I intend to build on Weiss Lake in AL. If I use 8x8 posts, can I span 10 ft with 2x10 or should it be 2x12 for the floor joists?

Also was wondering for the foundation can I just sink the wood post or should I pour a concrete footer to set post on? The shore is a peninsula and the dirt is fine clay.

  • Do you have any plans or diagrams you can share? – Machavity Aug 22 '18 at 12:38

How far you can span with a given joist size depends on the load, the spacing between joists, the type / grade of wood, etc. - but not on the size of the supporting posts. There are lots of tables you can find online such as http://www.southernpine.com/span-tables/

I'd see what's done locally but I'd bet you're way better off with piers made of concrete poured into sonotubes rather than sinking wood posts below grade. But if you do, you do need a footer under the post.

  • 2
    I've seen cases where the concrete in sonotubes idea is used to come up above grade some and then wood posts are set on top. Metal post retainers can be set in the concrete when it is poured. – Michael Karas Aug 22 '18 at 11:56
  • @MichaelKaras - thanks for the clarification that's what I meant - just coming above grade with the concrete, then setting the posts on the concrete. – batsplatsterson Aug 22 '18 at 12:18

I live where the ground is mush. We use two kinds of foundations: 1) piles (tree logs) driven into the ground to refusal (usually 100’ - 120’ down), or 2) piles driven until acceptable for friction.

1) Piles driven to refusal are designed to bear on bedrock.

2) Friction piles are designed to resist a load based on the ground “grabbing” onto the side of the pile.

Because it’s so expensive to drive piles, they’re designed to be spaced 20’ - 25’ apart. Then large grade beams (concrete beams) sit on the piles.

Wood piles will rot due to bugs. Bugs can only live with oxygen, which is in the top 10’ (due to tidal action), so we only treat the top 15’. The remaining pile is a “green” tree trunk.

Caissons (concrete piers) are also used, but they are so heavy they often sink under their own weight.

I’d recommend treated wood piles, but you need to know the characteristics of your soil. I’d hire a geotechnical engineer to take soil samples and let you know the best foundation to use.

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