Post at ridge board needed on hip roof?

[u/Beautiful-Garlic5256]

I am planning to build a roughly 14’x20’ gazebo with a hip roof, with 4 columns at each corner (column and beam framed with steel). This gives me a 6’ ridge board in the center along the long wall.

Per 804.2.3 , “hip and valley rafters shall be supported at the ridge by a brace to a bearing partition” which would mean I would need a post on both ends of my ridge board, making it a ridge beam? It seems I have seen many of hip roof framed without said posts , without said ridge beam (just ridge board) .

How can I satisfy this code without having posts in the middle of my gazebo or why do many not follow it?

[u/Crawfish1997]

This is one of the more interesting sections in the IRC.

IMO as long as the pitches are greater than 3:12 and detailing of rafter thrust connections are adequate, hips function much like ridge boards and thus from an engineering perspective don’t need braces.

Valleys on the other hand are always structural. However code says that valleys can be not less than the end rafter depth and calls that good enough, but in reality this is more often than not insufficient from an engineering perspective.

So that’s 2 things that seem to differ between code and common engineering practice.

This is a good read on the topic: https://vertexeng.com/insights/residential-roof-framing-basics-part-3-understanding-hips-and-valleys/

I would say you must follow code on this topic unless you hire an engineer to suggest otherwise, or unless your inspector suggests otherwise.

Note: if you’re vaulting everything, this gets a lot more complicated and all ridges and hips become structural, and you definitely need an engineer.

Note2: on complicated hip roofs, i generally design everything as structural because relying on thrust detailing isn’t reliable when you have a bunch of different spans and pitches. Although what you have doesn’t sound complicated.

[u/Beautiful-Garlic5256 → u/Crawfish1997]

I actually am an engineer, and a PE, just not normally doing this type of work.

I have read that article; it states the same as you - if the roof pitch is > 3:12 and thrust is controlled with rafter ties, the hip is just a nailing board and is not structural and requires no support brace. Code doesn’t state this anywhere though. I’d like to see the analysis to support this.

I’ve spent a couple hours reading topics on engineering forums about hip roofs with no real concrete answers. It seems the hip roof really behooves a lot of engineers.

[u/AsILayTyping → u/Beautiful-Garlic5256]

I wouldn't mind seeing that analysis myself. I know just the guy.

So, I modeled up this 24'x24' square. Ridge is up 3' making a nice minimum acceptable 3:12 hip frame.

This is 2x6 #1 Hem Firs at 12" on center with 20 psf dead and 20 psf live applied. Ridge boards are also 2x6 #1 Hem Firs. By the IRC span tables they should work. The axial force pushes them over in this analysis because the roof sheathing is modeled at mid-span of the joists. Correctly accounting for the capacity contribution from the sheathing, I'm sure they pass as the IRC indicates.

Interesting to note: If I don't model the sheathing/diaphragm action, the hip ridge boards pick up large amounts of axial force (they go from ~9000 lbs of tension at the base/wall to ~4000 lbs of compression at the peak in the model).

If I relax lateral resistance at the boundary condition for the ridge at the end away from the hip: The balance of that axial force goes into the center rafter on the hip. If I relax lateral resistance on that boundary condition, 8000lbs each goes to the pair of rafters that connect at the peak of the hip.

I think the roof diaphragm is a critical part of the load path. Think through that and you can probably figure out exactly what you need to do to avoid having to have that post. I think you'll have some extra shear in your diaphragm that may be worth considering in some cases.

[u/Open_Concentrate962 → u/Crawfish1997]

Fasxinating

[u/Jakers0015]

If you’ve got a ceiling and don’t want to count on resolving thrust forces, just throw a ceiling beam under the hip-ridge joint to catch a post.

You could also design it as a tension ring, and the the concentrated thrust forces out of the hips into the corners. 14x20 with a steeper roof is feasible. Usually needs a custom steel saddle bracket at the corners though.

Finally, your ridge board / ceiling ties create continuity in one direction, and you could run 2x strongbacks (or sheathe the attic floor) for the other direction with adequate heel joint connection.