How high do collar ties need to be?

[Nathan W. Swilling]

We're refinishing an attc of a home built in 1915. It has a front gabled roof, that is constructed of 2X6's (real 2X6's, not 2X5.5's), and planked sheathing underneath a slate roof. It currently has 1X6 collar ties on every roof rafter, which are about 20" apart.

The idea is to put in new collar ties that would double as ceiling joists (there will also be knee walls at 5' high per code). My question is, how high can the new collar ties be? The current collar ties around about 7 feet off the floor. I would think by using a stronger material, we could move the collar ties up a little bit and still maintain structural integrity. But, how high? Can we go up to 9 feet? Only 8.5 feet? How do we figure out what's possible?

[Larry D. Kage]

A little engineering wouldn't hurt...

[Richard A. Hetzel]

The rule of thumb I was taught is that the collar ties should be no higher than the upper third of the height between attic floor and ridge. Any higher and you take the chance that they won't funtion to keep the outer walls from spreading. The rafters may bend and allow the walls to move, and the ridge may in turn sag. Stay at the upper third or lower.

If the knee walls are made load-bearing, perhaps you could cheat a bit on the height, but that introduces another structural problem...what would support the weight on the knee walls? The ceiling joists are probably barely adequate to become floor joists, if at all, and probably can't carry the concentrated load from load-bearing knee walls without deflecting excessively.

One way to eliminate the collars strucutrally is to install a load-bearing ridge beam, but again, that introduces other issues, with large concentrated loads coming down in random places with nothing below to carry them.

There are ways to do it, but they all require some engineering.

[Nathan W. Swilling]

Another issue is that we'll be subdividing the space with some walls that run perpendicular to the collar ties. So, for a good portion of the space, we can run a lower "collar tie" that wouldn't be much more than 7 feet high. So, can we "cheat" a little with the collar ties/ceiling joists in the other sections?

For example:

Wall - exterior
8' - small room
Wall
3' - stairway
Wall
6' - bathroom
Wall
2' Closet
Wall
12' room
Wall
12' - room
Wall - exterior

[Nathan W. Swilling]

Here is a picture

[Nathan W. Swilling]

A few more pictures

[Kenton H. Shepard, CMI]

Rafter ties- keep walls from spreading. ceiling joists often serve as rafter ties. The lower in the roof they are, the more effective they are. Typically nailed to the base of opposing rafters w/5- 16d through each lap and toe-nailed to the top plate.

Collar ties- go in the upper third of the roof and prevent uplift. Typically installed every other rafter when rafters are installed on 24" centers. Often 1x6 is spec'ed, typically nailed w5- 8d through each lap.

See Larry's graphic in post #2

[Nathan W. Swilling]

So, do I have both - one piece of wood acting as both the rafter tie and the collar tie? The ties are right now about 7' off the ground, which is just over the midpoint (the roof is probably about 12' at its highest, and the floor is about 25' wide).

[bzimbelman]

If it was my house, I'd spend the $100 or so and get an engineer or architect to tell me the best way to raise the boards without giving up structural integrity.

[Nathan W. Swilling]

I wish it was $100 where I live...

[Erby Crofutt]

It's just a semantics mixup, Nathan.

Collar ties are high in the rafters to prevent them from uplifting. They do not prevent wall spread at the bottom of the attic. That is the job of:

Rafter ties are at the bottom of the rafters and are used to prevent wall spreading.

From what I understand, you now want to use those upper "collar ties" to put on the ceiling drywall for the finished attic room. As long as they are in the upper third, they are still doing their job of preventing rafter uplift. So, how high do you want your attic ceiling.

The existing "rafter ties" will be the floor of the room you're building in the attic.

[Christopher Currins]

Originally Posted by Nathan W. Swilling

So, do I have both - one piece of wood acting as both the rafter tie and the collar tie? The ties are right now about 7' off the ground, which is just over the midpoint (the roof is probably about 12' at its highest, and the floor is about 25' wide).

That's not possible, read Kenton's definition again.

[Nathan W. Swilling]

Collar ties have to be in the upper third to be effective, but is there any limit on high they can be? For example, if the ceiling is 12' high, the collar tie shouldn't be lower than 8 feet. But, is there any reason why they can't be 9 feet high?