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I am planning to build a pergola, and I have a 6x6 post, and on each side I plan to notch in the 6x6 by 1", then use two 2x10s crossbeams. I am trying to determine which screws I should purchase. My pergola is about 8" x 20" and has six posts (two 10" long sections).

So in effect, I have a 6x6 reduced to a 6x4 at the joint, sandwiched between two 2x10s for a total thickness of 8" (pergola is fairly long, which is why I go with the 2x10s rather than smaller crossbeams).

Picture this, but I intent to notch 1" in rather than using the strongties.

enter image description here

My questions are:

  1. Will notching provide less strength than the strongties? (I could go with strongties, i just don't like the look that much. If I were using 8x8s I would notch 2" and fully recess the crossbeam, but with 6x6s that would only leave me 2" of post, so I think 1" notching on either side is best I can do.

  2. I imagine I should go with 6" screws? Also, 3/8, 1/2, or 5/8 diameter? Obviously the thicker the better I imagine, but the thicker the more I have to off-center them so they don't collide when drilled from the opposite side.

To address the comments:

  1. I am okay with hex bolts all the way through, if that is better. I am thinking Hex 1/2 9" bolts might work, if I can find some. I found 10", but they were only threaded for 1.5" from the tip, so that would not work, and also, that would be a lot of bolt cutting (about 24 of them).

  2. I am in a possible wind zone (gulf cost but inland). 60+ mph gusts are possible.

  3. No solid roof, just slats of 1x2s.

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  • Does your pergola have a roof ? Are you located in a high wind area or seismically active zone?
    – Lee Sam
    Commented Jan 10, 2022 at 7:47
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    My son and I notched the beam into the post by 2" for a 2.5" beam and secured it with 3 1" hidden dowels with waterproof glue. As the post extended further up above the beam the joint is sheltered.
    – Solar Mike
    Commented Jan 10, 2022 at 7:58
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    Have you considered through bolts instead of lags?
    – Aloysius Defenestrate
    Commented Jan 10, 2022 at 13:42
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    You don’t need to stick the bolts out far… I’d do the math for thick washers on each side and cut the bolt at the end of the nut.
    – Aloysius Defenestrate
    Commented Jan 10, 2022 at 20:52
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    Notching the post means that the beam is supported by the post, not the fastener. Since you're at the top of the post, there shouldn't be any need to worry about the thickness of post left. I'd fully notch for each of your beams, then use through bolts to hold the beams to the post, letting gravity support the beam on the notch seat. Use a longer bolt than necessary, then a nut, cut the bolt a few threads beyond the nut & put on an acorn nut to pretty up that side.
    – FreeMan
    Commented Jan 11, 2022 at 16:30

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Definitely use through bolts, both for strength and to provide a stress free clamping force. Large screws could end up splitting the tops of the post over time allowing water to penetrate an already vulnerable area, i.e. the end grain. If you can't find the right length bolts it is possible to use threaded bar with washers and nuts at each end.

Personally, I think you could notch out more than one inch. The resulting tenon for want of a better word, isn't doing very much, even it if ends up being 2"x6" (50x150mm), that's going to take some force to break.

I recently built an elevated deck and sourcing straight 6x6" (150x150mm) columns in todays market proved impossible. We ended up laminating three 2"x6" (50x150mm) together with the center one standing proud to accept the pair of cross beams. Bolted up nice and solid.

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