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what size lag bolts for attaching beam to post in pergola construction

I'm building a pergola and would like to know what size lag bolts I should use to attach the beams to the post.


I plan on sandwiching the 6x6 post with two 12X4x16 redwood beams.   My pergola is 16X16 and will have 6 posts.


Please advise

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Posted 2012-10-03T05:56:56+0000  by dmirza dmirza

Hello Dmirza,


Welcome to the community.


What an exciting project.


I think that 4" x 12" beams are a little over kill but if that's what your plans calls for than I would say that you would need to use at least 5/8" x 8" lag bolts. You will need to pre-drill and off set all of the lag bolts so that the bolts don’t hit each other.


Most of the time when you sandwich a post the beams are 2"x instead of 4"x. It will still be strong enough with the post being 8' apart. That way you can use a 10" bolt, washers, and nut to go all the way through both beams and the post. You will have 2 per joint and it will be very secure.


If you were to use a 4" x 4" post than you could set the 4" x 12" beam on top with a post cap/base. Attach it with (12) 16d common nails or #10 x 1-1/2 in. Strong-Drive SD screws. It all depends on the style of pergola you want.


The design of your pergola is what is going to determine what fasteners you should use. I personally prefer the bold going all the way through both beams and the post. This is how I build my deck 10 years ago and it is still very strong even with a spa on it.


Just make sure that you always use fasteners that are rated for outdoor use and they are large enough to hold the load that your putting on it.


I hope this has helped you choose the correct fasteners for your pergola.


Let me know if you have any other questions. I would love to help.

Posted 2012-10-03T19:39:15+0000  by Christine_HD_OC
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