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Single 2x12x24' vs 2 2x12x12'

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Apr 24, 2013, 11:34 AM
ScottieSLG
Single 2x12x24' vs 2 2x12x12'
In building my deck, I've been advised to go with a single 2x12x24 beam instead of attaching 2 2x12x12' boards together. However, the 2x12x24 is quite a bit more expensive and they are going to take 2 weeks to get here.

How much structure do you lose by attaching 2 boards together vs using a single board? Also, is there more risk of the boards being deformed somehow? I'm worried that if I special order these and they warp that it's going to take me an additional 2 weeks to return them and get new ones.
Apr 24, 2013, 12:54 PM
swschrad
whoa, there, cowboy. a splice in the middle of "structure" hanging in the wind, why, that just ain't done in these here parts.

if it's for show, and not for load, big rip, put it up in 6 inch pieces if you're inclined.

otherwise, there is something called "clear span load" that comes into play. it's the engineering rating of how much load the beam can bear before failing. you don't get the inspection passed without meeting the standard, and people fall through the deck and get injured or die if the inspector didn't catch it.

and there is one little gotcha... you cannot splice two 12 footers and have any strength. by definition and in fact, it violates span load.

death and destruction.

you don't want that.

if there is another support at the 12 foot mark that is clearly capable of bearing all the load that will be held in that 24 foot run, then you can use 14 or 16 footers that overlap and are joined with fasteners (and hopefully connecting plates) on top of that extra support. it would be like an I-beam down the middle of the basement holding up the house, running at right angles to that 24 foot span.

otherwise, the deck is going to fold up in the middle.

-0-

warping... a sign of wet lumber and/or no cross-bracing. you typically will see cross-bracing in the form of additional 2x12s with joist hangars every several feet. many cities post deck construction guidelines on their websites you can download to get their recommendations for that. the deck surface cannot be relied on to keep the entire structure aligned.

This message has been edited. Last edited by: swschrad,


sig: if this is a new economy, how come they still want my old-fashioned money?
Apr 24, 2013, 03:33 PM
ScottieSLG
Well that answers that! I shall order the 24' beams.
Apr 24, 2013, 04:18 PM
joecaption
Really need to see a drawing of what your talking about building.
Your saying beam, there are no "beams" in a deck.
Do you really mean rim joist, girder, ledger?
I've built hundreds of decks and have never once used a 24' anything.


joecaption
Apr 24, 2013, 05:21 PM
redoverfarm
I think this follows a typical word. DEPENDS. That meaning if is is free standing or will be attached to an existing band board of the house? Even free standing will have to have some intermediate support. Attached to existing not so much at least at that intersection of the joist to the ledger. If it is being attached to that existing band board of the house in an apprioate manner then you could get by with shorter material to make your length as it will be supported continious by the lag bolts in the proper spacing.

Maybe this will help. It is code compliant.
http://www.fairfaxcounty.gov/d...ns/decks/details.pdf

This message has been edited. Last edited by: redoverfarm,
Apr 24, 2013, 06:41 PM
CommonwealthSparky
quote:
Originally posted by joecaption:
Really need to see a drawing of what your talking about building.
Your saying beam, there are no "beams" in a deck.
Do you really mean rim joist, girder, ledger?
I've built hundreds of decks and have never once used a 24' anything.

I have never seen a beam that measures 2" in width either. Our definition of a beam is multiple pieces of lumber [usually 2] fastened together with nail, glue measuring at least 3" in width.


Popeye only reached for the Spinach can as a last resort...
Apr 24, 2013, 07:31 PM
Jaybee
However you slice it (pun intended) a single 2x12x24 will be stronger than 2-2x12x12's. However, the question is: Does it matter?

Even with 2x12, you are not going to have a 24' open span. If they are used as joists, then you could get away with a 12' span. As joists, the single 24 will be slightly stronger than the two 12's even though each would have the same three points of support. But not by much.

If the 2x12 is used as a ledger, you are looking at a maximum of 8' span. The dimensions would suggest using a 6' span. In this case, since each 12' segment would have three points of support, the advantage of the single 24' would be minimal.

If the 2x12 is used as a beam, then it too will be supported no more than 8' OC.

It would help if you had a sketch of your deck framing but from where I'm sitting I would say you will be fine with 2x12x12'.


Jaybee
Apr 25, 2013, 06:01 PM
ScottieSLG
Yeah, when I said "beam", I'm referring to the girder.

Here is an extremely crude drawing of my deck plans.

Deck Image

I forgot to add that I'll be spacing my joists 16" OC and use 18 of them.

Edited:

As I was looking at this drawing, it seems to me that I can use a 2x12 8'2" and then another at 14'10". That would line them up on the center of the 3rd post. Then I would do the same thing, but reversed and line them up on the 2nd post, and then glue/nail them together to make a single girder.

Girders

Good idea?

This message has been edited. Last edited by: ScottieSLG,
Apr 25, 2013, 09:54 PM
Jaybee
quote:
As I was looking at this drawing, it seems to me that I can use a 2x12 8'2" and then another at 14'10". That would line them up on the center of the 3rd post. Then I would do the same thing, but reversed and line them up on the 2nd post, and then glue/nail them together to make a single girder.
Girders


You are exactly correct. With four piers per beam you have more than enough support to use less than 24' long 2x12's. Just make sure that all joints are over support and that the beam layout is reversed - just like you have stated.


Jaybee
Apr 28, 2013, 06:35 PM
joecaption
Using constrution adhesive will do no harm, but is rarely done in this case.
Just using 3" long ring shanked ACQ approved nails every 12" in sort of a W pattern will be fine.


joecaption