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Salt Box Roof and Rafter Thrust

Trusses, or the structural equivalent thereof.
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In the example above, the long rafter has to be engineered for both the uniform live and dead load and also for the midspan point load introduced by the collar tie.
 
jar546 said:
That appears to be inadequate. I see an upper third collar tie but not a real rafter tie.
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Again, it can all be engineered and checked for bending, shear, connections. Top plates of the walls can be checked to se if they can act like horizontal "beams" to resist lateral splay, etc.
It's not the easiest way to prove compliance, but it theoretically can be done.
 
jar546 said:
Is there a way to build a saltbox-style roof, let's say, for example, a 4/12 pitch on one side and a 12/12 pitch on the other, without using a structural ridge?
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Don't saltboxes usually have the same slope on each side, just with the ridge off-center? Seems like you could do that with a structural wall/beam on the long side at the same elevation as the top of the tall wall, letting you install ceiling joists/rafter ties in the bottom third of that span as usual.

If the ridge is off center but the two slopes are unequal as you stated, then you could arrange for the two exterior walls to have the same top of wall elevation, and install ceiling joists/rafter ties as usual. Wondering if this could be done prescriptively using the usual tables; the span and presumably member size on one side of the roof would just be larger than the other.

Cheers, Wayne
 
jar546 said:
No.
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Hmm, the pictures in the Wikipedia page https://en.wikipedia.org/wiki/Saltbox_house suggest that equal slopes are common, e.g. the picture below of a 1672 house in Connecticut.

Some of the pictures do have multiple slopes on the long side of the roof, but to my eye they almost all have equal slopes at the ridge.

Cheers, Wayne

Nehemiah_Royce_House%2C_Wallingford%2C_Connecticut.JPG
 
jar546 said:
That is not the same slope on both sides in that pic.
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Sorry, you are mistaken in your interpretation of the picture showing inline above. It has the same slope on both sides of the ridge. If you look at the upper triangle on the gable end on the right, just the region above the second floor where the siding plane is proud of the first and second story siding, that's an equilateral triangle, with the base level (the bottom-most row of that siding, level with the front eave). Looks to me like the slope on the roof extension in the back is same as on the main roof at the ridge, but it could be ever so slightly less.

Cheers, Wayne
 
Short answer….no…not prescriptive…same as a cape with a shed dormer. The good news is most of them don’t fall down. I’ve built 2 and we put in rafters ties every 8’ or so to keep the openness but have something for thrust.
 
wwhitney said:
Sorry, you are mistaken in your interpretation of the picture showing inline above. It has the same slope on both sides of the ridge. If you look at the upper triangle on the gable end on the right, just the region above the second floor where the siding plane is proud of the first and second story siding, that's an equilateral triangle, with the base level (the bottom-most row of that siding, level with the front eave). Looks to me like the slope on the roof extension in the back is same as on the main roof at the ridge, but it could be ever so slightly less.

Cheers, Wayne
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Fake news. My belief system tells me the slopes are different, therefore it is a fact to me. ;)
 
wwhitney said:
Hmm, the pictures in the Wikipedia page https://en.wikipedia.org/wiki/Saltbox_house suggest that equal slopes are common, e.g. the picture below of a 1672 house in Connecticut.

Some of the pictures do have multiple slopes on the long side of the roof, but to my eye they almost all have equal slopes at the ridge.

Cheers, Wayne

Nehemiah_Royce_House%2C_Wallingford%2C_Connecticut.JPG
Click to expand...

It goes beyond that -- or "went" beyond that. Structurally, a colonial saltbox consisted of the main, two-story house with a symmetrical roof and then a shed-like addition tacked onto the rear (the saltbox portion). The slope of the rear shed roof wasn't always the same as the slope of the symmetrical two-story roof portion. It was always framed independently of the upper roof.

Today designers use the saltbox roof shape without necessarily providing a bearing wall (or a bearing beam line) at what should be the eave line of the rear portion of the two-story roof, and that's where the problems arise.
 
I envision the roof described in the OP as a long shed dormer. The simplest way is to provide a beam or bearing wall with its top at the same height as the outside wall. It can be done the way Yikes showed in post #2. The collar ties would actually have to be raised ceiling joists. The moment in the rafter where the ceiling joists tie in at 1/2 span is triple the moment if they tied in at the base!
 
Paul Sweet said:
I envision the roof described in the OP as a long shed dormer. The simplest way is to provide a beam or bearing wall with its top at the same height as the outside wall. It can be done the way Yikes showed in post #2. The collar ties would actually have to be raised ceiling joists. The moment in the rafter where the ceiling joists tie in at 1/2 span is triple the moment if they tied in at the base!
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Right, and the original post only asked if it was theoretically possible, not if it was prescriptive or efficient.
 
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