Span of roof rafters, how to fix improper structure

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I'm building a carport that is 30×24 ft including a 20×24 gable and 10×24 lean to. It currently has 2×4 roof rafters spaced 24" with collar ties about a foot below the gable peak, so 5-6 ft long ties. Seems a 2×4 SPF #2 should really only span about 6 ft horizontally, using the American Wood Council online calculator. Why on earth do I have 2×4 rafters 24" on center? It's a long story.

The rafters on the project are spanning about 9.5 ft horizontally, both on a gable with almost 7/12 pitch (19 ft total gable span) and spanning the same 9.5ft on a side lean to with about 2/12 pitch.

This does not seem structurally sound.

My question is: Will adding 2×6 rafters in between the 2×4's make it structurally sound? So final spacing will be 12" between rafters, but alternate 2×4, 2×6. I would put 10 ft collar ties on the new 2×6 rafters, so about at the halfway mark down from the gable peak to the top plate on the posts.

Of course the answer to the question depends on the overall structural elements, but just assume that everything else is ok. Any other issues are a separate question, and you might see another from me regarding this project. I just want to know thoughts on 12" spaced rafters alternating 2×4 and 2×6.

Other info:
On top of the rafters will go 0.419 OSB, felt, and metal roofing.
There are other issues like adding additional bracing to the rafters, but that may or may not be relevant to this question. I'll give more details if anyone requires.

Best Answer

I am guessing the big concern comes from the fact there are 2x4's that are 24" on center and that is either old construction or from something else.

Depending on what will reside on that roof (shingles + snow etc..your loading will change) - the 24 inch centers seem to be fine .. now on to the real deal 2 x 4's vs 2 x 6's and the placement.

Using 2 x 6's between the 2x4's will give you that spacing of 2 x 6 on 24 center. So you will have the structural support as if the 2 x 4's were not there - Of course I am assuming you are just adding the same components in 2 x 6's . A problem will arise for you when you connect the ridge beam unless it is already a 2 x 6.

As noted in one comment it was recommend you get engineered trusses, so I thought to make sure there is clarity here because my answer depends on your terminology matching - so I included a link to describe. Trusses are faster to install .. There is a difference in traditional Rafters versus trusses ...

Look here for details: Differences Rafters and Trusses

Depending on Building codes in your area and the inspectors it might look odd enough that they give you a no pass - even though structurally you might be ok. Weirdness sometimes causes pauses..

While I do not see any problems with what your intent is ; except for the Ridge beam connection ....

An old saying I have :

Cheaper to do it right the first time than it is to do it right the second or third time..

If you must disassemble to do it right - Just do it right, you will be GLAD you did.

Related Solutions

Should I pre-built rafter units, or build them in-place

Since you're putting a gable roof on it, I can only assume this is being built for storage purposes, not for aesthetics.

Building it with a glulam ridge beam would be more difficult, but it would be much stronger. The main issue with a ridge beam design would be lifting the beam into position. If you can manage that pretty easily, the rest would be a breeze. I'd put some ceiling joists up at either end, lift one end of the beam over one of the joists, pushing it far enough to clear the ceiling joist on the other side, like so...

B
   B
      B                 
  JttJttJBttttttttttttttJttJttJ     
  S         B                 S        
  S            B              S
  S               B           S
  bbbbbbbbbbbbbbbbbbbBbbbbbbbbb

B = ridge beam
J = joists at the ends of the barn
S = studs at the ends of the barn
t & b = top and bottom plate

... and then lift the bottom end of the beam up with some ropes. The joists will need some strength to support the ridge beam, and you'll need to be able to stand on them, so I'd use three 2x6's @ 9" OC on either end (where the outermost "2x6" is actually the top-plate of the walls on either end of the diagram) with some 5/8ths plywood on top of them. I'd toss in some temporary studs at the center point to make sure the joists hold up to the weight of two people and the ridge beam. Once the beam is in place, put in the ceiling joists and plywood on top of them to make walking around easier (and to create an attic for extra storage,) then put the rafters up.

Grab some metal brackets for the joists, rafter heels, and rafter/ridge connections (it'll cost you an extra $125, but it will make the structure a lot stronger) from a HI store and your little barn thing will be able to withstand some nasty winds.

Raise a sagging shed roof by winching the walls back into place

Your plan is exactly right, except that you don't need such heavy-duty hardware to do the jacking/winching. Even a pair of motorcycle ratchet straps or a come-along is going to do the job, especially if you jack up the ridge board. I think it'll all move easier than you anticipate. Just make sure you have solid connections to the walls so you don't have any sudden surprises if something lets go. Maybe lag some loops to the top plates.

One or two rafter ties, well-attached, will prevent the problem from happening again. You could probably get away with keeping them a foot or so above the wall plates, too, if headroom is at a premium.

Good luck.

Best Answer

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Should I pre-built rafter units, or build them in-place

Raise a sagging shed roof by winching the walls back into place

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