Joining beams and posts for non-square pergola

31 Jul 2018
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United Kingdom
I'm building an odd half D-shaped pergola and would like some advice on joining the 6"x2" beams and 6"x6" posts, where they do not meet square or flush with the side of the post. There is 100 and 1 ways to do this in theory, but I wonder if there is a "preferred way" to deal with this?

Once the pergola posts are fixed into the ground, for the square joins I will cut a 50 mm deep rebate in the posts, to rest the beams and drive two coach bolts trough. I do not want to rest the crossbeam just on the bolts, and would like to avoid brackets. However, for the non-square joins the same method would produe very odd shaped rebates, which would be hard to cut at height, and the lip to rest the crossbeam might not be sufficient. Also fixing coach bolts or screws into this contstruction is not very straighforward. Is there a preferred way to join beams to the post at angles? I had to go with a very thick posts (150 x 150 mm2/ 6"x6") to rest the angled crossbeams with this method. And due to >3.5m span of the top joist, I've decided to have double cross beams on both sides to rest the joists as I would like to avoid warping.

Istead of rebate I could go with a metal brackets to rest the beams and then on the angled joins use some wedged insterts, but then the beams won't rest on anything but bolts, which is not ideal, as brackets won't work due to odd angles. Or I can join all 4 beams using a wedge, as one would do on the decking/shed base, cut the post flat to the heigt below the beams, and then put 2 bolts from the top into the posts. Any suggestions?
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Have a look at a mitred bridle joint, horizontal
Oh, thanks. As I understand you are suggesting instead of simple butt joint to interlock two crossbeams using mitred bridle joint, and then attach them to the post. This would spread the weight, lock the beams together and require less attachents to the posts. Thanks. Though this would be a bit tricky as two double beams need to be aligned and leveled with preinstalled posts, but is a good idea. Even if this is an angled tongue and grove joint this would work. What would the best way to attach these two angeled beams to the post? Using the lap method as before?
Or another way would be do not join all three beams in one place, just join the post and beams from one side, carry the beams over outside the post and then do the angled joint of two double crossbeams (bridle or t&g), is this a better way?

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