Blocking between Joists for Post Anchor Installation

by Bruce
(B.C.)

What is the best way to block the deck in-between the joists for the posts. What is the best way for building the blocks?

Would 2 boards laid flat (horizontally) be best or 4 boards laminated and installed vertically?

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Aug 23, 2011
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Laying 2x8 flat and blocking
by: Editor-Rich Bergman

After much engineering testing where our firm Intertek has built many assemblies we can say for certain that a 2x8 laid flat between the joists, screwed from opposing joists will do the trick. At least for the common wood species like pine.

We used to go overboard and quadruple laminate 2x8s and install them vertically between joists only to learn the top end performance was still achieved by a single 2x8 blocked horizontally under the deck boards between the joists.

Where this would change is if you were using a much harder post material like Ipe as an example, which by the way is more difficult to work with. But the performance for load rating goes up.

So with really hard wood, the deck screws become the weaker link and tend to pull out before the anchor deforms and the post begins to crack. In that case using a coarser deck screw or a longer screw is required and blocking should be thick enough that the screw do not go through the bottom.

I still like to install 2 - 2x8s flat and that is just my habit. I just think it stiffens up the entire joist and rim joist area a bit more even though testing has shown the weaker elements are typically the post material or anchor at maximum performance.

But even with a single 2x8 the 3" screw does not extend below the underside of the 2x8.

Anchor base - 5/8"
Decking - 1.25"
2x8 blocking - 1.5"

That is 3-3/8" and the screw is 3" so it works.

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