I purchased a home with a car port attached to the detached garage. The current carport rafters are 2×4 spanning 12' (24" o.c.) with CORRUGATED FIBERGLASS panels for roofing material. The corrugated panels have held up well however the 2×4 rafters can't support much of a snow load so the previous owner put vertical supports at 6' to support the rafters. The carport can't be used with those verticals in the way. For budget reasons I was going to sister new 12' 2×6 to the current 12' 2×4 rafters and remove the vertical support but I'd like to know how/if that makes them as strong as 2×8?
Sistering a 2×6 to current 2×4 rafter
loadroofing
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That crack is not checking. It's called "double bending" resulting in "horizontal shear failure".
Truss chords do not react to loads like joists do. Trusses are designed with each chord in tension OR compression, but NOT both. When the roofers dropped the bundles of shingles, (or worse, when they stacked the shingles in a concentrated area,) they put the chord in double bending. That is to say, they made the top chord bend. When this happened, the top half of the chord is put in compression and the bottom half is put in tension. This action increases the allowable compression and causes "horizontal shear failure"...tearing the wood fibers horizontally along the length of the wood chord.
Now, just because the chord has failed in horizontal shear, doesn't mean the truss has failed...especially because it is being held in line by the plywood nailing. The key to the TRUSS failure will depend on how the crack ends. That is to say, does the crack end in the middle of the chord, or does it veer off to one side (top or bottom) of the chord BEFORE it reaches the end of the chord. If so, you have a major problem.
I'd suggest you contact a STRUCTURAL engineer (not a CIVIL engineer) and have him review it.
So, I'm curious. Does the crack end in the middle of the chord or not? If not, does it veer up or down?
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.
Best Answer
It does not make it as strong as a 2x8, but there's a way you could make it stronger (with some additional materials...)
Beam strength goes directly as width (twice as wide is twice as strong) but as the cube of height (or depth) so twice as deep is 8 times as strong.
Assuming modern planed lumber, 3.5, 5.5 and 7.25 are your actual height for 2x4, 2x6 and 2x8 material, unless they shrunk them again while I wasn't looking.
For a comparison, we'll look at the height cubed, without trying to relate that to anything.
7.25 cubed is about 381
3.5 cubed is about 43 and 5.5 cubed is about 166, for a total when sistered of 209 - quite a bit smaller than 381.
If you put your 2x6 under, rather than beside, your 2x4s, and then plate them together with some OSB or plywood, you get roughly in the ball park of 729 for the depth cubed.
Indeed, you could come quite close to a 2x8 just by putting 2x4s under the 2x4s, if they are well-bonded with plates, and if the plates are large enough they also add to the stiffness. You can also follow that path to building a small truss in place, which I'd not advise for a house, but it's quite feasible for a carport unless you are in the sort of jurisdiction that will require permits and engineering drawings for reinforcing your carport roof, (that seems unlikely, since it was underbuilt to begin with.)