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I'm building a carport that is 30x24 ft including a 20x24 gable and 10x24 lean to. It currently has 2x4 roof rafters spaced 24" with collar ties about a foot below the gable peak, so 5-6 ft long ties. Seems a 2x4 SPF #2 should really only span about 6 ft horizontally, using the American Wood Council online calculator. Why on earth do I have 2x4 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 2x6 rafters in between the 2x4's make it structurally sound? So final spacing will be 12" between rafters, but alternate 2x4, 2x6. I would put 10 ft collar ties on the new 2x6 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 2x4 and 2x6.

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.

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  • If you're building it now, why would you add rafters "in between"? Why not order properly engineered trusses or use adequately large lumber? Hand-framed structures with no collar ties at the wall line are a bag of worms and I doubt you'll get much satisfaction from random strangers on the web who can't actually see your project.
    – isherwood
    Oct 9 '17 at 19:06
  • @isherwood, I've clarified the questions and details. Does it make more sense now? The problem is in order to order properly engineered trusses, would mean to take the structure apart and rebuild it. The framing is essentially complete. It's not my work, I'm just looking for a way to make it stronger as 2x4 rafters spaced 24" each spanning 10 ft horizontally (12 ft long 2x4's) seems like a disaster waiting to happen.
    – jdods
    Oct 9 '17 at 21:10
  • Well, the normal approach is to add lumber to the existing rafters, not drop them in the gaps. Then, your question really asks for an engineering signoff, and as I hinted there are factors at play that we can't see. I think you need to get someone on-site.
    – isherwood
    Oct 9 '17 at 21:24
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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.

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  • Metal roof. Snow load of a foot would be extreme, but 3-4 inches a few times a year. I'd worry more about ice and hail, strong winds. This is a botched job by a hired contractor, new construction. I'm trying to figure out what to tell him. The ridge beam is 2x6. I'm consider the load being born on both the 2x6 and 2x4 rafters. Of course that will require some playing at the top plate, probably a larger notch on the 2x6 so they all have the height up against the own..
    – jdods
    Oct 9 '17 at 21:59
  • @jdods well if you are putting in 2 x 6's at 24 centers - and already have a 2 x 6 ridge beam you should be fine with that (unless there is a code in your area to prevent it - you can remove the 2 x 4's if you wanted leaving them is not going to hurt the structural integrity after adding the 2 x 6's at that point it becomes overkill - but check your codes and perhaps $150 for a building inspector to look at what you have specifically is WELL worth it. Hope you have a permit, You might even get away with talking to your county building code office and get a freebie ok take a picture with you.
    – Ken
    Oct 9 '17 at 22:10
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    Permits aren't necessary here, rural unincorporated. I even checked with the county. That's a good idea though to show a photo to the county office.
    – jdods
    Oct 9 '17 at 22:48
  • On second thought, I don't think they even do inspections here for building code stuff though. They will assess for taxes of course!
    – jdods
    Oct 9 '17 at 22:56
  • @jdods WHAT??????? "Permits aren't necessary here..." They're required in every state. You just haven't found the jurisdiction in charge yet.
    – Lee Sam
    Oct 10 '17 at 3:20
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I'd suggest a different approach. I have a 23 foot span with a 50 lb per square foot load capacity built entirely of 2x4 stock on 24" centers - the trick being, it's a properly engineered truss, that happens to be built from 2x4's

You could get an engineering consult on how to turn your inadequate rafters (though in fact the collar tie makes them start to perform more like a very simple truss) into a proper truss, and follow the input provided by the engineer who can actually look at your building and run calculations on it.

Or, given your followup comments about no inspections, etcetera, you could perhaps read up on home-made trusses (agricultural extension is usually the best source of information) and build something that looks right (Follow applicable diagrams from your reading) underneath the rafters (now top chords) you have. Even without proper engineering it will probably beat adding 2x6 in spades, if done in the proper manner (thus the reading is required.) Cheaper, but not quite as certain as hiring an engineer. Where "if it falls down, that's your problem" is the building code of choice, it may suit, or it may not - that's up to you; I suggest the engineer.

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  • Well, part of the problem is that the there are no bottom chords. The ceiling is to be vaulted. I'm trying to keep it open. Additionally I want to create a pulley system for lifting canoes on/off vehicles which will put additional strain on the roofing. Currently there may only be collar ties on every other 2x4 rafter, I believe. I'll check when I get home from work today.
    – jdods
    Oct 10 '17 at 14:15
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    Look at scissor trusses.
    – Ecnerwal
    Oct 10 '17 at 14:16
  • Yep, I thought about doing something like that. And that should still allow plenty of room for hanging canoe storage.
    – jdods
    Oct 10 '17 at 14:19
  • so, assuming the lower structural elements are sound, if 12ft long (9.5 ft horizontal span) 2x4 rafters spaced at 24" were instead ALL some type of engineered truss, instead of just all collar tied, it should be good? Or every other one a truss? I just want a way to make the structure sufficient without tearing it down. So I'm going to add on to the existing elements somehow. Just need to figure out smartest way to do that...
    – jdods
    Oct 11 '17 at 13:42
  • Turn them all into trusses, not every other one. The truss structure makes the effective depth (and thus strength and stiffness) much more than that of the individual 2x4's it's made of, or a 2x6, 2x8 or 2x10 for that matter.
    – Ecnerwal
    Oct 11 '17 at 17:27

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