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Old 10-23-2007, 04:40 PM   #1
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A Beam for a Pergola


I am redoing parts of a pergola I constructed three years ago. Along with replacing some 2x8 x 10' joists from across the top, I am replacing the cross beam from the front of the pergola. This existing beam was an 18' 2x8 sandwiched to a 16' 2x8 on the inside and ran a span of 16 feet. Well, the obvious happened. after about 6 months to a year, the entire beam began to sag in the middle. I also used only deck screws instead of good nuts and bolts. What was I thinking??

So now I am replacing it and I want to do it right this time. I am now thinking of using two 2x10s sandwiched around some kind of particle or press board in the middle, and using galvanized nuts and bolts to attach it together. I am thinking this will make a very stout beam, but still wonder if I should add some more 6x6 post support along the span.

Thoughts guys??

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Old 10-23-2007, 09:54 PM   #2
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A Beam for a Pergola


RV; NO NOT NEVER particle board or pressed board outside.Since I see you have 6 x's for "legs just triple the girder from PT lumber, SS thru bolts.
triple gets ya 4 1/2 ur 6 x is 5 1/2. I presume you have no center support.

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Old 10-24-2007, 12:47 AM   #3
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A Beam for a Pergola


Skymaster,

So you're suggesting no sandwiched particle board here... just triple the girth, with I suppose an additional 2x10 to make three of them. The span across here is 16 feet with no center support (6x6 post). Are you saying then that properly attached together (w/SS bolts,nuts), that it won't sage with approx. 13 2x8 x 10' rafters spread out over that 16 feet?

And very curious here... why not sandwiching something like particle board? Is it because of outdoor weather exposure?

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Old 10-24-2007, 09:42 AM   #4
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A Beam for a Pergola


....how 'bout sandwiching the 2x10s with some exterior grade plywood. Would that help or not?

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Old 10-24-2007, 09:46 AM   #5
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A Beam for a Pergola


Why are you wanting to make a sandwich? Is it because you think it will increase the strength? I think the stronger thing to do would be to use 3 2x10s.
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Old 10-24-2007, 11:42 AM   #6
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A Beam for a Pergola


RV; particle board cannot, will not , never meant to be exterior material; A triple 2 x 10 16' with only a Pergola set of rafters on top will NOT sag. As far as a sandwich of ply center and 2 outers makes no real sense. Doesnt add much strength wise, is more work, and unless it is exterior grade ie CDX it also will crumble from the weather. PT plywood is fine but a sheet of that costs more than a 2 x 10. take 2 8's end to end to make 16, the center one take a 8' CENTER it on the first joint, makes 4' overlap, make 2 ends of 4' each, the outer one is same as first, or just get 3 16's if you can get them is better. Also CROWN all of them, keep all crowns UP. Next question;
rv8tor as in Van's Rv pusher?
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Old 10-25-2007, 01:35 AM   #7
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A Beam for a Pergola


Yeah, I guess I just thought it would be cheaper to sandwich the 2x10s. In my first attempt at this beam, I used two 2x8s for this span (instead of 2x10s) and only attached them together with deck screws, and it sagged badly. I simply don't want a repeat of that. Seems someone I worked with once told me about the "sandwich" idea.

So 3 2x10s it will be....wouldn't you know it though, we haven't had rain in about a thousand years, and now I have to wait for it to stop raining here in N. VA !! We need it baaaaad!

Skymaster, I am in fact building a Vans RV-8 ( middle of the wings now). You building an -8 too??

rv8tor

BTW........GO SOX!!!
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Old 10-25-2007, 10:55 AM   #8
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A Beam for a Pergola


RV; Nah too old, too ugly, too cranky to build one :} Besides got a sever case of the Italian Disease, Muhfundsalow.
A triple is way more than adequate for your purposes.just remember Crown UP.
I did my flying years back, havent been in an aluminum can in about 6 yrs now. I miss it so much. I went up the ladder a bit; SEL,Comm, Inst. Just got past 3000 hrs , about 300 hr mulit in there also. Hence the name "Skymaster". The Ole Cessna push me pull you, Skysmasher LOL C-337
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Old 10-26-2007, 12:11 AM   #9
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A Beam for a Pergola


I am going to guess the wood is PT? If so that would be some of the problem as that stuff likes to move and twist and bend and turn ugly. And if you were crowned down that just adds to it. A trip 2x10 would span it just fine unless it is a solid roof with a snow load.
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Old 10-26-2007, 01:26 AM   #10
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Thanks Robertcdf for that response. My plan now is to use 3 PT 2x10s x 16' firmly bolted together with either SS or Galvanized hardware. Which is better here for exterior applications? Should I be using something other than PT? such as Cedar or Redwood? I just don't want it to sag again.

Mixmaster...I mean Skymaster... ye' olde push-pull driver....yeah, I know..you've heard 'em all.. Was your multi done in the C-337 or in a "normal" twin? In other words, is your rating "multi - centerline thrust" or just MEL. I have heard stories of guys trying to take off with only their forward engine running ( because that's the one they can see! ) I do hope you know I'm kiddin' around with ya'.. Ya know...when I first saw your callsign. I wondered abit about that airplane..

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Old 10-26-2007, 10:47 AM   #11
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LOL I never liked the one in front, rear much better no spinning prop to block the view ROFLOL
I spilt my multi time between the 337, A B-55 Baron, and a B-58 Baron. I was totally stupid in not getting my multi ticket when it was "cheap" regretted it forever. prevented me from workin with the commercial ticket. At that time I was very busy in construction and didnt make much of it, youth and stupidity seems to go hand in hand :}:}
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Did ya know there is a neat trick in a skysmasher to be darn shure ya got both turnin and burnin? ALWAYS lead with the rear throttle and taxi on the REAR engine :}:}:}:}:}. And Oh yes it does fly reasonably well on the front alone but slows down to a slower brick FAST. Can ya guess how I know that? LOL LOL

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Old 10-27-2007, 12:55 AM   #12
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Yeah, it's funny how you learn to adapt when you have a unique situation like the ol' push pull Cessna, such as taxiing with the rear engine. I originally got my multi in an old Piper Apache with a 180 Lyc on each wing. It was generally recognized that if you lost an engine with the gear still hanging out, then the best you could hope for would be to slowly pull the goodengine back to have a controlled glide to an off field landing!
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Old 10-27-2007, 12:33 PM   #13
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RV :}:}:}:}:} that same program is still alive and well, even with the B58 and a pair of 300's If You havent made Blue Line ( VMC ) b4 one quits, uh huh only difference is we get to the crash site a bit quicker than the Ole Apache.
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Old 10-27-2007, 01:21 PM   #14
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A Beam for a Pergola


Quote:
This existing beam was an 18' 2x8 sandwiched to a 16' 2x8 on the inside and ran a span of 16 feet. Well, the obvious happened. after about 6 months to a year, the entire beam began to sag in the middle.
Who told you to use 2-2x8's the first time?

Quote:
Originally Posted by rv8tor View Post
Yeah, I guess I just thought it would be cheaper to sandwich the 2x10s. In my first attempt at this beam, I used two 2x8s for this span (instead of 2x10s) and only attached them together with deck screws, and it sagged badly. I simply don't want a repeat of that.
Instead of worrying about it being cheaper, you should worry about it being done right the first time around and structurally sound. I know that this is a DIY'er forum, but certain things that you want to do yourself isn't meant for a DIY'er, and this is one of them. If you have know idea how to size a beam, what makes you think a guess is correct?

So now that you were trying to use the particle board because it was cheaper, how much is it costing you now because you have to replace this beam since you didn't get professional advice first?
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Old 10-28-2007, 11:01 AM   #15
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A Beam for a Pergola


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Originally Posted by Joe Carola View Post
Instead of worrying about it being cheaper, you should worry about it being done right the first time around and structurally sound. I know that this is a DIY'er forum, but certain things that you want to do yourself isn't meant for a DIY'er, and this is one of them. If you have know idea how to size a beam, what makes you think a guess is correct?

So now that you were trying to use the particle board because it was cheaper, how much is it costing you now because you have to replace this beam since you didn't get professional advice first?

Joe, First of all, I'm not afraid to call in a professional if one is needed. I recently considered doing my own work on installing a solar heating system for my pool, but thought better of it. The idea of me walking around on my fairly steep roof three stories up gave me the heebee jeebees. I called in a company that did a first class job and were bonded to boot!

Secondly, I am a DIYer from way back and am not afraid to try things. So if I occassionaly make a mistake, then I only have myself to blame. I learn from it and go on. As far as to why I used 2x8s in the first place for this beam....you got me there. I can't remember who told me these were okay. But a sub contractor told me about the sandwiching the particle board idea. Hence, your so-called "professional advice".

This latest round, I want to do it right therefore I am here soliciting advice from all you pros. The "professional advice" I just received from the contractor at the lumber yard tells me that 2 2x10s are more than adequate for my application, and that a third one added to the beam begins to take away strength simply because of the weight it adds to itself. So which is it ? Three or two?

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