Author Topic: ridge beam and rafter  (Read 11294 times)

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Offline rakuz66

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ridge beam and rafter
« on: March 13, 2008, 09:55:16 AM »
When you guys build a cathedral ceiling using a ridge beam, do you like placing the rafters on top of the ridge beam? or fastening to the side of the ridge beam?  is there any benefit one way or the other?

Offline John_M

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Re: ridge beam and rafter
« Reply #1 on: March 13, 2008, 10:00:22 AM »
I have never seen them placed on top!!

Everything I have built and read about has had the rafters attached to the side.

I don't think you gain anything from putting them on top.  I think structurally it makes it weaker?
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Offline glenn kangiser

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Re: ridge beam and rafter
« Reply #2 on: March 13, 2008, 11:05:49 AM »
Mike T put them on top - Looked good to me.

Kind of a birds mouth or notched connection- I don't know the proper term - .  It doesn't take the full width of the board to get full strength of the joist, and in fact some of the minimum bearing requirements are really very small.  Birdsmouth should be limited to 1/3 of the joist depth if I recall correctly.
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Offline davidj

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Re: ridge beam and rafter
« Reply #3 on: March 13, 2008, 12:32:03 PM »
The drawings I had done have them on top too.  I kinda liked the idea is it leaves the expensive ridge beam in view but hides the cheaper rafters.  I was playing with the idea of doing some fake beams/rafters on the ceiling connected to the real ridge beam.  That, along with tongue and groove on the ceiling, should make it look more like board and beam construction (is that the term?) rather than light framing.

Offline MikeT

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Re: ridge beam and rafter
« Reply #4 on: March 13, 2008, 12:46:59 PM »
If I recall, the cathedral ceiling/ridge beam description in the Wagner's book on Framing shows the rafters on top.  In fact, that book shows them scissored at the top and nailed to one another as well as the beam.  I opted for having the rafters meet on the top and cutting a seat for the rafter in the beam.  That worked well on the main part of the house, but I mis-measured or something on the smaller, bedroom beam and was faced with 1-1/2" gap at the top of the beam between where the rafters were to meet.  The solution: a plywood gusset. 

mt

Offline MountainDon

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Re: ridge beam and rafter
« Reply #5 on: March 13, 2008, 02:24:06 PM »
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline Garrett In Tahoe

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Re: ridge beam and rafter
« Reply #6 on: March 22, 2008, 10:33:11 AM »
Yes the above picture is the preferred way to frame at your ridge beam. The lapping of the rafters gives a good lateral tie and the additional length (vs a plumb cut) keeps the end of the rafter from splitting at the seat cut. Solid block between the rafters on top of the ridge, notch them for ventilation so you get air flow from your eves to your ridge vent, which prevents condensation.

Normally a rige board is used where you have a self supporting roof and the opposing forces from each side are holding it up. Usually only designed on shorter spans with light roof loads. But if it is designed as a bearing ridge with a beam then you can still butt the ridge with a plumb cut rafter, but normally you're also required to have joist hangers on the rafters.

Offline Garrett In Tahoe

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Re: ridge beam and rafter
« Reply #7 on: March 22, 2008, 10:57:35 AM »
Oh yeah, I forgot the most important reason for a ridge beam in a cathedral ceiling. You have no ceiling joists to keep the walls from spreading from the weight of the roof. The joists act as ties to keep the roof from coming down, and the walls from spreading, creating a truss effect.

Offline PEG688

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Re: ridge beam and rafter
« Reply #8 on: March 22, 2008, 11:13:38 AM »
Ridge BEAMS and ridge BOARDS  are two different critters.

 A Ridge Beam is a bigger timber , generally solidly supported to bearing (foundation / ground ) at points (either end ) .

 A Ridge board is part of a roof system "that  one  part helps/ augments  another"  to create a greater stronger "whole" in the system.

  Ridge beam , as Mtn.D has shown can be placed below or partly  below the rafters ,


 

  rafters "birds mouthed " over the top of the exposed BEAM , beam solidly supported at both ends so not sagging / wall spreading can take place.


 Here's a ridge board , generally a 2x12 or so , although in the old days , 30 years ago and more a 1x was or could be used,  here's a ridge board 1x6 , 60 year old house , my own , a hip roof 2x6 rafter , some what over spanned in lenght , we'd never be able to build this house this way today , yet it's 60 years old and survived at least 3 earthquakes in this area. No "large" quakes just normal ones , no lahars either for that matter heh 



 



 Today it would look like this where rafters meet ridge should look much like this , as in no big gaps between the meeting members,

 



 Ridge board , dormer rafters , main roof rafters , and a strong back to "help" the ceiling joist ( that also in the dormers act as collar ties in many ways ,


 
 



  Basic rafter , collar tie , ceiling joist over view,



  


 So terms and words mean specific things, mixing words and terms leads to confusion. 
« Last Edit: March 22, 2008, 11:23:39 AM by PEG688 »
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Offline davidj

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Re: ridge beam and rafter
« Reply #9 on: March 23, 2008, 01:29:26 PM »
I guess "fake post, plank and beam" was the term that I was looking for.  Using the real ridge beam, fake "roof beams" (diy glulam from 2x4s or just some 1x made into a box) and fake roof decking (using 1x tongue and groove, rather than 2x or 3x).  Real post, plank and beam would obviously be even better, but I'm guessing a fair bit more expensive and time consuming.

Or maybe I should just copy my 1920's bungalow - 1x6 ridge board, 2x4 rafters spanning 20' (with the occasional purlin about 2/5 of the way along).  Or would that fall slightly short of modern code?

Offline MountainDon

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Re: ridge beam and rafter
« Reply #10 on: March 23, 2008, 01:48:24 PM »
Or would that fall slightly short of modern code?
rofl rofl rofl
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline davidj

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Re: ridge beam and rafter
« Reply #11 on: March 23, 2008, 02:06:01 PM »
Or maybe I should just copy my 1920's bungalow - 1x6 ridge board, 2x4 rafters spanning 20' (with the occasional purlin about 2/5 of the way along).  Or would that fall slightly short of modern code?
Oh, and I forgot to mention, 30" o/c! Fortunately most of them are closer to true 2x4, rather than nominal. The good side of this is that, when the "big one" hits the Bay Area, the whole roof can fall on us and we'll be fine!

Offline Ernest T. Bass

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Re: ridge beam and rafter
« Reply #12 on: July 16, 2011, 07:07:14 AM »
Okay, this is a major bump, but I've got a question that applies to a project I'm currently working on...

Ridge beam , as Mtn.D has shown can be placed below or partly  below the rafters ,

  
  rafters "birds mouthed " over the top of the exposed BEAM , beam solidly supported at both ends so not sagging / wall spreading can take place.

Does this violate the maximum notch depth of 1/3 on those rafter tops? It seems that if the plumb cuts were nice and tight against the beam then things would be okay, but wouldn't an inspector throw a fit?

I may end up having to notch my rafters over the ridge beam a bit deeper than I'd like to, and I was wondering if I'd need to add hangers or something..

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Offline PEG688

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Re: ridge beam and rafter
« Reply #13 on: July 16, 2011, 09:08:43 AM »
  IMO , and I haven't looked at a code book for this answer.


 But it seems to me the rafters could just be cut to land against the beam just like a "normal" rafter / ridge board connection. The beam being supported on either end can't come down , if the beam is sized properly.  If the rafters are attached to the ridge beam properly they also can't come down, as there is little weight or pressure on the rafters on a steep pitched roof.
 
  So the over the top notching we did in this case was to expose some of the beam , but not all of the beam.    

 It was done to "make it look good" which is one of those things I hate to hear coming out of the / any  bosses mouth!!    Of course when he came back he questioned "Why didn't you show more of the beam?".  

  The plans  where not specific to the beam / rafter detail.  The building inspector never questioned the detail , the home owner loved the finished product , the house still stands as far as I know.  At least I haven't heard any thing to the contrary.   I haven't been back there in at least three years, but I'm sure it's fine.

  


    Maybe I'll learn some thing new about this.   I do know that beauty is in the eye of the beholder , and when the boss / bosses / home owner / Architect  says ,  " Oh I don't care, how (insert detail) is done , just make it look good" I just really want to scream!!!

 My last boss was one who consistently did just that, of course when he'd come back he'd say , "You could have done it "this way"!"    

 "Draw a detail, I'll do it how ever you draw it!"     If it's do-able the way it's drawn.    


 
 ETA: I  seem to remember having a conversation with the home owner about only seeing the bottom of the beam , she IIRC didn't want to see much of it as the beam was  glue lam , made of D.Fir , it was smooth. The Red Cedar T&G was rough sawn ,  to limit the contrast  less would be more, in this case.

 It'll be interesting to see what Don P. has to say about the code requirements in this case. He's a walking / talking / typing code resource around here.  8)
    
« Last Edit: July 16, 2011, 09:33:51 AM by PEG688 »
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Offline Ernest T. Bass

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Re: ridge beam and rafter
« Reply #14 on: July 16, 2011, 03:21:41 PM »
It was done to "make it look good" which is one of those things I hate to hear coming out of the / any  bosses mouth!!    Of course when he came back he questioned "Why didn't you show more of the beam?".  

Wow, now that would be frustrating! Thanks for sharing your thoughts.. Nice looking ceiling--simple and crisp with no molding.

 As it turns out, I don't think I'll be notching the rafters more than 1/3 anyway, but I'm still curious as to what is "acceptable" in this case..

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Offline Don_P

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Re: ridge beam and rafter
« Reply #15 on: July 16, 2011, 06:33:18 PM »
1/4 depth max, R802.7 has rafter notching info.
But, it's not a good idea at all if you can avoid it. Bearing on a notch concentrates stress and invites splitting. In a ridgebeam situation it's much better to either sit the rafter entirely on top of the ridgebeam on a bevelled seat or reach around under the rafter and hold it up from there.

edit, remembered a pic;
http://www.aitc-glulam.org/shopcart/Pdf/aitc_104_2003.pdf
This is a tech detail paper for glulam but the physics is the same. Go to page 29, detail A7, also read the two referenced details from there.