I'm building a deck and designed it with Simpson's online designer. The design calls for beams made from triple 2x8s to be installed in a ABU66Z post base. A triple 2x will have a width of 4.5", and the post base has a width of 5.5". Therefore, there would be a 1" gap when setting the beam inside the post base.

What is the proper way of fastening the beam inside of the post base? Should I leave the gap open, add blocking, or something else? I don't see any info on Simpson's site for this.

Here's a chart from Simpson's post base knowledge base stating a triple 2x can be used inside of a ABU66Z post base.

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  • Scott - what did you end up doing in this situation. I have the same problem and am keen not to have inspection problems.
    – bhspencer
    May 23, 2023 at 22:03

2 Answers 2


A Simpson ABU66Z is not adjustable for your width difference. Typically the ABU would be attached to a 6x6 post that connects between your ABU and your deck beam (the 3- 2x8's) with some additional hardware connecting the post to your 3- 2x8's.

If you want to set your 3-2x8's directly into the ABU66Z then just cut some plywood to fir out the beam- preferably on the inside of your deck where it wont be seen. Either 2 pieces of 1/2" ply or possible a piece of 3/4" ply as 3- 2x8's fastened together will most likely be thicker than exactly 4.5". A hair loose is better than too tight so you don't have to fight getting it in the bracket.

  • This is what I had in mind when I mentioned adding blocking to the gap. However, I didn't know if this would pass inspection. I'll contact my city. Is it typical to use ply for this application? I feel like this would not pass inspection since it's untreated; or are you suggesting using treated ply?
    – Scott Lin
    May 27, 2022 at 7:17
  • 2
    Use treated lumber. It'll pass. Doesn't need to be plywood. It could simply be a series of strips cut from 1-1/2" stock lined up like soldiers.
    – isherwood
    May 27, 2022 at 14:21
  • Hmmm- my apologies- it definitely should be treated- I prefer plywood as it has less chance of splitting. All the plywood we use where I am is treated.
    – Kyle
    May 28, 2022 at 2:46

That post base is also known as Simpson’s Adjustable Post Base. I think the base can adjust to your width.


Edit Note: The term “Adjustable” is in the title:


  • I saw those installation instructions as well. I do not see any specs for what is adjustable. Do you have more details?
    – Scott Lin
    May 27, 2022 at 6:29
  • I think the adjustment in the "Simpson’s Adjustable Post Base" is the slot in the bottom that allows the position on the anchor bolt to be adjusted - not the width of the base.
    – bhspencer
    May 23, 2023 at 22:02

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