Author Topic: fake rigde  (Read 6336 times)

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Offline redc52

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fake rigde
« on: February 15, 2013, 09:37:15 AM »
anyone have a detail where dormer rafters fly through to a ridge pole and a higher ridge is faked in above it??
thanks :-\

Offline Don_P

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Re: fake rigde
« Reply #1 on: February 15, 2013, 10:51:58 AM »
  w* Red
Hmmm, ???, I think that's gonna take a picture. Are you talking about a shed dormer who's rafters land below the main ridge on a header?

Offline redc52

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Re: fake rigde
« Reply #2 on: February 16, 2013, 04:50:15 AM »
exactly
i'll try and put up a dwg of the space i'm talking about
thanks

Offline MountainDon

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Re: fake rigde
« Reply #3 on: February 16, 2013, 06:30:18 AM »
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline PEG688

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Re: fake rigde
« Reply #4 on: February 16, 2013, 06:01:13 PM »

 Similar to this ,


 

 
 


 
When in doubt , build it stout with something you know about .

Offline Don_P

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Re: fake rigde
« Reply #5 on: February 16, 2013, 06:55:18 PM »
If I'm visualizing right, that's close but I think your shed rafters land against the ridge PEG?
What I think Red is talking about doing is to double or triple or whatever is needed the common rafters on each side of the dormer, drop down from the ridge to the desired pitch break location and put in a header. The shed rafters land into the header and there are short common jacks that run down from the ridge to the header. That was alotta typing for a guess  :D

Offline PEG688

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Re: fake rigde
« Reply #6 on: February 16, 2013, 07:19:23 PM »
If I'm visualizing right, that's close but I think your shed rafters land against the ridge PEG?
What I think Red is talking about doing is to double or triple or whatever is needed the common rafters on each side of the dormer, drop down from the ridge to the desired pitch break location and put in a header. The shed rafters land into the header and there are short common jacks that run down from the ridge to the header. That was alotta typing for a guess  :D

 You're probably right, but those photos are as close as I've got to what you described for the original poster.

 But with your verbiage and my photos I think the meaning will be clearer.   
When in doubt , build it stout with something you know about .

Offline rick91351

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Re: fake rigde
« Reply #7 on: February 16, 2013, 08:00:19 PM »
I was wanting to do the something like that on one small house or cabin we were thinking about.  Peg I like those tails stuck out there and away from the house.

However one of the old timers up there said he never seen one that worked right in all his time.  So we opted out, changed the design and had it drawn without the shed dormer.  It was all done I said something to the engineer about opting out of the shed dormer for this design after we picked up the plans.  He said that we should have said something he could have put it in very easily and would never fail.  Then he had to twist the knife.  "That what my job is!"  He said.......       
Proverbs 24:3-5 Through wisdom is an house builded; an by understanding it is established.  4 And by knowledge shall the chambers be filled with all precious and pleasant riches.  5 A wise man is strong; yea, a man of knowledge increaseth strength.

Offline Don_P

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Re: fake rigde
« Reply #8 on: February 17, 2013, 07:02:58 AM »
What is the failure Rick?

The garage roof here is what I described. This was done with attic trusses. They were tripled on each side of the dormers (the setup is the same as PEG's, there are 2 back to back dormers for a bed and a 3/4 bath, these are just dropped below the ridge.


In this case I saw no reason for it and it cost a good bit of money to frame and then put standing seam on that detail. I suspect the archies ears were on fire.

Offline rick91351

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Re: fake rigde
« Reply #9 on: February 17, 2013, 07:59:32 AM »
One problem cited was a shed dormer as in your photo.  Ice falling straight down from the shed dormer to the roof below. 

Another problem cited was snow drifting in and packing in a huge block of ice along side the leeward side of the dormer.  In his view and experience up there, that tended to cause damage to the siding material and roof itself.  One reason was because it is most likely to be there until late spring melting and glaciating.  Over time once again in his view causing leaking siding and weakening walls and trusses.  Granted most of this old guy's experience was hardly with modern siding materials and very little engineering.

Another concern was 120 pound snow load on the almost flat roof shoving straight down with little to no support without requiring a wall and piers lining up that might have interfered with the lower floor plan.    ???         
Proverbs 24:3-5 Through wisdom is an house builded; an by understanding it is established.  4 And by knowledge shall the chambers be filled with all precious and pleasant riches.  5 A wise man is strong; yea, a man of knowledge increaseth strength.

Offline redc52

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Re: fake rigde
« Reply #10 on: February 17, 2013, 08:05:19 AM »
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how would you frame the fake ridge area?

Offline PEG688

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Re: fake rigde
« Reply #11 on: February 17, 2013, 08:51:03 AM »
How wide is the dormer?

  If you used trusses for the dormer roof , the truss company could design the truss to carry the added load  .

 If it stick framed I'd add collar ties , 2x6's or 2x8's right under the dormer ridge , then I'd just place a ridge above with short cripple rafters to create the triangle effect back down to the dormer roof.

 If the dormer was 12' to 16' wide I'd be confidant it would work unless you live where 4 or 5 feet of snow is a possibility.

 A metal roof would help shed snow off the low slope of the dormer roof.

  Some where inside the building  you have some interior walls , with some thought you might be able to create some added point load location to "post" up the dormer ridge if the span is longer than 16' , which would create more flat roof section = great snow loads on the ridge. 
When in doubt , build it stout with something you know about .

Offline Don_P

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Re: fake rigde
« Reply #12 on: February 17, 2013, 02:52:34 PM »
The 4/12 dormer roof is self supporting as drawn, the little witches hat is decoration that could be overframed above the fully sheathed dormer if you want.

Offline PEG688

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Re: fake rigde
« Reply #13 on: February 17, 2013, 03:55:53 PM »
The 4/12 dormer roof is self supporting as drawn, the little witches hat is decoration that could be overframed above the fully sheathed dormer if you want.

 You wouldn't add collar ties?  What his snow load probability / location?  Do you know?
When in doubt , build it stout with something you know about .

Offline Don_P

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Re: fake rigde
« Reply #14 on: February 17, 2013, 06:15:22 PM »
 Not only that, I don't want to know the loads, then he'd want engineering and I'm looking at an irregular building.  I sense Red is not an owner builder he's either one of us or he's from the dark side  ;D. I wasn't sizing anything, span and load don't matter yet, the geometry works for me.

Build a 4/12 roof on a set of walls as a stand alone structure. Put a cute light hat on it, that's trim.
The CJ's provide the thrust restraint, it does need collars or straps over the top, they are only there to tie the rafter on one side of the ridge to the rafter on the other side. Their job is to keep the roof from unzipping at the ridge in high wind... remember the pics of roofs opening up and coming apart at the ridge then 2 big planes of roof flying away during Andrew, that's when this came in. The CJ's are your daily drivers for load and thrust restraint. If my head is on right 50 lbs per bay of hat framing up top = 75 additional pounds of horizontal thrust at the 4/12 rafter heeljoints, throw a couple more nails in each end of the ceiling joists into the rafter.

Offline PEG688

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Re: fake rigde
« Reply #15 on: February 17, 2013, 06:43:01 PM »
I don't worry to much about "unzipping" roofs,  maybe I should but if you nail your sheathing properly , having a window blow out isn't going to take your roof off.

  Hurricanes , tornado's, tsunamis  and big earth quakes are going to , one way or another,  destroy some buildings. I don't worry about my roofs blowing off , I do worry about gravity, unlike engineers who design "hold downs" I figure uplift isn't my biggest enemy, downward pressure is.

 Of course I do have to comply with what I view as foolish engineering rules, but I do so with a smirk, generally.
When in doubt , build it stout with something you know about .

Offline MountainDon

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Re: fake rigde
« Reply #16 on: February 17, 2013, 07:12:06 PM »
But doesn't ensuring that the roof panels don't readily start to peel at the ridge help reduce the potential for damage in very high winds?  A house may not survive a direct hit from a big # hurricane or tornado, but if the roof assembly stays on, more or less intact, I would think the entire structure stands a better chance of surviving. That could be very important if the owners are taking shelter inside the house.
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline MountainDon

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Re: fake rigde
« Reply #17 on: February 17, 2013, 07:28:13 PM »
Roofs and hurricanes

a quote
"When homes fail under exposure to hurricane-force winds, the order of failure is usually roofs, openings, and foundation (Perry, 1995). Roof systems are exposed to higher loading than any other building element (Smith and McDonald, 1991). Field observations of damage from hurricanes Hugo, Andrew, and Iniki confirmed that once the roof of a home was breached, failure of other building elements usually followed. Roof failure followed the following scenario. First, cladding was lost at a roof corner due to the greater uplift there, followed by loss of sheathing. Once sheathing was lost, the building envelope was effectively breached. Wind pressure was now exerted against the inside of the gable end-wall, leading to its possible failure, and against the underside of the remaining sections of the roof, increasing the likelihood that the reminder of the roof would be lost. If the roof became detached, gables collapsed, and the remainder of the structure, now much weakened, often failed.

Roofs are subjected to wind forces from many directions. Direct wind pressure can loosen shingles and tiles. Suction forces on the surface of the roof and vortices on the roof corners can lift both roof cladding and sheathing. Internal pressure generated when windows, doors, or sections of the roof itself are breached can lift and separate the roof from the rest of the structure. A properly designed and constructed hurricane-resistant roof must be able to withstand all these forces."
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline Don_P

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Re: fake rigde
« Reply #18 on: February 17, 2013, 07:55:04 PM »
 The sheathing typically does not go to the peak so does nothing to tie the rafters to the ridge or to each other, barring a collar tie or strap the only thing holding the rafters to the ridge are a few nails in withdrawal. 

Rick, your observations are spot on. My neighbors dormer drops it's load of ice and snow on the porch roof below. It hasn't leaked from the pounding but it sure takes the hits. Look up the sidewalls of most dormers in snow country and you'll see water damage on the siding. I've turned ice and water up the dormer sidewalls quite high on some roofs prior to flashing. I can reside one easily if they don't lose the sheathing and framing.

 

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