Ridge board (not beam) w/o rafter ties

[Bammer25]

I am trying to talk my contractor friend out of doing this. His client wants the high ceilings but the span is quite long for the ridge. They do not want to incur the costs of the large LVL which would be required to eliminate the ceiling joists/rafter ties.

Has anyone had any experience with this? Are there special connections that can overcome potential wall thrust?

 

[hokie66]

Scissor trusses, perhaps.

 

[jayrod12]

Perhaps talking them into discrete tension ties spaced out along the space. I did one where we provided steel channels at roughly 10 feet o/c, designed the eave as a plywood box beam spanning between the channels. The architect ended up using the channels for hiding the conduit and hanging the lighting for the area. Worked a peach in the end.

 

[phamENG]

The LVL is likely the least expensive option. Scissor trusses can work, but they tend to be expensive, too, have long lead times in some areas right now, and you end up with less vaulted height than you would have with a ridge beam.

They could compromise on the height and put their rafter ties in the at the h/3 mark. You need more nails everywhere, but it gives you some extra height.

If there are perpendicular walls (assuming they'd need to be on the exterior of this room), you could use those walls to brace it and design the top plate as a beam resisting the thrust. This can be tough to do, though.

You could also put one of these in every 4 feet or so, though I doubt it'll be any cheaper than an LVL: (you're welcome, XR250)

 

[XR250]

What is the ridge span? As Pham described, the LVL is likely the best option.

 

[Bammer25]

Thanks guys. About what I expected

 

[BridgeSmith]

There are also some options for tension ties that present a minimal visual interruption - rods, cables, etc.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[IceNine]

If you want to get really cute, you can design the roof as a folded plate with all of the thrust delivered by the plywood diaphragm to the end walls, or tension ties at the ends of the roof.
There is a great paper from the 1970s by the American Plywood Association called 'Report 121 - Plywood Folded Plates, Design and Details' that details how to do this, with examples.
It can be hard to get to work with lower slopes.

 

[XR250]

If you want to get really cute, you can design the roof as a folded plate with all of the thrust delivered by the plywood diaphragm to the end walls, or tension ties at the ends of the roof.
There is a great paper from the 1970s by the American Plywood Association called 'Report 121 - Plywood Folded Plates, Design and Details' that details how to do this, with examples.
It can be hard to get to work with lower slopes.

I can't imagine it ever getting built correctly. I have not looked at the paper but do they account for long term deflection due to nail slip etc?

 

[phamENG]

Neat. I look forward to reading that someday. I found it in case anyone else wants it - link below.

 

[IceNine]

I can't imagine it ever getting built correctly. I have not looked at the paper but do they account for long term deflection due to nail slip etc?

There is a deflection equation that includes consideration for nail slip.
It's definitely a system that requires careful detailing and QC during construction.
I've used it once, with a relatively short span, 10+ years ago. So far so good.

 

[phamENG]

IceNine - was it in North Carolina? If so, can you share where? I wouldn't mind driving by it sometime if I'm in the neighborhood. I've never heard of anyone using that system intentionally before. It's usually only used to explain why something is standing when other analysis methods suggest it's unstable.

 

[IceNine]

phamENG-
Yes, NC. Unfortunately it's in a gated community.

 

[IceNine]

I've attached a google earth shot. Multiple folds with no interior columns, ridge beams, or tension ties (except at the ends of the gables)

 

[phamENG]

Shucks. Thanks for sharing the Google Earth shot. Looks pretty neat.

 

[manstrom]

It is possible on paper to treat the panels of the roof like a plywood box beam. But there is a lot of thrust to account for and tricky calcs that will make your head hurt.

I had a contractor ask me to do a pavilion with no collar ties or ceiling (16'x24' hip roof). I could make it work in tekla, but the thrust loads were huge (5000#) requiring custom steel to make all the connections work. We ended up adding collar ties. In your case, I would push scissor trusses.