' intervals. Any thing spec'ed by an architect or engineer is probably a rafter tie. I know it is semantics but the two terms are wrongly interchanged.
If you were to build the structure in your pic. I would have to assume that the tie would be placed at every rafter. Therefore it would be called a collar tie.
If you were to build the structure in your pic. I would have to assume that the tie would be placed at every rafter. Therefore it would be called a collar tie.
They are placed on every rafter @ 16" oc and sheetrocked. We call these clipped ceilings though. I've always called them collar ties and everyone else that I know calls them collar ties and never even heard of the word rafter ties until these forums.Joe Carola
By the right defination (not mine the codes) they are rafter ties or ceiling joist. I think that most of the arguements are caused by the terms collar tie and rafter tie being interchanged. I am in no way saying you are wrong or doubting your ability, so please don't take offence. Like I said it's just semantics.
Here in Las Vegas someone asked me to get the double jack out of the gang box. Being a carpenter of 18yrs from Michigan, I felt really stupid having to ask what that was. The foreman also looked at me like was an idiot, when he answered it's a sledge hammer. Again semantics.
I'm not taking offense at all. This is just another example of using different terms. Like I said before, I've always called them collar ties and everyone else does. My definition of ceiling joists are when they sit directly on the top plate and nailed into the rafters.
As far as you feeling stupid about the double jack thing, I would've asked how can you fit the double jacks for the header in the gang box.........;-)
Joe Carola
Joe - when the tie is installed at the right height to prevent wall spread, it is a rafter tie not a collar tie. The collar tie is in the top third of the rafter space and does almost nothing to prevent wall spread
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entirely different than I was taught. How can rafters sag with proper coller ties? If rafters don't sag, won't the wall be secure?
" How can rafters sag with proper coller ties?"
A collar tie is placed in the uypper third of the rafter space. How in the world can anyone imagine that it can prevent the rafters from sagging there?
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>> How can rafters sag with proper coller ties? <<
I tried to explain this in another thread, but let me put a different angle on it this time. Basically I'll be expanding on the last sentence of the 4th paragraph in my previous post.
Look at this web page. It's an excerpt from the NC 2002 residential code. Look at the "a." note at the bottom of the first table - R802.5.1(1)
What "note a." is saying is that as the ceiling joists are moved up further and further above the wall top plates the allowable rafter span is decreased, effectively meaning that you have to use larger rafters for a given span. Looking at the note, if the ceiling joists are moved up to 2/3 or greater of the roof height (thereby becoming collar ties) then the rafter span is reduced by 1/2. In the a. note they call it "rafter adjustment factor" and the number given is .50, meaning that you multiply the allowable rafter span by .5.
Example:
So, looking at the code table I referenced above (in this example I'm still using the first table and the a. note below) if you had a building that is 20' wide (rafter span of 10') and the ceiling joists are right on top of the wall cap plate, 16" OC, SPF #2 rafters, 2x6 rafters would be required. (the span for these 2x6 rafters in the table is 14-4).
If the ceiling joists are moved up 2/3 of the roof height (thereby becoming collar ties) the "rafter span adjustment factor" is .5, so now we need to find a rafter on the rafter table that satisfies a 20' rafter span. Looking at the same 16" OC, SPF #2 rafters, we have to go with 2x10 rafters. (the number given on the table is 22-3).
I'm going to make a EWAG that 2x10s are roughly 2x the cost of 2x6s. This matters when you are the one paying for the lumber.
Summary:
So, yes collar ties alone can be used to prevent wall spread, but the rafters have to be upsized significantly to compensate for the fact that there are no ceiling joists. This will add significant expense to the roof framing. The normal intended purpose of collar ties is different than that of ceiling joists (rafter ties) which prevent the walls from spreading. The intended purpose of collar ties is to hold the rafters together at the ridge.
Excellent post. Probably the best description I've seen yet of how collar ties work.
Be nice to your kids. They'll choose your nursing home.
Thanks much Boss! A compliment coming from you.
I think I've told you that I work with a few of your types on a regular basis - most of the stuff I build is trussed. Sometimes I feel like I work with some DAes but these guys are more refreshing... actually one is a girl....
Applause!
You get to teach fifth grade too now
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Actually I though t I was more on the 6th grade level... That was when I first started really noticein' "goils"... ;-)
This whole collar tie thing started in a few other recent threads (one by this thread's OP) - one about rafter spans and the other about a garage ceiling. I was waiting for you guys to jump in...
thank you.
Well, again, not trying to cause an argument here or saying one person is wrong or right. Let's say a rafter spans 11 ft horizontally. House width is 22 ft. If a collar ties are placed on every rafter pair in the upper 1/3 of this roof, wouldn't that then create a very strong triangle in the ridge? If so, It seems that the rafter lentgh within that triangle ( let's say 1ft 6"- coming from ridge down to center of collar tie) could never sag if the collar tie was appropriately sized. If that's the case, wouldn't the rafter span then be shortened from the wall top plate to center of collar tie, it would now be 9"6". I only write this becaue I have seen this exact same scenario in construction books.
BTW Matt, nice summary, but don't you mean that the collar ties have to be upsized ???
"If that's the case, wouldn't the rafter span then be shortened from the wall top plate to center of collar tie, it would now be 9"6"."
Not at all. A collar tie that's used as a rafter tie would actually add a lot of stress to a rafter.
That's why Matt was saying that it MIGHT work in some cases, but the rafters would have to be upsized.
"I see" said the blind man, as he picked up his hammer and saw.
In order to illustrate what happens when you put in a rafter tie or collar tie, I ran a couple of trusses.
They both have a 2X10 top chord and are a 24' span. Both have the same loading criteria.
It this first case, the bottom chord (or rafter tie) is 2' above the plate line. The tension in the bottom chord is 1714#.
The top chord (or rafter) is at about 80# of capacity. So this setup works O.K.
View Image
In this case, I moved the bottom chord (or collar tie) to 4' above the plate line. The tension in that member is now more than doubled, to 3,954#.
The top chord (or rafter) is at about 120# of capacity. So this setup is not viable. The lumber size would have to be increased or the collar tie lowered.
View Image
Keep in mind that you have to connect the collar tie to a rafter with enough nails to resist the forces created under full load. If you assume a 16D nail is worth abuot 100#, it would take 17 nails per joint in the first case.
Above all else: Sky.
Very cool Boss, now what if in pic #2,( with collar tie in upper) you also had ceiling joist running at plate level every 4 ft across ?? Wouldn't that make for an even stronger roof sysytem?
Add something at the ceiling level and you have a whole different ball game. But since the original point of this thread was to talk about collar ties, I'd rather skip that for now.
What do you call a boomerang that doesn't work?
A stick.
I understand, but that's my set up. Wouldn't it make it stronger??/ Just to satisfy my curiousity??
There are several things to consider here.If you put joists across at ceiling level, that will certainly keep the walls from spreading.If those joists are only on every other pair of rafters, you're depending on the roof diaphragm and/or the strength of the ridge board to hold up the other pairs.Collar ties will NOT make the rafters stronger or reduce their efective span.
Leaders go down in history -- some farther down than others.
Boss, just learning here. Doesn't code state that c.joists can be spaced every 4 ft??
What code?Keep in mind when you talk about a building code, there are a heck of a lot of different ones out there. I design trusses - Not stick framing. So I have no idea if any particular code says you can or can't put ceiling joists any particular way.
Q: Why can't Chinese Barbecue?
A: Because the rice falls through the grill
I'm still building on my Gambrel cabin/shed that 16'x32'. Rafters are 2x6 on 16" center, with a 2x4 glued flatwise (like a "T"), ring nailed and 3" screwed. Ridge beam is a doubled 2x6 with 2x6 blocking between the rafters at the ridge beam and at the floor plate.
The angle of the widest part of a gambrel roof is 22-1/2 degrees and 8' long. The center angle is (22-1/2+22-1/2 degree) 45 degrees and 68" long. Between every rafter, it is blocked, PL glued, and .131x3-1/4" ringed nailed to the ridge beam. Center angle joint was PL glued and held in place with Simpson 3x5 cleats on both sides and 1/2"-12"x24" plywood gussetts PL glued and nailed. The rafters are 10' high at the Ridge beam.
How many, what size, and where would you put the collar ties and still support a 2nd story ceiling that's tall enough?
Whew...Thanks,
Bill
As low as possible!
;)
question - are those rafters a T withthe added 2x4 on the top instead of the bottom?
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question - are those rafters a T withthe added 2x4 on the top instead of the bottom?
Piffin,
Here's a bit of history, first. Using culled lumber from those Lowes' cull carts, and to allow for those bows and cutting out good lumber from longer culled boards, I added a 2x4 on both sides of the wall studs.... The Hardi website recommends doubling the wall studs at the panel seams, so with the outside and inside 2x4, I have the equal of a 4x6 every four feet...
On the second story, I added only the top boards with all the adhesive, nails, and screws because I was running out of time as freezing rain was heading our way. To get the roofing crew to install the 5/8" decking and 30yr Architectural shingles before it hit, it was quite a challenge. I'll be adding another 2x4 to the bottom of the 2x6 rafters and the collar ties this week.
It's been in the 60's and low 70's the last few days, here in Texas, so I've been adding insulated windows and Hardiboard siding to the dormers as another bout with freezing rain and cold temps hit tonight. I'll be in the cold, but at least I'll be dry as I add the bottom of the I-beam rafters.... ;>)
Bill
Just a tip to throw out there. If you need them exposed, but want to disguise them. case them in wood. turns out nice
Ductape can fix EVERYTHING!!!
Sweet!
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Man you opened a can of worms by posting you were a truss designer. LOL
I design trusses, not stick framing. I know nothing about your codes or loading requirements. And you have other issues there, such as the dormer in your picture. So there isn't really anything that I can tell you.
A pessimist counting his blessings: 10 ... 9 ... 8 ... 7 ...
Doesn't code state that c.joists can be spaced every 4 ft??
Code states that collar ties can be applied to every other rafter pair.
Ceiling joists need to do two things, typically, depending on roof system:
carry live and dead loads
tie rafters together against out-thrust
If they are only carrying ceiling and attic storage loads, spacing doesn't matter as long as they (and the intervening sheathings) are sized appropriately
If they are acting as rafter ties, then they need to be on every rafter pair (16", 24" or 8' if it's a timber frame) unless their is a structural member (e.g. rafter plate) sized to take the thrust load between ties.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Boss this is off the subject, but not worth a new thread. I have been told that a 16d has a shear of 300#. Is this wrong, or maybe confused with a 16 common? Just courious, being a truss designer you might know.
"I have been told that a 16d has a shear of 300#."
I'm not an engineer, so I go with what I've learned from the engineers I work with.
The 300# number you heard may be an ULTIMATE number - the point at where a 16D will actually fail.
But most structures are designed with a 3X safety factor in mind. So if take you 300# number and divide by 3, you get the 100# number that I tossed out.
The number is not exactly 100# - It depends on several factors, such as the species of wood. It's just a rough working number I threw out to illustrate that a connection with a lot of force would need a lot of nails.
"Man you opened a can of worms by posting you were a truss designer."
Not really - I've hung out here for about 10 years, and the regulars all know who I am and what I do.
I know my limitations all too well, so I stick with what I know.
The bottom line remains: Force rules. Therefore, if you allow the government to have a monopoly on force, then you can never purport to be a free person. [Charley Reese]
The shear strength of a nail is irrelevant, unless you're nailing into steel.
It's the lateral strength of a particular nail in a particular wood.
For instance, a 16d common nail has a lateral strength of 80-120 lbs in SPF (depending on which standard you use).
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 2/15/2008 2:26 pm ET by Riversong
Can you please define the difference between the two types of strength. To me "lateral" means to the side and "shear" means to break or cut across.
Edited 2/15/2008 2:06 pm by dovetail97128
The shear strength of a nail is the amount of lateral force it can resist when it's clamped in a vise.
The lateral strength of a nail-in-wood connection (which is all that matters to us) is quite different and depends upon the density of the wood, the diameter of the nail, and the embedment length of the nail in the wood.
Standard numbers assume an embedment length of 11x the diameter. A 16d nail has a diameter of 0.162", and requires an embedment of 1.78".
View Image
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 2/15/2008 2:25 pm ET by Riversong
Understood, thank you
They can't get your Goat if you don't tell them where it is hidden.
Yes I understand, I was just curious as to what the shear strength was in simple terms, without variables. Not using info for anything in particular. The answer you gave was still informative. Thanks
This was the only formula I could find:
Problem:View Image Let's find the force necessary to shear a common 1" nail used to attach a hurricane clip to the rafter and header board. The nail is made of a low grade steel, with an elastic limit of about .3 GPa. The diameter of the nail is about 2 mm.
Using this formula and the above shear strength of soft steel, since a 16d common nail has a diameter of 0.162" (or 4mm), its shear strength would be approximately 850 lbs.
This is close to what I remember being told years ago, that a 16d common nail had a shear strength of 1000 lbs. Of course, it depends on the quality of the steel.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 2/16/2008 10:47 am ET by Riversong
My memory is the same as yours, 1000lbs, in shear. Someone earlier said 100, i8started to question my memory.
I was told 300# but that was obviously wrong. The guy that said 100# was factoring a 30% fail safe. But me thinks he was just throwing out numbers for an example. Rivers' post was real in depth I would be inclined to use his number. I once had a foreman who always used say " Just gotta do the math son"
now what if you also had ceiling joist running at plate level every 4 ft across ?? Wouldn't that make for an even stronger roof sysytem?
From your description of a "half vaulted" ceiling, it sounds as if the ceiling joists run from one side to the center bearing wall only. If this is the case, then they add nothing at all to the strength of the roof system.
Rafter ties need to do exactly what their name implies - tie the rafters together across the roof span to form a thrust triangle. The connection between the ties and the rafters and the connection at the center wall between tie and tie (ceiling joist lap or butt joint) are critical and often the weak link in any gable roof system.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 2/13/2008 3:15 pm ET by Riversong
Of course they tie into the opposing ceiling joists. Come on now. My interpretation is that ceiling joists can be spaced every 4 ft max , if there is no structural ridge beam supporting the rafters.
Better yet, why not have sort of a "W" looking thing between the ceiling joists and the rafters. Give it some special name--maybe even special nailer gizmos to hold it all together. See what Boss thinks about that! ;-)
Edited 2/15/2008 4:36 pm ET by Danno
Not only do collar ties not reduce rafter span (the total dead and live load of the roof system is the same), but the limiting factor in any tie preventing out-thrust is not the wood but the connection. And, as you can see from the chart, the lower the roof pitch, the more stress there is at the connection and the more fasterners are needed. Can you get 8 or 10 nails at each end of a 2x4 tie without splitting it?
View Image
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 2/13/2008 3:03 pm ET by Riversong
Edited 2/13/2008 3:05 pm ET by Riversong
Hey - I'm all about discussion - no argument here...
>> If a collar ties are placed on every rafter pair in the upper 1/3 of this roof, wouldn't that then create a very strong triangle in the ridge? << I'm not doubting that for one minute. I'm just trying to put into perspective what the intended use of collar ties is.
That's why in the other thread I said something like "Add some collar ties. If there already is some, add some more. " I'm all about whatever is easiest. It would take me longer to get out the compressor, hoses, and gun than it would take to install some collar ties (if the access was OK). In the possible over spanned situation like your were asking about in the other thread I wouldn't even be opposed to the attached sketch :-)
And BTW - yes - here code requires (depending on the situation) collar ties ever 4' and rafter ties (ceiling joists) every 4'.
COLLAR TIES DO ABSOLUTELY NOTHING TO STOP RAFTER SAG OR OUTWARD THRUST ON WALLS. The whole purpose of a collar tie is to stop the roof from blowing apart in a wind storm.
Very true by prescription or code. BUT!!!!
They can and do prevent outward thrust.....just need to be engineered properly.
The upper 1/3 or lower 1/3 rule of thumb for collars or ties covers the minimum design by code.
Lots of stuff works outside the code....cantilevers, point loads, shear walls etc. But bldg. codes can't address every situation by prescription........hence the necessary evil.. The know it all engineer ;-)
Great post Boats, only problem is each engineer that I spoke with gave me entirely different opinions!!!!!!!!!!!!!!!
Yes I agree. But having something engineered is not simplistic. If Framer were to build the structure in his drawing, he would have to present that drawing to the B.I. with the engineers stamp. Because it's outside code.
Edited 2/12/2008 6:23 pm ET by Bridge_Dog
Yep, the PE came out to look at my repairs and gave their stamped approval, so I guess it's ok, short rafter span and all. As I stated, they said the collar ties and large ceiling joists did help compensate for that. It looked like some homemade truss job that the owner then decided to cut out the bottom chord to vault the ceiling in L.Room. Engineer said Beam it or Tie it, so I tied it.
As long as the PE stamped it you have no worries.
Yes I agree. But having something engineered is not simplistic. If Framer were to build the structure in his drawing, he would have to present that drawing to the B.I. with the engineers stamp. Because it's outside code.
Everything I build is from Architectural or Engineered stamped drawings approved by the town and this type of roof is not outside of code. I frame this type of roof all the time.Joe Carola
absolutely. Unique designs call for unique engineering, not cut and paste work
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Ever get tired of going over this?I sure do....
I went to a restaurant that serves "breakfast at any time." So I ordered French Toast during the Renaissance.
does a fourth grade teacher ever tire of teaching the multiplication tables?
Follow your chosen destiny!
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"Does a fourth grade teacher ever tire of teaching the multiplication tables?"
I don't know. But I'm dating one, and I'll ask her this weekend
(-:
Age and wisdom don't always come together.
You didn't get the fuill lesson from your engineere teaching you. it is the stremngth of a structural riudge that supports the roof rafters in a cathedral ceiling. It is proper placement of the other elements that does the job other wise. doesn't matter how strong the individual elements are if they are not designed and placed to work together.
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I'll jump in here too, since I teach Engineering for the Home Builder.
There is much confusion between the function of high and low ties, so it behooves us to use different terms for each.
A tie, in general, is a tension element (whereas a strut is a compression element).
A collar tie is a tension tie in a gable roof placed in the upper 1/3 of the rafter rise. It's primary purpose, as some here have stated, is to prevent spreading of the rafters at the ridge due to pressure differential (which can also be caused by wind over the ridge - the Bernoulli effect that lifts an airplane wing). Collar ties were also used, however, to limit rafter sag. Struts and purlins were used for this purpose as well.
A rafter tie is a tension tie placed in the lower 1/3 of rafter rise, and it's primary purpose is to resist roof thrust. The ceiling joists in modern framing serve this purpose, which is why code requires solid fastening to the rafters.
View Image
Solar & Super-Insulated Healthy Homes
Edited 2/12/2008 7:09 pm ET by Riversong
Good description and drawing.
rafter ties/wall ties serve to tie the walls and prevent roof rafter/ridge sag and wall spread.
That is an entierly different purpose than that of collar ties which is to keep the roof from openning or hinging at the ridge in high wind uplift conditions
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