Rafter horizontal reaction on the exteriro wall 1

[m1208]

I am working on a roof project with 4:12 roof slope. The ridge beam span is 30'. The ceiling is exposed and there are no ceiling joists to tie the roof rafters to the ceiling joists. The windows in the exterior wall have cut into the wall top plate partially. The exterior walls are adequate to resist the vertical reactions from the roof rafters. I am concern about the horizontal reaction from the roof rafter pushing the existing exterior wall out. The owner does not want any trusses in the ceiling area. Any suggestion is appreciated.

 

[msquared48]

The only folded plates I have ever designed are paper plates. Some even fly!

Mike McCann, PE, SE (WA)

 

[oldestguy]

Mr. Kootk:

Evaluating the beneficial effect of the sheathing as a from of beam certainly would be time consuming especially if you take into account the shearing resistance of the sheets on the rafters, depending on nails and friction. That alone is pretty far out. It's just the "belt" that is known to help some but exactly how much is difficult to even come close to. So the rafter and ridge beam "suspender" numbers come easy even if the inputs and computed stresses are well on the safe side.

With this being a very common residential roof system it would seem someone would come out with design charts based on actual tests, to serve as a simple design guide, preferably on a spreadsheet.

I was just now looking at some old photos of the neighborhood where I was born in Western NY. All the houses there have 1:1 roof pitches and still are there today. Most were built 1910-20 with spaced boards as sheathing, wood shingles then, no ridge beams of consequence, and northern states snow loads. Of course with attic "floors", probably only 2x4's (full dimension), as a form of tie. My son's house in Milwaukee is same and his rafters are 2x4's full dimension stuff. I kinda shudder looking at them.

 

[KootK]

The attic level tension tie makes a world of difference to this system. I got my start working in Wisconsin and reviewed an number of similar buildings that were doing just fine.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.