I’m hoping to correct a sag where a gable dormer ties into the main roof, while also maintaining some headroom. The span from the dormer wall to the bearing wall is 14′ . Would two 2×6 SPF (nailed and glued together) perform as well as a single 2×8 doug-fir beam? I have tried the two 2x6s, and notice some sagging when I lower the jacks. Will it continue to sag? Would I have the same sag if I used two 2×6 doug-fir?
I’m grateful for your consideration!
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Replies
It will depend somewhat on the quality of the fir but in general, Doug fir is stronger than SPF grades.
IMO, it seems like you are doing this by trial and error process which is not really the most kosher framing engineering there is.
Excellence is its own reward!
Given identical grade with no serious defects in the span, the DF will consistently be stronger.
But in your situation, you might be better off getting an LVL beam spec'd for your application, or even a small header-sized lam beam...shouldn't break the bank on either one.
The variations found in a lot of the "common" lumber manufactured today make choosing a size as opposed to a hand-selected board a little chancy unless you know what you're doing. (And even then, a lot of 2nd growth wood can be disappointing).
I'm aware of DFs strength over SPF. I'm wondering, though, do I gain anything by doubling the width of an SPF beam, as opposed to a single DF beam?
I know I should have just ripped a 2x12 DF for this application. As Piffin remarked, it is a bit of trial and error, but I'm learning a lot in the process.
I'll look into the LDL.
Thanks.
DF is something around 10% stronger. It's not a dramatic difference. Like the difference between SPF and something like redwood.
Although my plans called for spruce, I built a newbie boat mast one time out of redwood, thinking that it looked strong enough and anyway it was clear, so it was safe. And anyway I was having a hard time getting Sitka spruce.
First time I got out in a stiff wind that mast sheared off clean at the base. I keep the stub of that mast on my desk as a reminder. Seat of the pants engineering.
The point of my remark is that you should figure out what you need and then use that instead of this patchwork process. LVL might do - definitely better than SPF, but no way for me to know from here..
Excellence is its own reward!
I just finished installing a pair of 2x10 LVLs and no deflection. Bigger is definitely better, and still plenty of headroom under the dormer.
Thanks everyone!
A 2X6 spanning 14' isn't worth much of anything. You need more depth than that. Even a 2X8 isn't deep enough.
The size and type of beam need to be determined by someone who can look at the job and figure out what you need. No way can we tell from here.
A pessimist's blood type is always b-negative.
A 2x8 has about a 30% greater moment of inertia as compared to two 2x6's nailed together. If both the 2x8 and built up 2x6 were made of the same material, the deflection with the single 2x8 would be about 30% less than the built up 2x6's. The moment of inertia for a beam of rectangular cross section is equal to the width times the cube of the depth, all divided by 12. That's why a 2x8 offers a lot better performance than even two 2x6's (because of the additional depth).
Also keep in mind that any beam will deflect under even the smallest load. Admittedly, your 2x6 configurations are probably way undersized, but whatever you select, you may want to overjack the ridge a little so that when you remove the jacks, the load simply straightens the beam out.
If you're concerned about headroom, maybe a pair of 2x8's with a sandwich of flat stock steel in between them would work? If you come up with an alternative beam configuration that offers the same or better moment of inertia than the beam configuration listed in a span table, you should be fine. I don't mean to turn this into an engineering homework problem but it really is. Probably the better solution is to either use what a span table recommends or consult a structural engineer for an alternative solution.
Edited 8/27/2003 1:26:51 PM ET by stonebm
Your reply is sensible. A given beam may be able to "carry" the desired load, but the deflection may become unacceptable. The problem with some span tables is they offer sketchy information at best. To use a span table correctly, one must have the calculated values of Fb that must be supported, with the Fb values listed for the species, and the grade of lumber used to support the load. In addition to the Fb values, you should also have the E values before you even look at a span table. The E values are criticle in determining accepatable deflection. Than you can compare different solutions for the beam, and be reasonably confident the selected member will perform as intended.
Thanks for clarifying my question about doubling up 2x6 versus a single 2x8.
I considered using a layer of steel, and even a steel I beam. I didn't know where to begin looking locally for such material. Thankfully I ran into a long-time carpenter friend who directed me to a pair of LVL 2x10s. I glued and nailed them together. Now installed, I find the headroom issue wasn't as serious as I earlier thought being a dormer and all. The new beam handles the load and span, sag is out of the roof, the exterior dormer wall is plumb, and I am happy to move on.
Thanks for sharing your expertise and your excellent advice.