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JoeT 11-09-2010 09:41 PM

Collar Tie for attic rafters
 
It's been suggested that I place 'collar ties' in my attic rafter to help strengthen an under built roof structure. I could either use two 1X6's on either side of the rafter or one 2X6 that would be attached to just one side. I suppose I could also use to 2X6s on both sides instead of 1X6s but that would be overdoing it. I it think the two 1X6s are better because the opposing force would be evenly applied if the rafters are trying to spread out. Where a 2X6 on one side seems like the opposing force would be twisting / torquing on just one side. Is there any obvious reason to go with one way vs. the other ? The one 2X6 would be less expensive but I'm more concerned about what is stronger.

Thanks.

http://s114920633.onlinehome.us/misc/two.png

jlhaslip 11-09-2010 10:06 PM

Collar Ties are used to keep the Rafters together at the top end only. They are not to be used to resist rafter foot spread.
Collar ties are in the top 1/3 of the rafter. Using single 2x6's would be enough, I would think. Without the details, we can't say for certain.

Ceiling joists are used as a Tie Beam at the bottom of the rafter or top of the wall to resist rafter foot spread.

Just Bill 11-10-2010 05:53 AM

Why do you say you have "an under built roof"?? Collar ties are usually not on every rafter, but every 3rd or 4th rafter.

Daniel Holzman 11-10-2010 08:13 AM

Jhaslip is correct, the function of collar ties is to tie the rafters together near the top, primarily to equalize uplift pressure due to unbalanced wind loading. Collar ties ARE NOT INTENDED, nor do they function, to increase the strength of a roof constructed of undersized joists. If in fact your joists are undersized, use of collar ties will not correct the problem.

If your walls are spreading, use of collar ties will not correct the problem. If the ridge is sagging, collar ties will not correct the problem.

All that said, it does not matter if you use a pair of 1x6 boards on either side, a single 1x6 on one side, or a 2x6 on one side, they will all perform the limited function that collar ties are capable of performing. Make sure you use adequate nailing to keep them in place.

JoeT 11-10-2010 07:58 PM

Thank you for your replies.

A structural engineer recommend collar ties. I assumed this was to prevent the problem of wall spread. It's difficult to say what is going on in my house. Some cracks are appearing in walls and ceilings. They grow slowly. It's as though the foundation is settling. But it's pretty odd that this would start happening 60 years after the house was built. A contributing factor could be having three layers of roofing. But that was torn off last year and I now have a much lighter new roof. Still the cracks seem to be growing. The house is a ranch.

The roof construction has ceiling joists that are perpendicular to the rafters. So the joists do not create the bottom of a triangle with the rafters. To prevent wall spreading force their are flat 2X4s nailed across the joists which are in line with the rafters but not attached. The rafters and joists are all 2X6 with the exception of the area where the two roofs come together (i.e. the valley ?) which is 2X8. There are no trusses. There are vertical 2X4s placed every 10 feet or so which help hold up the roof.

One radical idea I heard was to somehow install trusses in the attic and then forget about ever using the attic for storage again.

Here is a partial diagram I created when I had too much spare time. The red lines indicate where the cracks are. The red square is a closet which has cracks in ceiling and in the corners where the walls come together. The green line represents where the structural engineer recommended installing another 16' long 2X4 horizontally to help prevent the wall spreading force.
http://s114920633.onlinehome.us/misc/attic.png

Wildie 11-13-2010 05:18 PM

I would go along with the engineers suggestion, to a certain point.
Only, I think that the rafters need to be tied together, birds mouth to birds mouth.
This may require using 2X? material, installed on edge and lap bolted together in the middle.
If the ties don't travel all the way across, the load will pull the ceiling joists laterally.

JoeT 11-14-2010 01:22 PM

Not being a carpenter I get confused easily by the terminology. But I can understand a picture.

Here is what I think Wilde is suggesting:
http://s114920633.onlinehome.us/misc/rafter7.png

Where the rafter is cut so the tie can fit in:
http://s114920633.onlinehome.us/misc/rafter8.png

and the tie is then attached to the rafter doing something like this:
http://s114920633.onlinehome.us/misc/rafter9.png

Did I get any of this right ? (more or less)

jlhaslip 11-14-2010 02:31 PM

Tie Beams are the same as collar ties, but placed low on the rafters.
As close to the bird's mouth as possible. They stop the rafter foot from pushing out the wall and causing the drywall cracks.

Daniel Holzman 11-14-2010 04:05 PM

Help me out here. You (hired?) a structural engineer, who made specific recommendations for repair. Are you now questioning those recommendations, or did you implement them and they are not working?

Based on your drawing, there are no rafter ties in the structure, except apparently for a few 2x4's running perpendicular to the joists. Apparently there are some vertical 2x4's as well, which may or may not be properly attached to the ridge or the floor. This is unusual framing, to say the least. I believe I would have that structural engineer back to prepare a comprehensive report on the exact cause of your trouble, and have him prepare a plan to repair the problem. Or if you don't like that structural engineer, get another one.

Wildie 11-14-2010 10:58 PM

Quote:

Originally Posted by JoeT (Post 534153)
Not being a carpenter I get confused easily by the terminology. But I can understand a picture.

http://s114920633.onlinehome.us/misc/rafter9.png

Did I get any of this right ? (more or less)

Nice pics Joe, but the rafters need to tied together, as low as is possible.
Ties near the ridge will prevent the roof from wind lift, but they must be in the lower third to prevent the walls from being spread.
In my area, ties between rafters are considered to be collar ties. They are usually installed on 4 foot centers.
Ceiling joists can perform as collar ties, but not in this case where the joists are perpendicular.

Gary in WA 11-15-2010 02:31 PM

More to chew on: http://books.google.com/books?id=1fI...20ties&f=false

http://www.finehomebuilding.com/how-...llar-ties.aspx

Bermuda Roof Repair - Powered by Google Docs

thesda.doc - Powered by Google Docs

FHWA-HRT-04-098-Chapter 6. Ancillary Features-Covered Bridge Manual-APRIL 2005

The Development Of The English Timber Roof. Part 6

As mentioned, the rafter tie needs to connect at the lowest part of the opposing rafters (just over the ceiling joists), nailed well: http://myconco.com/ComEngProb.html

Common Nails Required for Rafter Ceiling Joist Heel Joint Connections - Engineers Edge

Gary

Wildie 11-15-2010 04:40 PM

That CONCO site was really informative! I thought that this paragraph was perticularly, pertinent.

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.

JoeT 11-15-2010 08:04 PM

Thanks again for the comments and thanks GBR for all the links.

Quote:

Help me out here. You (hired?) a structural engineer, who made specific recommendations for repair. Are you now questioning those recommendations, or did you implement them and they are not working?
Yes I hired a structural engineer. Actually two of them. Why two ? Mainly because I wanted a second opinion. Why ? Because (1) I'm not too crazy about permanently taking away from having my attic as an open area where there is lots of storage space, and (2) the first engineer didn't come across as being very knowledgeable about home construction, and (3) I really wanted to know what the root cause of the problem.
Quote:

I believe I would have that structural engineer back to prepare a comprehensive report on the exact cause of your trouble, and have him prepare a plan to repair the problem. Or if you don't like that structural engineer, get another one.
Engineer #1 totally missed some pretty obvious areas of concern that jumped right out for engineer #2. Engineer #2 needed to come back for a second visit after I pointed out that the location of the one of the main problem area was not properly identified. Engineer #1 and #2 both recommended collar ties that that were of the type that are located below the rafter peak. #1 recommended using 2X6s where #2 thought 1X6s would do. These guys are not supernatural. They can't magically see where all the stresses are in a structure. They can only take a best guess. In an older home where settling has occurred I'm guessing it's even harder to really figure out what is going on. I have already implemented some of the suggestions from their reports but will avoid going into details (unless someone really wants to know).

The last remaining item to install is some type of ties. I'll hire a pro to install them if that is something an amateur could easily mess up (but after all the $$ I spent so far on engineers I need to be sure it's really necessary!).

Quote:

This is unusual framing, to say the least.
Pretty much what engineer #2 said.

Quote:

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.
Maybe my plan should be to first install the upper ties and wait a few years. Hopefully the problem will be solved. But if it still persists then install the lower and wider version of 'rafter ties'?
http://s114920633.onlinehome.us/misc/attic10.png

Wildie 11-15-2010 10:26 PM

Quote:

Originally Posted by JoeT (Post 534978)
Thanks again for the comments and thanks GBR for all the links.

Yes I hired a structural engineer. Actually two of them. Why two ? Mainly because I wanted a second opinion. Why ? Because (1) I'm not too crazy about permanently taking away from having my attic as an open area where there is lots of storage space, and (2) the first engineer didn't come across as being very knowledgeable about home construction, and (3) I really wanted to know what the root cause of the problem. Engineer #1 totally missed some pretty obvious areas of concern that jumped right out for engineer #2. Engineer #2 needed to come back for a second visit after I pointed out that the location of the one of the main problem area was not properly identified. Engineer #1 and #2 both recommended collar ties that that were of the type that are located below the rafter peak. #1 recommended using 2X6s where #2 thought 1X6s would do. These guys are not supernatural. They can't magically see where all the stresses are in a structure. They can only take a best guess. In an older home where settling has occurred I'm guessing it's even harder to really figure out what is going on. I have already implemented some of the suggestions from their reports but will avoid going into details (unless someone really wants to know).

The last remaining item to install is some type of ties. I'll hire a pro to install them if that is something an amateur could easily mess up (but after all the $$ I spent so far on engineers I need to be sure it's really necessary!).

Pretty much what engineer #2 said.

Maybe my plan should be to first install the upper ties and wait a few years. Hopefully the problem will be solved. But if it still persists then install the lower and wider version of 'rafter ties'?
http://s114920633.onlinehome.us/misc/attic10.png

I would install the 'rafter' ties first. To my mind there's only a 5% chance that collar ties will work and a 95% that the rafter ties will.
If the collar ties don't work, the damage may be irreversable.
Everybody seems to agree that ties need to be added. Why not add both sets and set your mind at ease!

albanello 11-17-2010 12:18 PM

Rafter ties shown in post #14 need to be connected at the middle ! And I am assuming there will be many more... than one.


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