2×6 or 2-2x4s to support a truss?
I’m just wondering if it is better (stiffer & carries more weight) to use a single 2×6 as the stud in an outer wall of a house (under each truss), or to use double 2x4s. This is in a 9′ high wall, and my gut feeling is that the double 2x4s will be better, assuming that lateral deflection of the stud is the key element to worry about. The house will be 44′ wide and will have a center, load bearing wall. Snow load is 60 psf. A single 2×6 is about $5, whereas double 2x4s are about $4. The cost difference is not terribly worthy of worry, but if double 2x4s also get better stiffness and load carrying capacity, then that may be a swaying factor. Maybe this is a question for an engineer???? Thanks. j
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2 - 2x4s have less of a chance of buckling than a single 2x6 - so structurally - 2 2x4s is the answer. But if this is in an exterior wall and assuming in a cold climate due to the snow loads you mentioned, I would recommend the 2x6 so you can get more insulation in the wall and less thermal bridging.
I agree with Danny Kelly. You will make up the $1 savings in the long-run with added insulation. Also, the exterior wall will really only hold 11' of the 44' roof load. !/2 of the total will be supported by the interior wall, which would be the place to add extra support if you are worried about it, and it's inside the thermal envelope.
Thank you both for the input. I'll be using some kind of double wall system, about 12-15" thick, so I am not worried about the thermal bridging, but glad you pointed it out. The interior load bearing wall will be 2x6. I will give due consideration to the double 2x4s, though that will make fitting insulation a small PITA. It has been suggested, understandably, on other threads to blow in cellulose or 'glass, but that is pretty spendy, I just found out today. For comparison, a 2x6 wall will run $1.70/sf to have 'glass blown in, whereas good old 'glass batts are about $0.44, plus the fun of installing them. I will also check into whether or not we can make the interior wall (of the insulated double wall) also load bearing, but I think it will have to remain non-load bearing as much as possible. Thanks again. john
With a continuous 2-span beam, which is what the roof trusses will be in this case, the center support will carry 5/8 of the total roof load, so be sure there is a continuous load path to footings in the ground under this center wall. And make sure the roof trusses are engineered for a center bearing.
Do not make the interior part of the exterior walls load-bearing unless there is a direct load path to ground beneath them. This is one of the liabilities of a double-wall frame, as the interior wall will tend to take some ceiling/roof load whether intended or not. I would keep the interior double wall ¼" lower than the exterior and either connect it with gussets or a plywood secondary top plate to the outer wall or to the trusses with floating truss clips.
John,
Make sure to check insulation pricing on the double stud wall before you go with fiberglass batts. In our area it's a minimal upcharge to go from a 2x6 to a 12" wall with dry dense-packed cellulose. Most of the cost turns out to be the labor in netting of the walls, not the product itself.
And honestly, I wouldn't use fiberglass batts, regardless of the cost. They are practically impossible to install correctly, and unless you have an expensive flash coat of spray foam, fairly risky from a moisture management standpoint in a well insulated wall.
Robert: Center wall is on a footer; did not know it carried 5/8 of the load, but that makes sense. The truss builder is the one who mandated that the center wall be load bearing. Interior part of the exterior wall "floating" is exactly what I was feeling, but unsure of how to secure it w/out making it load bearing; I will check into those floating truss clips. Thanks. john
Jesse: Point taken; I did not think to ask about the upgrade to thicker wall cost, just assumed.... Trust me: I've installed 'glass batts in 5 structures and you are dead on w/ the concern about installing them perfectly. My present house has been working well for 30 yrs, but I know the insulation could be better ('glass batts in walls). Cellulose: It sounds great, if I can get it installed. The first place I called won't do it. He does 'glass instead. I'll keep calling, but we have few installers here. Thanks for the thoughts. john
A single 2x6 stud in a sheathed wall has almost 3x the bearing capacity of a single 2x4. As far as I can gather it's because the primary failure mode is buckling inward or outward, and the deeper stud has about 3x the stiffness in that direction.
http://www.eng-tips.com/viewthread.cfm?qid=92407
KW: Excellent. That was my question; which way does a stud fail, typically (if there is a "typically"). If the stud is acting like a horizontal beam does, and failing across the 5.5" surface, then roger that on the 3x strength. I believe that the load capacity (uniform load, anyway) of a horizontal beam is a function of the square of its height. Thanks. john
Thanks. I originally found that link while looking at advanced framing, and I was surprised to find that it's a lot stronger than the usual type, at least for vertical loads.
Here's another article on failure modes; I didn't pay for it, but the abstract has some interesting info: http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JSENDH000132000005000745000001&idtype=cvips&gifs=yes&ref=no
(From Jesse T) "Make sure to check insulation pricing on the double stud wall before you go with fiberglass batts. In our area it's a minimal upcharge to go from a 2x6 to a 12" wall with dry dense-packed cellulose." Jesse: I checked on that, and prices are approx $1.80 for a 2x6; $2.20 for a 2x8; $3.10 for a 13" double wall. So, a tad cheaper/cf for the double wall, but not as much as I'd hoped. F-glass batts would run about $1.10 for that wall, and mineral wool will be a bit more. The $2/sf difference will have to be thought about. Better R, no fusing w/ batts, but $4k more.....????
If, in fact, the center bearing wall is carrying 5/8 of the roof load, then a stud directly under a 24" oc truss will support a maximum design load of 1155 lbs. A single SPF#2 9' 2x4 that's restrained laterally by sheathing or blocking can support 1820 lbs and a similar 2x6 can support 7061 lbs without buckling (almost 4 x as much).
A double 2x4 can support 3639 lbs, or about half that of a single 2x6. The only advantage of the double 2x4 is, in the event it loses lateral support, it can support 5 times as much weight without lateral buckling as the single 2x6 (2674 lbs compared to 525 lbs).
Robert: Excellent numbers. If it is a simple calc, I'd like to know how you arrived at the 1155 lb load. I figured each half truss supports approx 50 sf of roof. A 70 lb snow load gives 3500 lbs. Add the 1/2-truss and roofing weight (400 lbs?) to get 3900 lbs the half-truss carries. 3/16 of that will be on the outer stud, or 731 lbs. Is that too simple of an approach? I'll make sure the CDX is well nailed, glued w/ water proof glue, and has fire-stops as well. john
Just remembered; when you add sheet rock, that must add another 100 lbs to each half truss. I also forgot overhang on the truss, which would add 6 sf, or about 300 more pounds. Result: 806 lbs. Is there a wind load that is typically used that I did not account for?
Not including any roof overhang, I calculated 3/16 of the 44' span times the tributary width of each truss (2') times the snow (live) load of 60 psf plus dead load (framing, roofing & drywall) of 10 psf. 8.25 ft x 2' x 70 psf = 1155 lbs.
Wind loads need to be added to the lateral buckling forces on studs, but normal wind forces contribute only about 10-12 psf, depending on maximum wind speed, height of building and site exposure. They can exert -1.2 to +1.1 times static forces on vertical framing members, depending on wind direction and wall orientation.
But the design buckling resistance of constrained studs is sufficiently below the critical rupture stress that normal wind loads are insignificant.
Buckling resistance is calculated by a variation of the Euler formula: 0.3E/(L/d)² psi.
And the maximum allowable axial loading is the buckling resistance of the column times the cross-sectional area.
Shameless plug:
For those who love to play with formulas and want to learn to calculate loads and resistances of framing in residential construction (as well as tons of other useful engineering info), there's room in my upcoming Engineering for the Home Builder class at Yestermorrow Design/Build School in Waitsfield VT on March 20-21.
http://www.yestermorrow.org/courses/detail/engineering-for-the-homebuilder
You're burning the midnight oil, I see. OK, and thanks. I will digest all that. I would love to take your class, but the distance is a killer. R U making a CD of it?
This has been a great string to follow. Echoing Robert R's original comment, you have to be careful about making the interior wall the bearing wall since the sheathing is attached to the exterior wall. This sheathing is likely the thing that is going to resist wind load PARALLEL to the wall. If your roof load and walls are disengaged from this shear plane, you have yourself a recepie for disaster in a high wind condition. if you're lucky you'll only crack some sheet rock or windows. I recommend consulting a structural engineer.
(I also vote for the thicker wall with packed cellulose ;)
Chris: Thanks. Interior wall will not be load bearing, now that I know better. john
If you are concerned weight baring qualities of the 2x6 then you can shorten the distance between your studs. Instead of every 16 you may go with every 12. I used it on my house and many others and the upper base plate then becomes twice as strong. Seriously in the long run you will be better insulated going with the 2x6
Charles: I have learned a lot about insulation in last few months; much, much reading. I will definitely be using dense-packed cellulose, and now wish I would have put it in my walls in 1980. No one (that I knew) up here was familiar w/ the stuff then, so I went w/ Nasty Batts; did blow in a foot of cellulose over R19 Nasty Batts in my lid, though. BTW: We have decided to add on to the present house w/ a 24' W x 22' D addition, so I think 2x4x8', 12" OC on the outer wall, will be fine. I will keep the inner wall 1/4" low as someone suggested above. We did that when we built in '80, and the house is still sitting here (28' span). I'll have double studs, 12" of dense packed cellulose in this addition. I am thinking, too, about tying the two walls together "every periodically", just for kicks, but that is another thread sometime. If I recall, Robert does that w/ plywood strips on his truss walls; I'll keep reading, as the wood is probably 2 yrs off. Thanks for the opinion. john