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I want to build a floating deck with a pergola but I don't want to set posts in concrete.

My yard is mostly shale with about 5 inches of actual topsoil, if that.

My question is, could I put treated 4x4 or 6x6s on the ground and anchor them with rebar, going straight through and into the shale? Or would that be too weak to hold it? So posts on the ground, held down with rebar, and then the vertical posts attached to the horizontal ones.

The highest winds we get in my area would be around 50mph, so I'm trying to figure out if the weight of the deck and pergola, plus the rebar anchors, would be safe enough.

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I suppose you could do what you're describing; but, my years of building deck and fence in college tells me it's a bad idea.

Even with rock and shale (and no topsoil) wood resting on the ground picks up water, which accelerates rot, even if the wood is treated. Also, nearly all deck framing assumes strong anchor points, so a wooden base (enforced or not) would have more lateral movement than most anchoring systems.

If you don't like posts in concrete (in my area there's enough water that these are problematic) I'd recommend looking at piers to replace the posts. These are concrete cylinders that the entire deck framing rests upon.

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If you get 50 MPH winds, then a gust would easily be a very temporary 70 MPH. Even with low winds of half of that, I can't imaging rebar directly in dirt having enough surface area contact with the surronding earth the hold back an 8 foot lever being pushed on it's top end with 50 MPH winds.

Put the posts at least 6" below the frost line, and minimally two feet deep. The cost of the wood that will go into the deck should be enough of an incentive to do the deck's foundation right.

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  • Would it be any better if I put down a gravel bed first? The only reason I don't want to do concrete is the labor involved in digging the holes. The shale is so bad that I initially had to have a skid steer come in and scrape a bunch out just to get grass to grow. If I just skipped the pergola and went for something that caught less of the wind, would that make it better? I appreciate the info, I think I'm just being lazy. My back can't handle as much strain as it used to Commented Aug 21, 2020 at 3:25
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    The stuff that keeps the deck from moving is the sides of the post against the earth. This means that no matter how hard your surface is, it won't matter as an anchor for your deck, because you can slide a piece of plywood across it just fine. It also means that rebar can be driven into the ground but it has such a small surface area that it will either bite into the soil, bend or break (none of which keep the deck from moving). You need something like a 4x4 or a post, which has lots of square inches of "side of post to earth" contact. Sorry about the back, mine's a mess too.
    – Edwin Buck
    Commented Aug 21, 2020 at 3:29
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    And keep in mind that all those mattresses that fly off their vehicles you've seen over the years were probably about six inches high. There's surface effects of the wind that can move the deck, even if it is heavy. Sure, I can't guarantee it will happen to you, but you don't really want it happen either way.
    – Edwin Buck
    Commented Aug 21, 2020 at 3:32
  • Well, that settles it. I might hire out for the posts just to get it done faster and make sure it's going to work out. Thanks for all the advice. Commented Aug 21, 2020 at 4:01
  • If the shale is solid (seems doubtful from some of what you have said so far) just scrape off the topsoil, drill anchor holes in the solid rock, insert rebar into holes, set your form tubes, and pour short concrete columns on top of and solidly connected to the solid rock. If it's not solid, you'll have to dig it to get a foundation you can trust.
    – Ecnerwal
    Commented Aug 21, 2020 at 12:23

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