Ridge beam and rafters

[rakuz66]

What are the role of rafters in a cathedral ceiling once a "ridge beam" is installed?  Do the rafters still carry any live loads?   

[PEG688]

#1: What are the role of rafters in a cathedral ceiling once a "ridge beam" is installed? 


#2: Do the rafters still carry any live loads?   

#1: They hold up the sheathing and / or roofing. If that make any sense as your question really does not make sense to me . What your angle /point here? 

#2: No live loads , roofers are NOT considered "really" alive , thier not people in the "normal" sense.

But no,  the  roof load would be considered a dead load , unless your splitting hairs for some code compliance question.


No roofers where harmed in the production of this post 

[rakuz66]

Sorry about the confusion.  I guess what I'm asking is , once a ridge beam is in place in a cathedral ceiling, does this ridge beam and posts carry all the snow load to the ground?  or do the rafters still carry any snow load to the outside walls?   Will the ridge beam help reduce the snow load on the rafters?

[PEG688]

I'd say , yes . Common sense sez they would carry some weight and transfer that to the exterior walls. depending on the roofing type IE Comp shingles (standard asphalt shingles) would tend to hold more snow than a metal roof so more weight could build up.

Your rafters have to be able to span ( read hold themselves up / stay straight over the length of there selves) are you angling for smaller rafters like say 2x4 DF like they used in MANY older buildings? Where or what is your real question or what are you trying to do? I guess ?   

Either you don't know what to ask or your wanting a answer that outside of the plans you have is my thoughts here

[PEG688]

 Will the ridge beam help reduce the snow load on the rafters?

Yes,  in a way. 

  Maybe it should be looked at as a ridge beam supported on both ends transferring the load to bearing would reduce the outward thrust on the exterior bearing walls , so the load / weight would be the same but the outward push would be reduced.

[rakuz66]

Well, you hit it Peg, my rafters are undersized and overspanned for the snow load in my area.   Anyway, I have half a cathedral ceiling without a ridge beam, and was considering putting in a ridge beam to help with the undersized rafters and snow loads.

[John_C]

Half the downward forces that are exerted on the roof are carried to the ridge beam by the rafters.  The other half of the load is carried to the walls by the rafters.

I too am a bit confused by what you are searching for.

If you have seen a building without rafters it most likely had stressed skin panels for the roof. That is an different engineering solution.

[John_C]

my rafters are undersized and overspanned for the snow load in my area.

A ridge beam wouldn't add much to the load bearing capability of the rafters.  You could triangulate things...  in effect creating a scissors truss, with the lower chord exposed. 

[rakuz66]

So John, what you are saying then is that a ridge beam carries 50% of snow load and the rafters carry the other 50% of snow load.

[rakuz66]

My exact situation is that my rafters are good for 9 ft span with my snow load.  I have 11ft.  My cathedral ceiling is only half of the 22 ft width of the camp, the other 11 ft is regular ceiling height.   There is no ridge beam in this cathedral ceiling, just collar ties and gusset plates at the ridge connecting the rafters.  I was considering putting in a ridge beam to help reduce the snow load on the rafters in the cathedral ceiling area.  Make any sense??? If not, I 'm used to it.

[John_C]

So John, what you are saying then is that a ridge beam carries 50% of snow load and the rafters carry the other 50% of snow load.

The snow load is a downward load spread evenly across the roof surface.

From an engineering standpoint the load from the midpoint up is transferred by the rafters to the ridge beam and then to some vertical load bearing member..... load bearing wall, post, lolly column etc. and so on down to the footings

The load from the midpoint of the roof down is transferred by the rafters to the exterior load bearing walls, or post in post and beam construction and so on to the footings.

Does that make sense??


Modified:
In a house without a ridge beam those downward forces on the rafters are all transferred to the outside walls.  But the rafters are trying to flatten under the downward load and spread the walls outward.  That's where the collar ties come in to resist those outward forces. The closer they are to the bottom of the rafters the more effectively they can deal with those forces.   BUT the load on the rafters is the same.

[rakuz66]

Ok, I put in beams ( doubled 2x10's) every 4 ft at plate level to help resist those forces.  Is there anything I can do then to help reduce this rafter span??  I take it a ridge beam won't do much to help reduce the snow load on theses rafters?  Snow load is pretty heavy 70#

[John_C]

A few things that came to me over lunch:

How are you measuring the span?   It is measured horizontally NOT the length of the rafters.  In a 20' wide house with a 12/12 pitch roof the span is basically 10' even though the rafters are a bit over 14' long.

How are you figuring snow load?  In areas with high snow loads the design loads may vary quite a bit from site to site according to elevation and other factors.

If you have a steeply pitched roof you may be about to assume a reduction in anticipated snow loads.

[NM_Shooter]

I thought that a ridge beam does not provide any support for roof loads.  It is only there to keep the roof joists lined up with one another and is in compression between the joists on either side?.

-f-

[John_C]

I think PEG addressed this some time ago and had some nice drawings, but here goes

Two different situations and a bit of confusion in nomenclature.

If the forces pushing outward on the walls are resolved by
    1. Ceiling joists spanning from top plate to top plate & attached to the rafters  OR
    2. Collar ties relatively low on the rafters.
Then there is little work done by that piece of wood between the rafters. It can be relatively small 2x10 or so, whatever is convenient to attach the rafters to.  I would call this smallish piece a ridge board.

If you want a cathedral ceiling then something else has to take care of those forces that would push the walls outward.  One way is by hanging the upper ends of the rafters on something substantial...  a ridge beam.  It needs to be sized according to the anticipated loads,  the span of the rafters, and the span between whatever is supporting the ridge beam and transferring those loads to the foundation.   Now we have a downward load of half the roof load supported by the outside wall and a downward load supported by the ridge beam and no significant outward loads.

Hopefully that make sense. 

[rakuz66]

Hi John, I'm figuring span the correct way and I know the snow load is 70#.  The entire width is 22Ft , so my span is 11 ft.  The area that has the cathedral ceiling is 15 ft by 11 ft.  As I read your last post, I again seem to think that a ridge beam might help.  If, a ridge beam transfers 50% of snow load, ( half the roof area), then ( to me) it seems that only 50% of the snow load would be on my rafters. 
Now, if there is no ridge beam ( as it is now), my rafters have to handle 100% of snow load and transfer it to the outside walls.  I'm not trying to argue, it's just how I am perceiving this.

[John_C]

The rafters carry that load with or without a ridge beam.  If you have 2x10's joining the walls at the top plate then you have taken care of the outward forces.  The forces you are trying to deal with are the overall loads that would cause the rafters to sag in mid span.

What size are your rafters? What kind of wood?

I'm having difficulty understanding how you keep going back to the notion that something is going to relieve the rafters of their load. 

If there is a blanket of snow on your roof the rafters will carry that load.

[rakuz66]

Your right, the snow load will be on the rafters regardless.  My rafters are 2x8 SPF #2 (24 in / ctr) Overspanned by about 18 inches to 2 feet.  The area is small: 15 in length by 11 feet wide.  I have exposed doubled 2x10's at plate level every 4 ft to help hold walls together. Giving me 4 beams.  ( Ifigured it would look better than ceiling joist every rafter).  The only thing I can think of to help reduce rafter span and snow load ,would be to build a knee wall about 12 to 16 inches out from the outside wall, from the doubled 2x10 beams up to the rafters???

[John_C]

I ran those numbers through a calculator on the American Wood Council web site

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

2x8 SPF  12" OC  70#live load 10# dead load deflection L/240 would be adequate for a span of 12' 4"

It would be acceptable practice to sister the existing rafters with another 2x8. 

2 - 2x8 24" OC

With that much snow load what is the roof sheathing?  If its something flexible like OSB  it might be better to just put another rafter in between so they are 12" OC

[rakuz66]

I considered putting another 2x8 at 12 in on ctr, problem is the opposite side of the ceiling is closed up, I would have to open it up again 
Sheathing is 1/2 in ply.  If I did put another 2x8 on 12 in ctr, wouldn't that put more weight and stress on the outside walls??  Remember there is no ridge board, rafters only connected by gusset plates, is that ok???   If I did this, would I need more ceiling joists from plate to plate??  Put in a ridge beam???
Actually, the 2x8 rafters were sistered to original 2x4 rafters.  So, I have 2x8's sistered to 2x4's spanning 11 ftr.  Maybe that would be adequate.    I could sister a piece of lumber underneath the origianl 2x4 to bring it flush with the 2x8? 
I really appreciate all your help. 

[John_C]

I'm still having trouble visualizing the construction and exactly what is going on there structurally. 

As far as   

If I did put another 2x8 on 12 in ctr, wouldn't that put more weight and stress on the outside walls??

No, not really.  The weight of the rafter is insignificant. The snow load on the roof remains the same... it just get distributed differently.  The collar ties  /  joists you have should be sufficient to keep the walls from spreading.  I assume the shingles are already on so you wouldn't be able to nail down through the plywood.  I'd use some construction adhesive and let it go at that.


Is this a residence?  How long was the structure in use with the 2x4 rafters?  If the 2x4's & 2x8's sistered together are in good shape I might be inclined to ignore the whole thing unless it started to sag under a snow load.

I also don't get the gusset thing.  Was somebody trying to make site built trusses?  It all sounds a bit cobbled together for an area with 70 psf snow loads.

[PEG688]

 So the issue is overspanned rafters , those gussets at the ridge aren't doing much . The "beams" you added , IF you've got a good positive tie to the rafters will stop any wall spreading , you said every 4' on those , essentially they are "low collar ties" at least in my book.

  And you've already sistered the "new " 2x8 onto existing 2x4's.

 
#1: So how many layers of roofing are on the place right now?

#2:  I assume comp. shingles   That weight is more than likely what caused the sagging , well that and gravity , at least on the old 2x4 rafters.

How "good" what type of wood are the old 2x4's ? Doug Fir ?? Are they pretty clear or full of knots?

#3: Looks don't seem to be a high priority with that added beams/ collar ties and the sistered rafters , is that right? Function is more the goal here?

  #4: If so how about a strong back , essentially a purlin mid way / span under the rafters  , a 2x6 D.F, straight grained ones. Prop it / them  up there some how, and  jack the sag out of the existing rafters .

Then use joist hanger at each rafter slide the hanger onto the purlin like a "normal" hanger would go on , use a bigger / longer say 2x10 hanger then nail off the "ears " to the 2x8 sistered rafters.

  That would "beef up " your over spanned rafters , it will look odd, BUT it will do what your asking better than any thing else you've come up with.   



 

[rakuz66]

Am I giving you guys a headache yet or what???
JOHN.  It was an old camp in upstate NY, that had trusses and the previous owner cut the bottom chord and webs out of half of each individual truss to vault half of the ceiling in the living area. The trusses were 2x4 construction.  He then sistered the top chord ( or rafter) with a 2x8 and left it at that.  In my haste, I bought it cause I loved the land it was on, probably why nobody else decided to buy it.  Anyway, UNBELIEVABLY, It sat like that for about 5 yrs without any MAJOR sag or wall thrust.   
I then sistered the opposite side of each cut truss with 2x8's and put in a 2x10 beam underneath them on one side ( the closed ceiling side) for the trusses to sit on.  I also ran 2x10 ceiling joists at plate level every 4 ft from opposite wall to 2x10 beam to hold walls together.  I left the vaulted side open cause wifey likes the look. 
It is cobbled together, but it's actually probably pretty strong.  I am trying to tell myself that the 2x8's sistered to the 2x4 are probably fine.  There is no ridge sag at all, just some sag in the roof sheathing between the rafters in some spots. 
If you search my name ( rakuz66 with beam ) you should find some pics. 

[rakuz66]

Peg, you probably remember my "overkill" and this camp.  2x10's every 4ft at plate level, good connections all around. 
1) 1 layer of shingles that are like potato chips.
2) Don't know what type, but pretty clear, not many knots.
3) Function and safety is the goal.
4) That was my one idea.  Place 2 lvls about midway of rafter span ( 6ft) underneath all the rafters.  (Sort of like a ridge beam except not at the ridge but midway.) Put in some extra 2x4's in the wall for a post for them to sit on, and dig two 3 ft deep  piers with concrete to rest on.
5)  Or, do nothing cause there's no sag now. 
Thing is this, I 'm ready to cover up the ceiling with t& g pine and put on a metal roof.  I do now  or forever hold my peace ( which I'm incapable of doing)>

[John_C]

If the sheathing is sagging between the existing rafters I'd put 2x8's in between.

Probably something at the ridge would be a good idea.  I'll search for pictures later and think about it.