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I have a 8" thick concrete foundation, and am wondering if I should use 2x6 or 2x8 PT for the sill plate.

I believe 2x6 would be substantial, but if I use 2x8 I would have something to fasten to on the interior side of the basement in the future, and I could have my anchor bolts in the center of the 8" thick foundation, allowing better strength, since it would be the center of the 2x8 instead of 1/3 in from the outside of the foundation for a standard 2x6 sill plate.

That's the only benefit I can think of? What am I missing?

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  • The wall will still be off set or have a step at the bottom so putting down a much more expensive bottom plate is not gaining in my opinion. The anchors are to keep the walls in place for earthquake and hurricane's not a lot is really needed.
    – Ed Beal
    Jun 7, 2017 at 16:07
  • @EdBeal So is there no real gain in strength from a 2x8 having anchor bolts in the center versus a 2x6 having the anchor bolts in 2.75" in the concrete edge versus the 3.75" for the 2x8?
    – CCCBuilder
    Jun 7, 2017 at 16:22
  • 2x8 in the center is stronger but it is not needed and it would cost more. You can build your walls with 2x8 but we don't because the extra strength is not needed but they are stronger. Hope that makes it a bit clearer.
    – Ed Beal
    Jun 7, 2017 at 18:43
  • This is the sill plate prior to setting floor joists on it. My only reason for considering 2x8 is added tear out resistance during high winds. We are in tornado alley and I rather over build if it makes sense. Sounds like it's a toss up, considering the j anchor bolt is only 1" apart between the two sill plate choices.
    – CCCBuilder
    Jun 7, 2017 at 18:52
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    It may be worth it being in tornado alley make sure to use "hurricane" clips to hold the roof trusses to the top plate or your wall may be there and the roof on its way to Ozz.
    – Ed Beal
    Jun 7, 2017 at 21:41

1 Answer 1

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A 2x8 plate is not needed. In calculating the strength of the plate/anchor we look to "develop full strength" for resistance. The key factors are the thickness of the plate, size of anchor, size of washer, depth of anchor AND edge distance from bolt to edge of wood plate.

Edge distance required to "develop full strength " is 4 diameters. So, using a 1/2" anchor, you only need 2" of edge distance. That's why a 2x12 plate isn't even better.

(However, make sure the anchor bolt is not pushed up tight against the edge of the concrete wall or up against the face shell of a concrete block. You need a minimum of 1 1/2" of concrete to flow around bolt to develop full strength in concrete wall and 3/4" of grout in concrete block wall.)

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  • Good to know. Not sure where the 4 diameters number comes into play? That's interesting and sounds like a good theory. I'm wondering if the L or J of the anchor should also face a certain direction for optimum strength. A J facing the center would have more concrete surrounding it, which is what I think I would do.
    – CCCBuilder
    Jun 11, 2017 at 4:33
  • Yes, always hook "towards" center. The 4 bar diameter is resistance for stress perpendicular to the grain of the wood (edge distance on side of plate).
    – Lee Sam
    Jun 11, 2017 at 4:38

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