This is probably a common question, but trying to sort it out has my head spinning....

I'm trying to work out the size of a post and beam foundation for a 10x10 shed I plan to use as a workshop. Shed will have 2x6 floor joists, 2x4 wall construction.

What I think I know:

I need 3 beams, as a 2x6 has a max span of +/-8ft. Three beams will also deal with any flexing issues in the 2x6 floor joists. Posts will be set 48" deep (yay, Canadian frost line!).

What I'd like to do:

Sink 2 rows of 3 posts each to support a double 2x8 beam, with posts set 8ft apart, creating a 1 ft cantilever on each end of each beam. Outside beams will be set 1ft inside the 10x10 shed perimeter to facilitate skirting foundation.

Here's a crappy pic of the foundation plan.

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  • Is a double 2x8 beam sufficient or should I upsize to triple 2x8 or double 2x10s?

  • 4x4 posts or 6x6 posts?

  • The roof ridge runs parallel to the beams. Should the outside beams be directly under the bearing walls or is the small offset ok?

Thanks in advance.

  • 1
    Thanks, batsplatsterson. The shed is a pre-fab item, so I'll have to see if I can special order 2x8 floor joists, but I like the two beam idea!
    – Charles L
    Mar 25, 2019 at 14:39
  • 1
    Looks like the shed manufacturer cannot accommodate 2x8 joists, so I'm stuck with 2x6. Sadly, I am not in a position to build from scratch or I would. Looks like I'm heading for 3 beams, double 2x10.
    – Charles L
    Mar 25, 2019 at 17:16

1 Answer 1


I would consider beefing up the beams a little and going to just four posts.

Using table 14-40 in this document:


you can use doubled No. 1 2x10's for a 10' beam supporting 10' long floor joists. Depending what's available where you shop, if it's easier to get No. 2 grade, you might want to bump up to doubled No. 2 2x12's.

For the joists - using this table


you see you can use No. 2 2x8's spaced 12 inches, or No. 1 2x8's spaced 16", or No. 2 2x10's spaced 16" (or even up to 19.2"), or No. 1 2x10's spaced 24".

I'd probably buy all No. 1 2x10s because I'd prefer to just use all one size and better lumber is easier to work with.

I'd use 6x6 posts without even thinking about 4x4s, even if you don't need it for the weight, it's worth it for the additional bearing surface, just makes life easier.

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