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I plan to build a pergola with some 4x4 (pic 1) and the horizontal "beam" is 14' long based on the design. The longest lumber my car can carry is 8', so I am thinking build the horizontal one using four 2x4x8 by gluing or using screws (pic 2). I know two 2x4 is not a real square but it doesn't matter aesthetically in this case. I wonder if this is strong enough to stay straight and hold all the rafters?

Thank you very much.

enter image description here

long beam

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Calling a local lumberyard and having them deliver lumber in the right length will be about 1000x less work even accounting for any delivery fees.

Also, an actual long enough 4x4 will definitely sag with that kind of span, probably even before putting on the rafters. The strength of a beam scales with its height cubed. A 2x10 will be 8 times stiffer and will almost certainly cost less. A 2x8 might be ok too.

I think your pergola design needs too be checked against some load tables before you think about how to fabricate beams.

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  • 4 inch wood at 14 feet is just asking for a good sag by itself. Any load and probably don't need to worry about it sagging.
    – crip659
    Sep 5 at 13:45
  • If your design aesthetic demands 4x4, then start calling steel suppliers. Box section would almost certainly be stiff enough. Sep 6 at 3:07

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