Hip Roofs & Large Dormers 2

[jerseyshore]

Normally when I design roof framing for a house it's all structural ridge/hip/valleys. If it was simple, they wouldn't be hiring an engineer (generally).

Example below of this house with a tradition 6/12 sloped main roof, 46 ft deep front to back, and a matching 6/12 46 ft wide hip roof off the back, flanked by a couple of dormers on either side. Attic floor joists run front to back as well.

Around here most architects do their own structural work and majority of the time it's typical gable-type framing with ridge & valley nailers, not beams, for a roof just like in this example. They wouldn't even think twice to do it that way, even if it doesn't work on paper. Then they try to make me feel like the crazy one for suggesting structural ridges.

So what rules of thumb do you guys use for determining when hips or dormers become too big to work without structural beams?

 

[TheDW]

I always wonder how the ridge is supposed to hold up a valley beam if it's just a 2x nailer and not a beam itself.

If I'm understanding you correctly, we don't have ridge plates supporting valleys. I always show post support at the ends of valley beams

 

[phamENG]

Out of curiosity, why does an internal support negate the use of ties?

It doesn't, really. I just use that as a sort of litmus test. If you need to break your rafter span, the loads are likely getting high enough that I may not want to trust in the magic of alternate load paths through a diaphragm compression strut. It's usually easier to switch to beams at that point, as any fancy blocking or strapping to account for higher loads is likely to be ignored by the framer.

but do other people use nailers for ridges and hips, but beams for valleys?

Yes. As TheDW mentioned, you have to put in a discreet support for the high end of the valley, but as long as you do the rest of the roof can be traditionally framed. The valley is holding up the rafters and ridge at that point, not the other way around. (Of course, you have to be careful sizing the valley beam in this case - some of the free software out there (looking at you, Forte) doesn't have an option to assume there's no ridge beam and would under size the beam if you're not paying attention.

 

[kissymoose]

diaphragm compression strut

ah, you're talking about taking the thrust of the rafter into the gyp ceiling through the ceiling joist? As opposed to taking the ceiling joist to the rafter on the opposite side of the roof, "tying" them together with the ceiling joist in tension.

 

[phamENG]

No, I never consider tension through the gyp to be sufficient, though that is probably an important part of the reality of these things in a lot of cases (especially if you don't have discreet ties for the hip). I'm just talking about the point at which I start to pay a lot more attention. If you have simple span rafters and a regular shaped roof, there's nothing really to engineer. As long as the rafter and CJ are sized right and there are enough nails at the heel joint, the roof isn't going anywhere. (Collar ties are also helpful.) But as the roof grows larger or you get some irregularities in the geometry, it needs a closer look and usually some beefed up connection somewhere. I'm often leery of doing 'custom' connections in situations that seem "normal." Most framers will ignore the custom connection and just do what they always do. So if it does need that, then I'd rather just switch to a ridge beam. It also gets the job done and is easier to inspect and ensure things were installed properly. And by making a sufficient divergence form the "norm" the framers are more apt to actually do what is on the drawing.

 

[jerseyshore]

Okay, no that makes sense; I saw an architect's plan recently that had a double 14" LVLs for a valley up to a single ridge nailer (no post) and was wondering how that is going to fare long term. Didn't know if that was a new trend I was missing, unsupported beam ends

pham, what is the issue with Forte with valleys?

 

[phamENG]

Forte assumes you'll have a ridge beam, so if you just put in a ridge board it'll underestimate the load on the valley. As a free program, there's no toggle to switch. I'm not complaining, just pointing out a possible issue since I know how popular it is.

 

[jerseyshore]

Good tip. I use Forte everyday and have designed hundreds of projects with it, but don't think I've ever used the hip/valley beam module so I'll keep that in mind if this scenario comes up, thanks!