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Ridge Board vs Beam

CAGIV's picture

My understanding thus far is a  Board is simply a nailer for the rafters, and the Beam post's down through the wall system and is designed to help carry the wieght of the roof.


Right so far?


so when is a ridge beam needed vs just a board? 


Wouldn't most roofs benifit from having the beam to help prevent the exterior walls from spreading under the load? 


Other than a roof that rests on wall that is bustrested from thrusting outward in some manner I'm trying to undersand why  ridge beams aren't used ?


 


Team Logo

(post #94652, reply #1 of 15)

Beams dont need ceiling joists, so you can have vaulted ceilings.  With a ridge board you need cj's to keep the walls from spreading.  Also if the pitch is less than 3/12, you need a ridge beam.

(post #94652, reply #2 of 15)

Maybe one of these days we'll get someone like Joe Fuscoe to make a web page with some graphics to explain this. It seems to come up fairly often.

The obvious answer to this problem is to use trusses and outlaw rafters.......................(-:

But since that's unlikely:

With a ridge board only, the rafters "lean" against each other. To get a good visual of this, put 2 pencils on your desk and lean them up against each other. If you let go of them, they fall.

Now add an imaginary ridge board that keeps 'em from falling over sideways, but doesn't hold the end (peak) up. Now the pencils are leaning against each other and can't fall over. But the bottoms of the pencils will probably slide out and they'll fall anyway.

Now add a ruler to the mix - Imagine it's a ceiling joist. The bottom end of the pencils/rafters are fastened to the ruler/ceiling joist to keep them from sliding out. And the ridge board keeps them from falling over.

.

Using the pencils leaned against each other again, imagine having a ruler as a ridge beam. The ruler/ridge beam is supported on each end by something like a coffee cup. Fasten the pencils to the ruler with some super glue, and you've got a solid structure again. No need for ceiling joists.

.

That's the best I can come up with for "virtual desktop engineering".



The hardest thing to understand in the world is the income tax. [Albert Einstein]

(post #94652, reply #3 of 15)

Now that I've re-read your post, I realize you were asking a different question than what I tried to answer. Sorry about that. But I put a lot of thought and effort into typing that, so I ain't gonna delete it. (-:

I suspect there are 2 major reasons ridge beams aren't used more. The first is cost. As you well know, people don't want to pay for something they don't need.

The second reason is that it complicates the structure. You can't have a ridge beam unless you have something to support it. On a long house you might have to have 2 or 3 places where you'd have to carry a point load from the ridge beam down through the house to the foundation. That would add cost and create problems with future remodelling options.



Bachelors should be heavily taxed. It is not fair that some men should be happier than others. [Oscar Wilde]

(post #94652, reply #4 of 15)

boss did such a good job of that. I thought he was answering your question too. After three years in here and four or more in collich, I'm sure you understand the physics. It looks like you are commenting on why the choice to avoid ridege beams is usually made.

It's mostly one of economics. If you are building a cathedral ceiling type ( not ignoring scissors trusses Boss) roof then you automatically need a beam most of the time.

Where I see roof failures most ofteen from not having a ridge beam is when there is an old cape or similar structure with steep roof, attic, and no ridge beam. Then somebody decides to let in a shed dormer and upset the equilibrium. Thgey land the upper end of the new rafters on the non-existant ridge and the cut takes away from the opposing thrust on back side of roof. The triangle is broken and the ridge sags starting almost immediately after the work is completed The new wall of the dormer gets pushed out and passers by say things like, "Quaint house ain't it?"

 

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(post #94652, reply #5 of 15)

So, how would you add a shed dormer to a roof with a ridge board instead of a ridge beam? (Oops - is the answer going to be "Consult an engineer?")

(post #94652, reply #6 of 15)

There are two ways to counteract the forces.


Use a ridge beam to resist the vertical force pulling down on the beam and rafters.  Of course this beam has to be supported by something, so there's a good chance that you will have to call an engineer.


Countercat the lateral thrust that is present.  Since the rafters are not continuous to the plate (and ceiling joists) you have a problem.  How to tie the plate of the first story walls to the plate of the dormer.  You could use cables and turnbuckles, but that might get in the way of the living space <g>.


The short of it is you usually need a ridge beam and sufficient support on each end of the beam.


 


Jon Blakemore

 

Jon Blakemore

RappahannockINC.com

Fredericksburg, VA

(post #94652, reply #7 of 15)

Size the beam, open a hole in the gable end, slide it in, Jack it up and hook it up

 

 <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #94652, reply #8 of 15)

Hey Piff,  I did a shed dormer on a Cape last summer.  Specs/plans from registered archy.  Was thinking of copying the plans on my own cape, but now you've got me thinking.  Here's the skinny:


About a 20' overall  rafter span (10' run)


96" shed dormer wall


12 pitch on front


Think it worked out to a 4 or 5 pitch on the shed (don't have my Construction Master in front of me)


Ran ceiling joists from the shed top plate into the 12pitch rafter w/ no ridge beam, just a 2X12.


I know we are not engineers here, but I'm asking your opinion/experience on this system.  Ever see one like this fail or survive?

(post #94652, reply #12 of 15)

I'm guessing that a 2x12 for an 8' span would act as a ridge beam, maybe slightly undersized but not likely to fail or sag visibly. Doubling this up would be a simple matter. The rafter-ceiling assembies adjacent to the dormer would transfer the ridge load to the basement. The other crucial element is the rafter ceiling beam connection in these adjacent assemblies though I wouldn't expect these to fail in a dormer of this modest size.

(post #94652, reply #13 of 15)

I had to read this three or four times to get a picture of what you were saying here.. The 2x12 is not a ridge beam but is what you used for ceiling joists to connect midspan to the rafters on the opposite 12/12 side of the roof, right? Rather than answer definitively, I'll ask you questions to focus your own analysis.

Your dormer shed penetration of this roof is fairly small so you probably don't have too much to worry about, but...

Here is the stress point where you can have failure -

Where those 2x12s connect to the 12/12 rafter. Are there enough fasteners to hold secure against the stresses?

And

Is the rafter itself strong enough to handle the loads being transfered onto it. If these rafters are like many in New England from way back in the dawn of colonial construction, they are the equivelant of a rough cut 2x4 and will probably develope a sag midspan, even tho it is a short span. But if they are 2x12s too, I'd sleep well.

I bet they are something in between - which gets you the standard disclaimer about seeing an engineer.

;)

 

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Welcome to the
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Knowledge FHB Campus at Breaktime.
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Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #94652, reply #9 of 15)

I think I just replied to a similar question recently.


My ol house has no ridge beam.RR's butt one another and they didnt even use any nails. 8/12 pitch.


Be pegged


                    andy

"My life is my practice"

(post #94652, reply #10 of 15)

OK .. now ask ...


why even a ridge board?


Jeff


Buck Construction   Pittsburgh,PA


     Artistry in Carpentry                

    Buck Construction

 Artistry In Carpentry

     Pittsburgh Pa

(post #94652, reply #14 of 15)

"why even a ridge board?"

I think it's a matter of convenience.

Without a ridge board, you have to put the rafters up in pair from opposing sides and toenail 'em together. Then space them correctly and brace them somehow so they don't fall over.

With a ridge board, you can just nail the ends of the rafters to it. No need to worry about spacing if you have layout marks on the ridge board.



I don't know what's wrong with my television set. I was getting C-Span and the Home Shopping Network on the same station.
I actually bought a congressman.

(post #94652, reply #15 of 15)

that's waht I was told way back in trade school ..


basically it's a nailer.


plus it gives ya something to lean the rafters against as you nail them.


my understanding of how 2x material became standard ... was 2x material is usually cheaper and more available thru framing.


plus ... give ya more room for error when nailing.


Jeff


Buck Construction   Pittsburgh,PA


     Artistry in Carpentry                

    Buck Construction

 Artistry In Carpentry

     Pittsburgh Pa

(post #94652, reply #11 of 15)

CAG,


I think it's all a matter of how you want to carry the load.  At my house that I am building, I used a ridge beam in my vaulted breakfast room out back because I didn't want ceiling joist and also didn't want to push the walls out.  As an added benefit, using a ridge beam also took half the roof load off of the walls which I was looking to do as my walls are cantilevered off the back of the house.  This is exactly what the other guys said I guess.


Also did a job on a very old century home where we added a portion of pitched roof over a badly done scabbed on flat roof (ok, there are many things about that sentence that don't sound great but that's another story).  Anyway, there was no real reason not to load the new roof to the walls but because the house was so old, and every exterior wall was, how can I say this, bellied out in the middle, my boss and I posted up from the foundation on either side of the house (tearing off the plank sheathing, setting in posts, and recovering with osb to add some shear strength) and put a triple LVL beam to carry the load. 


Like I said, I think it's a matter of application more than anything.  But then again, I am coming from a remodelers point of view.  New home builders might have other considerations when making these decisions.


Rob Kress