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Discussion Starter · #1 ·
I'm doing an addition on a house that involves a cross gable configuration. Due to the span, the roof framing over the main part of the room is using 2x10 rafters. My question is. On the Valley Jack rafters, the span is much less and lesser span each subsequent rafter. Can the jack rafters be done in 2x8 and still pass code? Thanks for any and all info.
 

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Joseph A. Capece
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There are many factors that come into play. Where you are located & what your codes are; what pitch the rooves are, what material the framing members are, etc....
 

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Discussion Starter · #3 ·
All valid points and appologies for not being more thorough in explaining the substructure. Ive questioned the local code official and his best answer was stay the same all the way. Which is what we were planning to do. But with the old ceilings being cathedral/open in style, and wanting to keep them that way, we were trying to avoid the added weight on the overlap/jack part if possible. West NCarolina is the jobsite.

The house is 60 years old, though solid and structurely sound, much of it is no where near up to code. However in doing this addition/remodel trying to make sure that all of the new work is. Will bring the old where needed up to spec.

Old structure is a 3/12 pitch. The new intersecting gable addition will also be 3/12. The addition is 10' with a span of 36'. New ridgeline to intersect old ridgeline. Outside walls to be framed 2x6 to match the existing structure and interior petition walls 2x4.
 

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What is the longest span, in plan view, for a jack?

Get that, then go here. http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

To get your gsl, the ground snow load, go here. http://www.groundsnowbyzip.com/

Actual snow load for design is computed using gsl. Factors include exposure types, and more, but if you just use the straight gsl you should be safe.

I am an engineer, but I am not your engineer. Get some local pro help if you need it.

If you want to really get into the weeds download the excellent structural analysis package from Weyerhaeuser, called Forte. It is free with your registration. Hip and valley sizing, rafters, flush beams, headers, most everything you need to size members for a wood framed structure.
 

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Discussion Starter · #5 ·
Willin, thanks for the Input.
Yes Ive used that awc link many times. Using it under member type (snowload) and the longest jack at 16' it says that 2x8 would fall within spec for the jacks. Using this link the other day is what gave me the idea to do the jacks in lesser size vs what the main rafters are. I just can honestly say I have never seen anyone use smaller, and myself have only framed a few cross gable roofs. All the ones Ive done and seen were all much smaller spans using either 2x6 or 8 rafters, and never over an open ceiling. Then I called the code fellow that has come out a time or two for his opinion and I couldn't get a clear answer. I figured I'd start asking around to see how others might have handled a similar situation. I do have a local engineer that has helped me with some of the specs, I'll probably give him a call as well. Many thanks.
 

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If he's not giving you a clear answer then it probably means he doesn't know the answer so he's afraid to tell you something that isn't right. It sounds like your thinking is correct, no reason to have a 2x10 as a short valley jack rafter. You just need to know how to talk to him, ask him where it says in the code that lowering the size is unacceptable, and why it wouldn't pass inspection. Say if it meets code then that's what I'm doing. I've had to spend about 20 minutes talking to an inspector that should take about 30 seconds for him to tell me, they tend to get afraid of telling you something that will make them liable so they go around in circles.
 

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Common rafters run from ridge to wall plate. Jack rafters are those that frame to a hip or valley. See pic.

He is talking about a cross-gabled roof and the valley jacks at the cross, which have shorter spans than the commons. What do the carpenters call those in California, near you?

By the time he gets in close, he can probably use 2x4s, since he apparently does not have a vault situation below, but his building inspector is having none of it.
 

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I've never heard it called that either, but by his description it's what we commonly call a "third gable" with a 36' wide addition extending out 10' from the existing structure. What confuses me is that he's doing it to conserve weight. I get that, it would lessen the dead load. But he mentions the old ceilings being "open/cathedral" and wanting to keep them that way. So if the ceilings are open, wouldn't he have to keep the rafters all the same for the rock to plane across? Doesn't make sense to use 2x8's and then furr them down with 2x2's.

I'd do a California, er, frame-over valley unless the whole exposed valley look is what he's after. Maybe he's taking out the wall at the addition with no header.
 

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Discussion Starter · #12 · (Edited)
Sorry for the slow replies on the asking side. Been busy working.

Kenn,
On the part about not getting a clear answer. I get the impression by the 2 different inspectors that have come out so far, neither is really willing to make any suggestions, but just inspect the NEW work I am doing. They are well aware the old part is not up to code but grandfathered. They would rather see my work be overkill to make up for what they have no control over, than agree to allowing a bit less in certain areas based on span charts and load limits. One actually made a comment about being liable IF there were to suggest. I plan to have copies of lumber span charts present IF they say anything about it when next out. However, something tells me that had I never asked, they never would have questioned it.

On the topic of Cross Gable and rafter. Ok maybe I should have used a comma in there somewhere. Here in the south I have always heard 2 gable roofs that meet perpendicular to one another referred to as such. I even have old carpentry manuals that refer to it as such. But then Ive been in the trade for near 40 years, and maybe terms have changed and I have not. Either way, the first few repliers knew what I was referring to. WIllin explained it, as well as depicted it well in his picture.

As for California, I have several freinds out there, one actually retired from the building trades. Many terminologies are different in all facets of life between east and west coast.

Willin,
the way I see it you are absolutely correct, IF I wanted to I could downsize each all the way to the ridge based on span and load charts, but I have no plan to do so.

Kiteman,
Yes I guess one could call what I am doing a third gable, as technically there is 1 gable on each of the house now already, but then if one were to add another on the other side of the house would we call it a Quad gable?

The section of the old house that is being tied into has no Ceiling joists, Just 2x8 rafters, occasional 4x6 collar ties near the ridge that are seen in the living area, and the bottom of the rafters covered with tongue and groove. Between the old ridge and the outer wall, (where the addition is tying in) Only 2 perpendicular petition walls are under this old roof. The owner wishes to keep the open ceiling and walls the way there are in this part of the house. Due to the lack of ceiling joists and vertical bridging commonly found in a truss configured roof, I figured it best to maybe lighten the load of the new roof IF allowable. Over the main part of the addition will be normal rafters,ceiling joists, collar ties and Vertical truss type bridging. Over the old roof, with be the Jack rafters that tie into a sleeper laid on the old roofing surface to create the valley that goes from ridge to the outer edge of the new addition. This new addition is the exact length as the old house, just coming out 10 feet. Many would have just done such an addition with an added shed type roof. However, they wanted to do the addition as well as change the exterior appearance of the place.
 

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So, yes, a California valley.
 

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General Contractor
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I never heard that term also, but I did read the article in Fine Homebuilding Magazine a while back, and if I remember correctly cross gable roof is mostly made up using jack rafters and is used in situations when you have one pair of gables being lower than another.
 
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