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Old 09-05-2010, 05:06 PM   #1
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Adding Collar Ties to make a ceiling


Hi everyone,

We have a large room, about 36 feet wide inside, by about 12 tall, and we'd like to make a regular flat ceiling at 9', instead of a peaked ceiling with exposed collar ties. Now, in this room we had three dormers installed, each with double trimmer rafters and double header at top and bottom, with hangers on the top rafters to top header. The side of the room with the dormers has a 4+' knee wall, and under the dormers we doubled up the studs of the knee wall at the corners.

My first question is, is there any issue with adding the extra weight of the ceiling? The collar ties right now sit at about 9' already, although they are all of a slightly different height--some as much as 9.5' high. It is about in the upper 1/3 of the roof, so is that high enough not to be an issue with the extra weight on the existing rafters?

Secondly, one of the issues I have is the house wasn't framed perfectly. To me, and a few other carpenters who have checked it out, the birdsmouth on the main roof rafters is a bit small. So, I originally thought that adding the extra ties at 9' (I was planning 2x6 ties; 2x10's are what's there right now) would serve to prevent the roof from spreading even more, and therefore help make up for the original poor work. But a little research tells me that the collar ties are only for uplift at the ridge, not spreading; 1/3 height rafter ties serve that function. So my question is, is this really the case? Would adding extra collar ties up top (at 9' in a 12' ceiling or so) really do nothing in terms of further preventing the possibility of spreading?

Would the knee wall I have be helping all of this, though? Perhaps I should go and reinforce the whole knee wall as well, adding double studs throughout. Even though the knee wall doesn't land on the lead bearing I-beam in the garage below (it just sits on the 2x12 joists below), should I perhaps add some sort of pulins to the bottom plate of the knee wall (maybe with some strongback on top)--or would this serve no function really, if not attached to something directly bearing below?

Any advice is appreciated.

Thanks,
Joe

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Old 09-05-2010, 06:39 PM   #2
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Adding Collar Ties to make a ceiling


Page 39-40: http://books.google.com/books?id=iwS...joists&f=false

On heel and toe bearing rafters: http://books.google.com/books?id=iwS...rafter&f=false

Page 257 on struts and purlins: http://books.google.com/books?id=1fI...20ties&f=false

Rafter ties: http://myconco.com/ComEngProb.html

The fasteners at the rafter/tie joint are critical: http://www.engineersedge.com/civil_e...onnections.htm


Gary

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Old 09-06-2010, 11:51 AM   #3
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Adding Collar Ties to make a ceiling


Hi Gary,

Thanks for the information, I appreciate it. I found some of those links, but others I didn't, so they were helpful.

I guess what I have to figure out is at what point on the roof rafters will the ceiling joists/collar ties serve to prevent spreading/lift somewhat without increasing the load on the rafters. I think if I put it at 8', as a framer suggested, I'd be getting too close to the center span. Just to note, the inside is about a 12' ceiling (pointed) right now, but the whole gable roof side, ridge to the gutter, is about 18'. So maybe 8' is not too low. I think I'd rather have the height at 9', though.

Maybe what I'll do is make it at 9', which shouldn't increase the load at that point. As far as spreading, then I'll maybe double the studs all along the knee wall that's there. The I-beam in the garage doesn't fall directly below, but the floor joists are 2x12, so it should be able to take the indirect load. What do you think? If this can take the bearing, I could also add some struts inside the wall, and then I don't have to worry about trying to do much tying with the ceiling joists.

Also, regarding the ceiling joist connections, would you recommend adding a few screws in addition to the nails?

Thanks,

Joe
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Old 09-06-2010, 12:44 PM   #4
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Adding Collar Ties to make a ceiling


Collar ties lower than the upper 1/3 of the roof height will not help with wall spreading. (They only keep the ridge together).
Here is the info from: http://myconco.com/ComEngProb.html
"Rafter ties above the lower 1/3 will not stop the walls from spreading and add extra weight to the rafters.
Many builders confuse collar ties with rafter ties. Both are horizontal framing members that connect rafters, but that's where the similarities end. Collar ties (which are required by the Southern Building Code and no other) function to resist the pressures of wind uplift on a roof by holding the rafters together where they meet the ridge. As high up as they are, collar ties have no leverage to prevent the rafters and walls from spreading outward. That job is best done by the ceiling joists. The wrong and the rights of rafter tiesTo prevent roof loads from spreading the walls outward, rafter ties (or ceiling joists) must be in the lower third of the roof pitch. Collar ties are too high to keep walls from spreading and instead serve to resist uplift by holding the rafter together at the ridge.
If there are no ceiling joists or if the joists run perpendicular to the rafters, then the code requires rafter ties. Similar to a ceiling joist, a rafter tie is typically a 2x4 that runs parallel to the rafters, from outside wall to outside wall, and ties the rafters together as close to the top plate as possible. Rafter ties need to be installed every 4 ft. down the length of the roof.
Rafter ties do not have to be at ceiling height to be effective, but they must not be placed any higher than the lower third of the roof pitch. In other words: Measure vertically from the outside wall's top plate to the bottom of the ridge, and place the rafter ties within the lower third of that measurement. Once they get above that point, they lose their most effective leverage.
I've seen builders compound their mistakes when they try to use rafter ties as ceiling joists in semivaulted ceilings. For maximum headroom or aesthetic balance, they place the rafter ties halfway up the roof pitch, near the center of the rafter span where they're too high to be an effective tie. Applying the insulation and the drywall greatly increases the load on the rafters at their most critical point: midspan (what engineers call the maximum bending moment). This added load can cause the rafters to sag, pulling the ridge down and also pushing the exterior walls outward.
To avoid this problem, you'd need to engineer the rafters to carry the point load created by the additional weight being placed on them. You'd also need to design a ridge beam capable of supporting the roof load, just as you would if it were a cathedral ceiling, which essentially it is."


The knee wall should be supported below on a wall or beam to transfer the rafter dormer loads down to the earth.


Could you post a picture please?


Just measure the height of the rafters, vertically, from top plate to peak, not on the slope.



Gary
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Old 09-06-2010, 01:36 PM   #5
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Adding Collar Ties to make a ceiling


Like GBR said make sure the rafter ties are in the lower third.. take a look at my garage. It was sagging badly, the ridge beam was 2 pieces and was separating and the rafters were being pulled out of the ridge beam and the walls were separating. Not pictured was a partial loft that was built that was attached to the roof near the ridge beam then tied into 2 walls in the corner, the loft almost extended halfway out to the garage, BUT, there was no support to ground so it was pulling mostly at the center of the roof.

You can see the labeled pieces in the pic

#1 there were 3 of these up there before I took 2 down, basically collar ties, I guess were added after the loft by prvious owner to try and stop the separation

#2 I added these 16 ft 2x4's in the lower third as rafter ties to help stop the walls from separating

#3 The original "rafter ties" built with he house in '64. They are halfway up, NOT in he lower third, and thus didn't help as much to stop the separating

#4 A cable I installed to help pull the walls together before I installed my rafter ties.

Before I jacked up the roof and cabled the walls I bet there was almost 3/4" inch separation at the 2 ridge beam pieces. After adding my rafter ties and cable its holding well for past 2 years.
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Old 09-06-2010, 03:35 PM   #6
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Adding Collar Ties to make a ceiling


Thanks for the responses, guys. Yeah, Gary I read that information on the site. I understand what they're saying about the important differnce in funciton between the collar and rafter ties. I can't install rafter ties here, because it's a finished room I'm dealing with. Plus it shouldn't call for it; they were built without them to begin with. The floor joists here are parallel to the rafters, and are 2x12s, so I think the walls should be okay in terms of not separating. I'm just trying to make up for some poor work here and there (such as the smaller birdsmouthes) by stiffening things up a bit. I'm not evevn sure it's really necessary to do so, but I'd like to to play it safe, while I have the room taken apart.

As far as the vertical height, I believe it is about 13 or so feet from top plate to bottom of the ridge.

Maybe I can take some pictures and post them later.

Regarding the knee wall, yeah I know it should be going straight down to bearing and to the ground, but the I-beam below is a few feet away across a finished floor, so I can't do it. I was just wondering if it would serve some purpose to reinforce the knee wall here and there, since the floor joists are pretty large.

Joe
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Old 09-06-2010, 04:22 PM   #7
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Adding Collar Ties to make a ceiling


The collar ties are not required if the ridge is structural, unless locally. Different types of connections with shear hangers better than any for lateral shear forces: http://books.google.com/books?id=E5S...raming&f=false

So, 36', is that wide or long? Rafter span? Beam size and span?



Gary
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Old 09-06-2010, 04:54 PM   #8
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Adding Collar Ties to make a ceiling


No, the ridge is not structural. Sorry, the length of the whole room inside is about 36-37', and the outside length (wall thickness and eave overhang) is about 39'. The rafter span is about 23-24', which makes the height about 14', given the 14/12 roof pitch I have. From inside, the floor to ceiling height is about 13). And again the rafters themselves are about 18 feet long.

The wall distance between the rafters, if the collar ties are placed at 9' high, is about 6' 4". The ties I plan to use are 8' long; they'll lap the sides of the existing rafters about a foot. So I guess that makes the span roughly 8' And I plan on using 2x6's, 16 OC. But the existing ties are 2x10s, so there will be 6 2x10's among the other 2x6's.

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Old 09-06-2010, 05:54 PM   #9
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Adding Collar Ties to make a ceiling


The knee wall is not structural. The collar ties may be required as your ridge is non-structural.

The rafter ties are required and should be in the bottom 1/3 above the top plates rafter bearing. Any higher up and you should consult a Structural Engineer for safety. I really want to see a picture of this.......

Gary
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Old 09-06-2010, 06:06 PM   #10
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Adding Collar Ties to make a ceiling


Okay Gary, I'm sending my photographer (brother) out now, ha.

I think I am forgetting to note one factor, which is probably important. The 39' or so roof length is only on one side, the other gable side has a large other gable section running into it perpendicularly. So one side is a full 39' length, where as the other side only has gable ends of about 10' on either end. The short gable sections there have no knee walls, and only go directly to the floors the wall. The middle section where the other part of the house runs into has what seems to be a large header carrying the load down to the I-beam in the garage.

I'll get those pictures right now, though.

Joe
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Old 09-06-2010, 10:18 PM   #11
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Adding Collar Ties to make a ceiling


Okay, here are the pictures under a photo album. There are some of the outside of the gable section of the house, from underneath in the garage (of the I-beam, etc.), of the dormers, and the whole room itself.

I appreciate any advice.

Thanks,

Joe
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Old 09-06-2010, 10:23 PM   #12
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Adding Collar Ties to make a ceiling


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Old 09-06-2010, 10:34 PM   #13
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Adding Collar Ties to make a ceiling


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Old 09-06-2010, 10:36 PM   #14
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Old 09-06-2010, 10:37 PM   #15
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