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Strength of 2x4 and 2x6

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66K views 33 replies 17 participants last post by  gargey  
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#1 · (Edited by Moderator)
Got a question for you builders.
I am desiging an awning off of my garage and am wondering about the difference of a double 2×4 compared to a 2×6. There isn't much room for a lot of pitch so it will have to be a 2/12. I need to use 2×4 rafters but am wondering about possible sag. If I double every other one with 2×4s, would it give me a strength that is close to a single 2×6? They will be 16" on center and the longest ones are 10 foot. The blue lines below would be the double 2×4s

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#2 ·
Our single car carport has virtually a flat roof- just enough pitch to get water to run. The house is almost 60 years old. The "rafters" are 2×4's 16" on center covered with tongue and groove 3/4" boards and rolled roofing (the roofing has been replaced several times. It has held up- with only minor sagging we live in south central Pennsylvania so there have been winters when the snow has been quite heavy.
 
#3 ·
In this picture, it appears to me that the studs are doubled. You are better off with one 2X6 versus two 2X4 sistered. Rafter size is dictated by your local code. In the past, I have used 2X4 rafters where now 2X6es are required due to the 1994 earthquake here in Southern California. I am not qualified by law to tell you what to do, but for that span, you should be OK with 2X6es. Depends also on what you use for a ridge beam.
 
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#4 ·
AtomJack, I think you are seeing this differently than intended. All of the area shown is the roof, viewed from above. There is no ridge beam as it attaches to the gable end of a garage. The odd shape is to take in as much of the patio area as possible.
Thanks lew, that leads me to think that the doubled rafters will work. And I'd suspect that a 60 year old 2×4 was much heavier and stronger than the stuff sold today!
 
#9 ·
A slightly different perspective for consideration - -

Usually, a 2×6 is preferable to 2 2×4s. There may be an exception if you are dealing with inferior material. A board is only as strong as its weakest spot along its length. If you are dealing with wood that has weak spots, sistering 2 boards together can be a good idea.
 
#11 ·
Since you're in Ohio, you'll be dealing with snow loads. You don't say why you need to use 2×4 rafters, but if this were my project, I would find a way to use 2×8 rafters. I would also have a rafter at the apex of the roof line (where your 10' dimension is located) so the facia boards aren't hanging in space.
 
#12 ·
As long as you have support posts you should be fine with 2×6s at that spacing. My house's roof is fairly steep which helps a lot but is made of 24' 2×4s set 16" on center with only one support each in the center and they have held for 93 years without any noticeable sagging despite being in upstate NY. The load could get heavy being flatter but if they're properly braced to the wall and supported at the open side a 2×6 roof can hold a lot. Just make sure your support posts are buried deeply enough for a proper anchor and the roof is properly attached to them or I'd be more worried about the roof blowing away in a good storm.
 
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#13 · (Edited by Moderator)
OK, this needs more splainin I guess.
This is NOT a clear span and not cantalievered into the existing structure.
There will be post supports at the 3 points opposite the garage wall and there will be headers between the posts, probably double 2×4s.
There will be a 2×6x12 at the top where the rafters tie in and I will use hangers.
There will be at least 5 horizontal ceiling ties that I may also use to help support the rafters. These will tie the garage wall to the headers.
My effort here is to keep the dead load to a minimum as I know there will be some snow load.
I calculated this to be about 5 pounds per square foot so with a snow load of the same I am within tolerance on some charts.
The reason for the 2×4 rafters is because going to a 2×6 would increase the tie in height and I would give up some of the pitch to do that. I cannot go higher as this will be attached to the gable end of a 24 foot wide garage. I plan to use vinyl screen doors around the perimiter so that is my height at the low end of the roof.
This is a poor attempt to see what it would look like. The reason for the weird angles is because the garage and house are not in line and the gable ends are in a V configuration about 12 feet apart.

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#14 ·
You could always make your own trusses. This would increase the strength without losing any height. Top member 2×4, bottom member 2×4, with webbing of 2×4 or plywood. It's important to have very strong joints, which is why I mention plywood. You could cut a groove and glue it in between the members. Of course this would more than double your lumber cost. Just a thought.
 
#15 ·
A doubled-2×4 is only half as strong in flexure as a single-2×6. The bending moment of inertia "I" of a rectangular section is proportional to the cube of the section's height. I = 1/12 [b*h^3] where b is the width and h is the height. Every little bit of additional section height counts WAY more than adding (a whole bunch of) width.

To put it differently:
Under the same weight per sq ft with identical spans, a roof made of ALL doubled 2×4s at any spacing will deflect TWICE as much as a roof built of single 2×6s at the same spacing. This is due to the section's bending moment of inertia described above.

Building codes aside, (!!) what deflection will you find acceptable?
 
#17 · (Edited by Moderator)
For what its worth, a 42" snow load on 10ft beams at 12" CL spacing, supported at the ends will impose about 4050 psi tensile stress on the extreme fibers (bottom grain) of a 2×4 beam and about 2750 psi on a 2×6. The 2×6 sees less load because of the height to length ratio..it works like a big lever..in reverse. Anyway, what I'm trying to say is two 2×4s will carry more load than one 2×6. Two 2×4s equal 4050 / 2 = 2025 psi; which is lower stress than a single 2×6 at 2750 psi. The tensile strength for wood is variable from species to species and depends on flaws, knotts, moisture content, etc. But, I believe anything but redwood, or similar soft, brittle woods, should be able to handle 2025 psi. If in doubt, use kiln dried, clear, quarter sawn white oak. I think it's good for about 7000 psi.

The triangulation in your example would greatly increase the capacity. By a factor of about 3 to 1.

I am not a PE and not a structural engineer. I have been a plant / project engineer for 39 years however. We are the guys who have to make it work, after the professional guys get through playing with the design.
 
#18 ·
That last drawing is what I'm talking about. That looks like a truss to me. Except, would you be bracing on the side, or having all the members in a plane? In a plane is the right way to do it. If you put a brace to the side you only get the strength of the nails holding it in place. This brings me back to plywood.
 
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#19 ·
A 42" snow load???? I get some 10-12" storms on rare occasion but not that much.
If we got 42" of snow, this awning would be the least of my worries. The house roof would collapes too at that load.
I would cut the proper angles and place them like a truss as that, as stated, would make it stronger. The probem is that typically the ceiling joist lays on top of the header, or beam, and is beside the rafter. The bottom of the brace would need to be caught by a joist hanger on the 2×4 ledger because of the slight out of plane condition at the wide end. The bottom of the brace would land beside the ceiling joist. A truss construction usually brings the ceiling member under the rafter and makes the height at the beam about the same as a 2×6.
I think I'll plan on some of these braces under the single 2×4 and use doubles on the others. If I see some sagging then I can either strengthen them with plywood or apply some 2×6s to the sides that extend below the 2×4s.
 
#20 ·
I live in South Central Tennessee. The biggest snow I have seen here in the last 60 years is 18"; and that set a record for all snow here since records have been kept. But our building code requires design for a 48 inch snow load. Go figure.

I ran the numbers on my house, because I was preparing to remove a load bearing wall and replace it with a truss to create a more open floor plan. I found that a normal house will in fact support a 4 ft snow load. Surprised me too.
 
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#21 ·
Crank, is that based on truses at 24" on center or for a certain size rafter member?
You guys got me thinking now that maybe I should consider building half trusses for this.
I could build them on the ground, where I an more comfortable working, and they would not be too heavy to put up. I mostly have to work by myself on this stuff!
 
#23 ·
I own a Design/Build construction company and I agree with nailbanger. Think of how a I-Joist is configured. They're made of 7/16 osb and span a great distance. In your situation I would simply cut your rafters, ceiling joist and stiff back (vertical 2×4 against adjoining wall) and screw a piece of 1/2" osb to the side. Do this on the ground. It won't be that heavy. We done this on several low pitch roofs. Most where for rubber membrane at 1/12 TO 3/12 pitch and as long as 20'.
Good luck
 
#24 ·
rusty, thanks for backing me up. I still don't like the idea of putting reinforcement on only one side, but screws would ease that worry.

I know this isn't what you are looking for, but I (and apparently rusty) have used these numerous times with great success. All you have to do is adapt the design to your situation.

http://www.gp.com/BUILD/product.aspx?pid=1390
 
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