Does Notching a Rafter Change it's Max Span?

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  #1  
Old 09-06-05, 12:54 PM
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Does Notching a Rafter Change it's Max Span?

I'm replacing 2x4 rafters with 2x6 to increase span and do away with a support post in the middle of a workshop.

In order to keep the level of the roof decking the same as the old, which is necessary due to other rooflines and siding already in place, I'll need to notch the new 2x6 rafters deep enough to have them sit on the outside wall's top plate at the same level as the old 2x4's.

Does this action essentially render the 2x6's to BE 2x4's since I'm taking some of their width away at the point of thier bearing? In other words, am I losing the very reason (greater span) for which I'm putting them in?

Also, is it possible to shave down the part of the new 2x6's that will be the overhang (soffit) without minimizing their strength? Siding is already in place that I need to match the new soffit to, so I can't just let the new rafters be 2 inches lower than the old.
 
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  #2  
Old 09-06-05, 04:34 PM
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Originally Posted by cakins
I'll need to notch the new 2x6 rafters deep enough to have them sit on the outside wall's top plate at the same level as the old 2x4's.

Does this action essentially render the 2x6's to BE 2x4's since I'm taking some of their width away at the point of thier bearing?
I believe the deep notch does compromise the 2x6. Have you considered sistering a second 2x4 to the existing rafters? This would beef them up considerably and not encroach the extra 2" of your inside clearance. If your soffit is existing you could cut the new 'sistered' 2x4s plumb at the outside of the bearing wall so they don't extend into the soffit. Otherwise you'd have to notch a birdsmouth in the new pieces, slide them into place, and "roll" them to vertical with a big hammer.
 
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Old 09-06-05, 06:13 PM
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Does Notching a Rafter Change it's Max Span?

From an engineering standpoint, a notch in the wood member near the support reduces the shear strength of the member, but it does not reduce the flexural strength. If it is at an end it does not reduce the deflection.

This means a single 2x6 will deflect less than two 2x4s sistered together if the member is carrying something like a snow or floor load.

I think you should have someone take a look at what you are proposing since the actual situation needs to be described and looked at in detail.

Dick
 
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Old 09-06-05, 07:27 PM
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>>Does this action essentially render the 2x6's to BE 2x4's

It sounds to me like you'd be notching the 2x6 so deep that the rafter seat would be more than 1/3 of the way into the 2x6. As Dick mentioned , it will still have the "stiffness" of a 2x6, but only if it doesn't fail! A board is only as strong as the amount of wood left above the heel of the birdsmouth.

Is the bottom of your current 2x4 rafter lined up with interior edge of the top plate of the wall? If so, dropping the 2x6 rafter below the top plate is a BAD BAD idea.

Dimensional lumber always contains heartwood in the center, and is surrounded by sapwood on the outside. Since this is the case, cutting through the sapwood on either side weakens the structure of the board, similar to the way lumberjacks fell trees, by making a notch in one side of the tree first. This being the case, ripping a 2x8 down the middle (through the middle of the heartwood into 2 pieces would make 2 pieces... 2x4, BUT they will not be as strong as a 2x4 that has heartwood in the center. Those 2 pieces would then have heartwood on one side with no surrounding sapwood, which is what helps give boards structural integrity.

As far as cutting the bottom off for your soffit, that wouldn't affect the rest of the board. It would mean that the portion you cut off would not be as strong, due to the fact that you're cutting off the sapwood from one side of the heartwood as described above. It sounds like if you are lowering the 2x6 by dropping the rafter, and you want to cut some off the bottom of the rafter tail, you're going to only have about 2" left on those rafter tails and they won't be able to hold much weight like that. The rafter tail will only be as strong as the vertical dimension of lumber that is left above the corner of your bird's mouth.

It sounds like you might want to get a building inspector's opinion before proceeding.
 

Last edited by XSleeper; 09-06-05 at 08:03 PM.
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Old 09-07-05, 05:19 AM
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[QUOTE=XSleeper]>>

Is the bottom of your current 2x4 rafter lined up with interior edge of the top plate of the wall? If so, dropping the 2x6 rafter below the top plate is a BAD BAD idea.

[QUOTE]

Can you explain the above statement a bit more? Why it's bad, etc?
 
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Old 09-07-05, 06:55 AM
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The Concrete mason is correct

A notch at the bearing surface reduces the shear capacity of the member. If you want to see if you have enough "meat" left use the formual 3V/2bd where v= reaction , b&d equal the breadth and depth. Once you calculate the required shear you can compare that to the value of the member type you're using.
 
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Old 09-07-05, 09:12 AM
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What you describe is replacing the roof structure without disturbing the roof in order to eliminate center supports under the ridge, is that correct?

Have you considered adding a ridge beam beneath the existing one and supporting the new beam with posts in the end walls? A bit tricky that is, since the new beam must sit where the support posts are now. Don't let the roof fall on your head. Gotta support the existing ridge board and everything connected to it from a side angle; remove the center posts; then jack the new ridge beam beneath it and add supports in the endwalls, reaching up into the gable. Having a crane handy would help.

This sounds like a dangerous challenge to attempt.
 
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Old 09-07-05, 09:15 AM
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Thanks, but can you clarify?

Originally Posted by A_unique_name
A notch at the bearing surface reduces the shear capacity of the member. If you want to see if you have enough "meat" left use the formual 3V/2bd where v= reaction , b&d equal the breadth and depth. Once you calculate the required shear you can compare that to the value of the member type you're using.
This is exactly the type of good info I'm looking for. However, I'm unclear on the above formula. Can you give me an example using 2x6 lumber?

Where do I find the "reaction" value?
Also, you say compare the formula result (the "shear") to the member type value. Where would I find that?

Are breadth and depth the same as thickness and width of the board (breadth =2 and depth = 6 for a 2x6?)
 
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Old 09-07-05, 10:53 AM
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Look at the following illustration and tell us how your current 2x4 rafters are sitting. http://pg.photos.yahoo.com/ph/thexsl...&.dnm=9b66.jpg

The reason I said "dropping the 2x6 below the top plate is a bad idea" is illustrated in the picture "dropped 2x6". When you drop the 2x6 in that manner, it is no longer a 2x6 in strength, and as I was referring to earlier, is actually a little weaker than a 2x4. A 2x4 would bend under load until it fails, but a 2x6 notched like that would first want to crack with the grain under load, with the crack starting from the inside corner- the deepest part of the notch- which would actually cause it to fail sooner than a properly notched 2x4 would.

The 2x6 would be stiffer than the 2x4, but only until the load it can carry is exceeded, then it would begin to break.
 
  #10  
Old 09-07-05, 11:55 AM
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here is some additional info

the reaction is derived from the span x psf loading. For example if you had a 20' span @ 50 psf loading and framing members @ 2' o/c your reaction would be 1000. The height of the 2 x 6 will be determend by the pitch involved. If you can remember back to school and the old theory of a(squared) x b (squared)= c (squared) then you can determine the height. For example a 5 /12 produces a c = 13. Now take 12 /13 and you get .9231. So then you take 5.5 (dimension of 2 x 6) \.9231 and you get the height.




The shear value comes from the lumber type. You should be able to find the value from lumber charts
 

Last edited by A_unique_name; 09-07-05 at 11:58 AM. Reason: adding shear info
  #11  
Old 09-10-05, 09:55 PM
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You may consider steel mated to the ceiling joists as a means to increase their span strength. You will need to talk to a structural engineer to make sure you have the proper thickness of steel.
 
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