Collar Tie Alternative Idea

[TRAK.Structural]

I've been asked by one of my contractor clients about alternatives to collar ties for rafters in a space with a desired vaulted ceiling (residential construction). The contractor doesn't want to use the prescriptive minimum 1x4 wood framing due to loosing some ceiling height. The rafters are connected to the ridge beam with just nails, so I'm not inclined to try to justify this (even with more nails) because of the withdrawal loading that would occur in the case of wind uplift. I am however considering the possibility of using steel straps in the same manner as a conventional collar tie. They would be positioned directly below the ridge beam, oriented horizontal, and connect opposing rafters just like collar ties. Straps are convenient, cheap, and low profile so there would hardly be any loss of ceiling height if positioned right under the ridge. In my head this works if the straps are installed without much slack because this is purely a tension member. Has anyone done this before? Is this a bad idea?

 

[jerseyshore]

If the ceiling is vaulted you would have a structural ridge beam and hangers from the rafters to the ridge, eliminating the need for collar ties.

If you have have a partially vaulted ceiling (tray ceiling) or something similar that still utilizes a ridge nailer, the IRC permits the use of 20 Ga. ridge straps overtop of the rafter-rafter connections in lieu of collar ties. That's what I used all over my house.

 

[TRAK.Structural]

Jersey - I still think there is a horizontal component of the rafter-ridge connection force that has to be resolved even with a true structural ridge. Wind loads being perpendicular to the rafter slope, the connection at the ridge has to resist some amount of horizontal force wanting to pull the rafter away. Most of this is already built in my case and no hangers were used, just nails. And I forgot to mention that the sheathing and roofing is already installed so the ridge straps aren't an option.

 

[jerseyshore]

There is some horizontal component sure, but it's rarely considered.

How did you resolve the vertical component at the structural ridge without hangers?

I have used straps just below the ridge before in a retrofit type situation like you are describing, but that was with conventional roof framing and ridge nailers. But as long as they are tight they'll do the job. I still would add hangers though for any structural ridge beams (unless it's a dropped ridge beam).

In rafter-ridge repairs I'll specify DTT's, but that's overkill for new construction.

 

[XR250]

So basically you have a vaulted ceiling with no structural ridge.
Even code mandated collar ties are not intended to do what you are asking.
Try a hand calc on the model and see what you come up with.
Adding a structural ridge under what you have is the easiest thing to do.

 

[TRAK.Structural]

XR - Not sure how this situation qualifies as one without a structural ridge, can you explain?

There is a double ply beam at the ridge that will resist vertical loads from the rafter ends at the ridge. The low side of the rafters are supported by wood stud walls. I have sketched this up below, showing only one side of rafters for clarity. What I am seeing is that the horizontal component of reaction force at the top end of the rafter needs restraint somehow. My thought is that using a collar tie (or similar) will provide this restraint instead of relying on dowel type fasteners in withdrawal.

 

[phamENG]

Some form of restraint of the rafters at the ridge is required whether it's a ridge board or a ridge beam. The beam only negates the need for rafter ties. With no collar tie, your toe nails for the rafter into the ridge beam are in withdrawal under uplift loading - gross.

TRAK - your idea is not original, but a little off the mark. What you want are ridge straps. Check out the IRC - Figure R802.4.5 and section R802.4.6.

 

[ChorasDen]

I agree with Trak here, I think what most are missing is that the uplift is usually resolved by some minor capacity associated with a facemount/topmount hanger, but because there are no hangers on this project, and the joists are simply toe-nailed into the ridge, we are missing that uplift resisting component. Trak, what about A35's alongside the rafter to the ridge, is there room to install that? It's a much better connection than a toenailed connection anyway, for vertical loads, I would make sure to mirror it on top and bottom though, depending on rafter depth.

 

[XR250]

Ah, got it. It sounded like you did not have a structural ridge.
I have used the DTT1Z idea many times.

 

[phamENG]

XR - why? All you need is a strap over the ridge...

 

[XR250]

For retrofits where it already has shingles on it.

 

[ChorasDen]

All you need is a strap over the ridge..

I believe in one of the OP posts they indicated that the roof is already installed, so this is not an option, but OP will need to confirm.

 

[phamENG]

XR - touche.

ChorasDen - don't think they say that specifically, but re-reading it I can see that interpretation. So I suppose the DTT1Z tight to the ridge would be the best option. Though it's only going to gain you what - 2" of ceiling height? Is it really going to matter that much?

 

[jerseyshore]

I would still add the hangers unless maybe you're in a no snow region. I don't like relying on toenails for anything.

 

[XR250]

Around here (central NC), it is toenails all day long.
They seem sketchy, but I can't remember if I have ever seen a failure of a toe-nailed rafter connection and I have been doing this since 1991.

 

[jayrod12]

What about a simpson strong-tie SA connector? I think you could install that on the underside of the beam that would tie the two sides together.

 

[TRAK.Structural]

Thanks all for the input here! Going to pitch the options of the DTT1Z or Straps to tie the rafters together against uplift.

 

[kissymoose]

I'm confused, is this what you're proposing?

You're doing all this to save 2" of ceiling height at the ridge?

 

[jerseyshore]

Yeah this is a weird request to save 2 inches or so. Not sure what the sheetrock would attach to anyway if there are just straps instead of wood ties.

 

[TRAK.Structural]

Agreed, It's silly.

But sometimes a client has their mind set on what they want, and in this case it can be rationalized structurally so I'm happy to provide a solution for them.