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We are building an outdoor pergola and foundation work was done by a contractor who built a cement foundation that is 22' by 12'. I have no idea why he made one side so much larger than another.

Will it be fine for me to join two 11' 2x8 and span it across 22' side?

It's an outdoor pergola, nobody would be standing on it but I just want it to withstand wind obviously don't collapse but never done span this wide.

edit: 5/23 18:05

Below is a picture describing what we have.

one side so much larger than another

I meant 22' side is so much longer than 12' side.

Is he building the foundation to the specs that were agreed upon in the contract?

No I'm not building a foundation, I'm building a pergola on top of the already built foundation.

the loads the beam has to carry, and the material.

It would not cary much load at all except a shade and some wirings for it. I'm open to any materials.

"will it be fine for me to join two 11' 2x8" How? Obviously the way you intend to join them would be critical to even a gut-level guess at whether this would be good enough.

I was thinking about doing an overlap board on each side or one side of the two 11' boards. But I'm open to any ideas.

Sorry I learned wood working from my father who doesn't speak English so I don't know if specific technique has an English word for it or not.

welcome to Woodworking!

Thank you!

enter image description here

FreeMan
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jk-kim
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    This would probably be a better fit over at [diy.se]. Of course, it would need more detail. What does "spaced cement foundation spaced 22' by 12'" mean? In what way is "one side so much larger than another"? Is he building the foundation to the specs that were agreed upon in the contract? A picture of what you've got, plus a drawing with dimensions would help a lot. – FreeMan May 23 '22 at 12:21
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    The only way to determine the specs for a beam is to be specific about the span, the loads the beam has to carry, and the material. So, as @Freeman says, you need to much more specific about your plans. That said, there is no way that you span 22' feet with joined 11' beams. Any wooden beam that actually spans 22' feet is going to have to be either a single piece of wood, or an engineered laminate. – Walnut Close May 23 '22 at 14:54
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    Well... I wouldn't say no way. The amount of overlap necessary to securely laminate the beam extension will, however, probably make it economically infeasible. – FreeMan May 23 '22 at 14:57
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    It's a pergola. the beam doesn't support anything but itself. I think if connected well, say with 6-8 bolts (depending on size of bolts), you could probably get away with 12ft boards having a 2' overlap. with no load to support it should be fine (IMO) I understand, 11 or 12 ft boards are less than half the cost of a single 22' board.... This is in the comments because I am not a structural engineer, but it is something I would do personally – bowlturner May 23 '22 at 15:20
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    "will it be fine for me to join two 11' 2x8" How? Obviously the way you intend to join them would be critical to even a gut-level guess at whether this would be good enough. Oh and since everyone else forgot, welcome to Woodworking! – Graphus May 23 '22 at 15:32
  • I adjusted the wording of your opening paragraph a little bit to help make it more clear that this is simply a 22x12' rectangle. The original wording made me think that it was, somehow, more irregularly shaped. Also, you state that " I have no idea why he made one side so much larger than another", isn't he working to your specifications? If not, why not? If this isn't what you hired him to do, you should have a chat to have him redo the work to be exactly what you want before you pay for it. – FreeMan May 24 '22 at 14:52

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I actually built a trellis that had an approx. 22' span using 2"x12"x12'treated lumber. It consisted of two thicknesses joined together with one face centering the 12' piece with 5' pieces on each side. The other face consisted of two 11' boards butt joined at the center. The two layers were secured to each other with nails at 12" 0c. top and bottom. The only weight it supported was a wood trellis with a flowering vine. This seemed plenty strong to begin with but failed after 10 years. The reason was that moisture would trap between the inner and outer beam and eventually rotted out the treated lumber. Determined not to have it fail again, I upped the ante by using similar 2x12" pieces using a 1/4"x 11'x 22' steel plate sandwiched between them. I used steel bolts instead of nails with the bolt ends tucked in drilled recesses. I then covered all the bolts with a piece of continuous trim. The most important thing was that I capped the new beam with an aluminum top cap flashing to prevent water getting between the new sandwich layers. This may be a bit of overkill, but after another decade, it still looks like new.

Ashlar
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will it be fine for me to join two 11' 2x8 and span it across 22' side

If this does not need to support load I think joining two boards at ends will work, if it is done with strength. It would be good to get longer boards if possible so you can create some type of overlapping joint.

I was thinking about doing an overlap board on each side or one side of the two 11' boards. But I'm open to any ideas.

It has always been needed to make longer boards than were available. Everywhere in the world "scarf joints" were created for this purpose, some of them very complex like puzzle pieces!

enter image description here

There are much simpler versions of scarf, and simplest is this:

scarf joint

From this question How can I join two boards at the ends? eBook linked in same answer has entry on scarf joints.

Later, in 19th century and into 20th century simpler joints or plain butt joint were sometimes reinforced with steel plates bolted through. This saved on timber and was also much stronger than plain scarf joint.

Wooden boards overlapping joint works in similar way to these steel plates. Boards must be on both sides. If boards longer than 11' can be bought create short scarf joint, overlap joint or half-lap joint.

Additional reinforcement

It would not hurt to add more reinforcement.

  • Hardwood dowels at least 12" long. Use 3-5. Because you are working with 2x8 boards dowels can be 1/2" or 5/8" diamter. Remember to bore holes deeper than needed so there is space for excess glue!

  • Angled braces or struts. Pergola construction should probably include struts anyway. In addition to direct support and stiffening of structure they shorten the span, reducing load on central joint.

enter image description here

Volfram K
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Double the beams using 2-16', and 2-6'boards. Put the joints at opposite ends, fasten securely, and you have a very strong beam.

Joel McLaughlin
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