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We’re building a pergola using 4” thick 7 ft fence posts. The uprights will be separated by the length of a post and fence posts will also run around the top as beams. This will mean that there’s a butt joint between the ends of two fence posts resting on the top of an upright post.

The beams will be secured to the uprights with 200mm coach screws, so they won’t be going anywhere. However, I also want to tie the ends of the posts together, but without visible fixings. The fixings that I can find seem concerned with securing the beams to the upright, but I’m worried that, over time, a gap will grow between the beams, because the top is in tension. The fixings I can find all tie the bottom part of the ends of the beams together, but this bit is in compression, so I’m not worried about that.

My thought was to get some metal plates and recess them into the top surface of the beam, across the joint. However, I can’t see anything that suggests doing this anywhere. Is there some reason that this is a bad idea and I shouldn’t do it?

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  • If you're using 4" lumber (is it really 4", or actually 4x4—which is really 3.5x3.5"?) are you gonna have one set of "beams" sit on the post and the perpendicular beams supported only by your fasteners? It'd really be great if you showed us a sketch...
    – Huesmann
    Commented May 11, 2023 at 13:15
  • So, "they aren't going anywhere", but "they might go somewhere"? The only way that can happen is if the beams sag, and if they do they're undersized. Also, you seem confused about the tension/compression thing. Beams are not in tension or compression with respect to each other.
    – isherwood
    Commented May 11, 2023 at 13:16
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    Essentially you've asked an XY question. Ask about your design instead of about a proposed solution to a problem with it. (Hint: your real concern is in diagonal bracing, which you haven't addressed here. If you handle that, the rest is fine.)
    – isherwood
    Commented May 11, 2023 at 13:18
  • @Huesmann, your question implies that there will be corners in the beams. This is not likely in a pergola, which usually consist simply of a pair of parallel beams and rafters on them.
    – isherwood
    Commented May 11, 2023 at 13:29
  • Chop up a few extra 4x4's into sections 8-12" long (use angled cuts and you might get an extra piece from each post) and set them between the post top and the beams resting on them (smaller side of this block down \ ___ / ) so you have more bearing area for the beams joint than just the top of the post. It's also a rather classic look. You can put an ogee on the angled ends for a fancier appearance.
    – Ecnerwal
    Commented May 11, 2023 at 14:57

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