large span conv. framed hip roof ceiling joists

[eng003]

Looking at a 34' span (short width) 2nd story renovation in which contractor wants to conventionally frame a hip roof. 4:12 pitch no snow but high wind. Maze of existing bearing wall lines below to use but unfortunately mainly parrallel to the potential ceiling joists/rafter ties. Thinking of desiging beam to have ceiling joists frame into (face mount hangers) with straps on top of ceiling joists to transfer tension/thrust of rafters...don't really like the idea but no better ideas at the moment....any thoughts??....Also interested in opinions on practical span limitations of conventionally framed roofs withough ridge beams before combined deflection action of ridge "board" and rafters gets excessive??

 

[dhengr]

Eng003:
I can remember the good old days when you could still get and use roof trusses, so as to avoid all the b.s. that you are talking about. And, except for the contractors ego about thinking he is a roof framer, I’ll bet the truss roof won’t cost any more if you can fairly account for all the other details which you will avoid and the savings in time and your unpaid efforts. You can’t just look at the difference in cost btwn. rafters and ceiling jsts. vs. the cost of roof trusses. All the framing time and labor should be included and you should be paid for all your extra hassle if they insist on framing the roof.

 

[AELLC]

As a general rule, every conventionally stick framed roof ever built is grossly overstressed on paper. So if you design one that works for Code, then the Builder always says he's never seen anything like that and it will cost far more than he bid the job.

 

[XR250]

@AELLC,

They are not overstressed if you have beams for hips that are supported by posts or if you have an attic floor diaphragm preventing the framing from spreading on all four sides.

 

[AELLC]

@Excel,

I should have explained that I meant all those stick-framed roofs in "pre-plans checking" days, all those roofs built w/o calculations.

I was saying it gets very frustrating to justify that construction in calculation, and I was agreeing w/ dhengr.

 

[tony1851]

@AELLC,
Out of interest, how would one actually calculate a traditional hip rafter?
I've had this discussion with inspectors and engineers over the years, and no-one seems to know whether the hip supports the rafters (ie as a beam) or whether it is supported by the rafters. Presumably if it is assumed the latter, then it calls into question the fixity of the rafters at each end.
Sometimes it's easier just to prop the hip somwhere then everyone's happy (even though they might not know why).

 

[msquared48]

tony1851:

"By" the rafters can happen if the shear to the top plate is achieved, there is a functional plywood roof diaphragm, plus the top plate is used as a ring beam to take the kick from the hip at the corner and transfer that to vertical shear walls under the wall double top plate. Kind of a special situation, but with a plywood roof diaphragm, it can be made to work as such.

Mike McCann
MMC Engineering

 

[tony1851]

@msquared,
Thanks for that. Our problem is that we dont use plywood/OSB for lining the roof. It's just breather felt nailed across the top of the rafters, then 2x1 battens to support concrete interlocking roof tiles.
This might make it heavier than a typical US roof, and without the diaphragming effect of the boarding.
Hence inpectors are always going on about the size of the hip rafters.
We uually put a short diagonal piece @ 45 deg. across the corner where the hip bears, to help tie wall plates together.

 

[CELinOttawa]

Interesting construction; Where do you practice Tony?

 

[XR250]

@Aellc

I justify it based on all of the sagging complex hip roofs I run across. Usually, they are more than 30 years old and of low slope. Seems like the high sloped ones seem to function pretty well.

 

[AELLC]

Wood design has a FoS 3.5 to 4, and our roof sheathing has a huge effect to help "belt and suspenders" effects (compared to what Tony is saying in UK), so all these old overstressed roofs I speak of mostly have no apparent problems even though they are overstressed to some degree.

Right now I have so much new construction design work I turn away all remodels involving old existing stick-framed roofs. They usually are money-losers because of the plans checkers we have here don't agree with my opinions of secondary load paths, etc., and it would be tricky to strictly prove all this in a courtroom.

 

[tony1851]

@CELinOttowa
UK; typical older cut (stick) roof, circa 1920; pitch approx 30 deg.
Welsh slate on thin battens (no felt - they pointed the gaps with mortar); 3x2 rafters; 12x3 purlins running gable-to-gable (propped mid span but not shown on pic.); ceiling joists 3x2 sometimes 3x1 1/2. Uprights are just thin offcuts of timber as hangers to stop excessive deflection in the ceiling. (The ceiling is obscured by that dreadful fibreglass insulation).

 

[charliealphabravo]

@een003,
To your second question...

The performance of ridge plates (vs. ridge beams) has more to do with the roof pitch and the rigidity of the supports than it does the span. I would think that ridge plates restrained with collar ties or ceiling joists should be workable on 8:12 or higher, maybe even 6:12. I would think that ridge plate systems would almost never work at 4:12.

I did forensic work in old Tampa neighborhoods where you could drive down the street and see the 4:12 ridge plate systems that were broken down. This usually happened when the house was being re-roofed and the squares of shingles were stacked on the ridge prior to being installed.